SparkRef — Electrician's Field Calculator

Tools

Sizing

Raceway

Protection & Tap

Motors & Power

Smart Wiring

Conductor Material

Insulation Rating

Cable Type

NM-B / AC / UF-B cap ampacity at 60°C regardless of conductor insulation rating (per each cable's article). SE caps at 75°C for typical service-entrance applications. Individual / MC / ITC / TC use the conductor's insulation rating directly (no assembly cap).

Ambient Temp (°F)

Max V-Drop (%)

Circuits

Each row = one circuit being sized. Total current-carrying conductors across all rows drives the raceway derate factor.

Apprentice tip: the wizard picks the smaller conductor that meets BOTH ampacity AND voltage drop. On runs over 100 ft, voltage drop usually wins — that's why long runs upsize even when the breaker is the same.

How It Works

The wizard walks every binding rule against every candidate wire size, picking the smallest size that passes all of them. The result shows the recommended size per circuit + the rule that drove the recommendation.

Rules considered: insulation ampacity, continuous load 125% factor, ambient temperature derate, raceway derate (counts CCC across all rows), voltage drop limit, NM-B 60°C cap, and small-conductor breaker-size limits.

For 3-phase circuits with a neutral, the wizard counts the neutral as a current-carrying conductor when the Nonlinear toggle is on (matches the harmonic-current rule). Balanced linear loads on a 3Ø wye don't count the neutral toward derate.

Wire Ampacity

Conductor Material

Insulation Type

60°C = 140°F · 75°C = 167°F · 90°C = 194°F

Ambient Temp (°F)

Load Type

Conductors in the Raceway

Add a row per wire size in the bundle. Total current-carrying count drives the derate factor — change a row's size to upsize and watch the derated ampacity update live.

Check Signal/control on a row when those conductors are signal circuits or motor-starter wiring — they're excluded from the current-carrying count for derate.

Apprentice tip: pick the ampacity column by the lowest-rated terminal on either end of the run. 90°C wire landing on 75°C lugs is a 75°C circuit — the weakest link sets the limit.

Watch out: parallel sets must all be the same size, type, length, and material — and they share derate factors as if they were one conductor.

Conduit Fill

Conduit Type

≤24" nipple

Conductors

Apprentice tip: fill % is only half the rule — you also have to stay within the conductor count for your conduit size. Both checks must pass before the inspector signs off.

Watch out: nipples (≤24 in. between bodies) allow up to 60% fill instead of 40% — different rule, the calc doesn't auto-switch.

Fill Rules — Max Fill by Conductor Count

# Conductors	Max Fill %
1 Conductor	53%
2 Conductors	31%
3+ Conductors	40%
Nipple ≤24" between enclosures	60%

Why these limits exist: they leave room for pulling without damaging insulation, and for heat dissipation. 53% for 1 wire (easy pull, lots of room) · 31% for 2 (tight geometry can jam) · 40% for 3+ (the workhorse pull). 60% in a nipple ≤24" — short, accessible, no real pull, no heat buildup.

Aux Gutter & Wireway

Equipment Type

Cross-Section

× in

Conductors at Worst Cross-Section

Count conductors at the busiest slice through the gutter — the cross-section where the most wires pass simultaneously, not the gutter total. Why?

Check Signal/control on a row when those conductors are signal circuits or motor-starter wiring — they count toward fill area but not toward the current-carrying count for derate.

This gutter section contains a splice or tap

Splices/taps must be accessible via removable covers or doors. 75% fill check applies at the splice cross-section.

Mixing Different Voltage Systems

AC and DC circuits ≤1000V can share a gutter or wireway — but every conductor inside must be rated for the highest voltage present. Most building wire (THHN, THWN-2, XHHW-2, USE-2) is 600V-rated.

Why 20%?

Heat is the enemy of insulation. The 20% fill rule leaves enough room around every conductor that ambient air can carry heat away before things cook. Push past 20% and insulation pre-heats before the load even connects.

Metal Aux Gutter Exemption

Sheet metal gutter walls dissipate heat. Metal aux gutters are exempt from the ampacity derate up to 30 current-carrying conductors at any cross-section. Nonmetallic gutters and all wireways don't get this — derate kicks in at 4+ CC.

Cable Tray Fill

Tray Type

Inside Width

Inside Depth

Cables in Tray

Add a row per cable type in the tray. The calculator picks the right fill rule for your tray type + cable population mix.

Apprentice tip: tray rules split by cable type — multiconductors, singles, and MV/MC each use a different formula. Mix populations in one tray and the smaller cables get their allowance trimmed; the calc handles that math.

Watch out: with mixed populations, the smaller cables lose part of their area allowance proportional to the bigger cables' widths (the “Sd term”). Don't compare against the simple area limit.

Annex C Quick Lookup — max cables by tray width

The informative annex tables (C.14–C.20) pre-compute max cable counts for the most-common cable types + tray widths. Same math the calc above does — handy for a fast field check when you already know the cable + width.

Why so many rules?

The fill rules split across 6 tray types and three population classes (multiconductor cables, single conductors, MV/MC ≥2001V) because heat dissipation and cable geometry differ for each. The calc picks the right rule for your population mix. Mixed populations get a special formula — the smaller cables don't get the full area allowance because the bigger cables sharing the tray subtract a width-proportional adjustment.

Solid Bottom + Wire Mesh quirks

Solid Bottom caps depth at 6 in. for the control/signal-only fill rules — anything deeper still computes against the 6 in. limit. Wire Mesh follows the same rules as Ladder + Vent Trough, but only when its depth is 50 mm (2 in.) or less.

Singles in Solid Bottom or Solid Channel?

Solid Bottom and Solid Channel aren't covered for single conductors in this fill calc — run multiconductor cable assemblies in those, or verify your AHJ's interpretation before pulling singles.

MV/MC cables ≥2001V

Type MV (medium voltage) and Type MC cables rated 2001 V or over follow a separate rule: single layer, sum of OD ≤ tray inside width. Triplexed and quadruplexed circuit groups count as one bundle for the diameter-sum check. Add a row in the calc with the "MV/MC cable ≥2001V" category for each high-voltage cable type.

Mixed populations + the Sd term

When 4/0+ AWG and <4/0 AWG cables share a Ladder/Vent Trough tray, the smaller cables don't get the full area allowance — the table subtracts 1.2 × Σ OD of the 4/0+ cables. Solid Bottom uses 1.0 × Σ OD. For single conductors, mixing 1000+ kcmil with smaller follows 1.1 × Σ OD. The 4/0+ cables also have to lay in a single layer with nothing on top.

Conduit Jam Ratio

Conduit Type

Conduit Trade Size

Conductor Insulation

Conductor Size

Apprentice tip: jam happens when three same-size cables twist into a triangle inside the raceway and lock together mid-pull. Stay outside the 2.8–3.2 ratio band and the geometry can't lock — pulls go smooth.

Watch out: the ratio only applies to three same-size cables. Mixed sizes can't geometrically lock into a triangle — no jam concern there.

About Jam Ratio

When pulling exactly three equally-sized conductors through a conduit with two or more 90° bends, the ratio of conduit inside diameter (ID) to conductor outside diameter (OD) can land in a narrow danger zone where the three conductors align into a rigid triangular wedge and physically lock inside the conduit. This can happen even when fill percentage passes.

Jam Ratio	Verdict
JR < 2.8	Safe — conductors too large to wedge
2.8 ≤ JR ≤ 3.2	Jam risk — can lock in bends
JR > 3.2	Safe — conductors have room to reposition

Applies only to 3-conductor pulls with 2+ 90° bends. For 2 or 4+ conductors, or straight pulls, jam ratio is not a concern.

Box Fill

Largest Conductor Size

Custom Box Volume (in³)

# Conductors

Mud Ring (in³)

Box Extension (in³)

# Equipment Grounds

# Devices

# Internal Clamps

# Support Fittings

# Terminal Blocks

# Fixture wires ≤14 AWG

Apprentice tip: don't forget the device yoke — every receptacle or switch counts as two volume allowances at the largest conductor in the box. It's the #1 thing apprentices undercount.

Watch out: internal cable clamps add one volume allowance per box (just one, no matter how many clamps). AC-style connectors that thread into a knockout from outside do NOT count.

Volume Allowances by Wire Size

AWG	Volume per Conductor
14	2.00 in³
12	2.25 in³
10	2.50 in³
8	3.00 in³
6	5.00 in³

Volume allowance rules (each based on the largest conductor in the box):
• Conductors — 1× each
• Fixture wires ≤14 AWG from a luminaire/canopy — first 4 omitted, anything past 4 counts at 1× each
• Equipment grounds — 1× for the first 4, then ¼× each beyond
• Internal cable clamps — 1× total (any number of internal clamps, all together; external clamps don't count)
• Devices on a yoke — 2× per yoke
• Support fittings — 1× each
• Terminal block assemblies — 1× each (sized by largest conductor terminated to the assembly)

Load Calculator

Calculation type

Branch-Circuit: face-value sum of every load for sizing branch circuits. Service Load (Pro): applies code demand factors (lighting, cooking, dryer, fixed-appliance 75%, etc.), A/C compressor sizing, and HVAC noncoincident demand.

Occupancy

Occupancy type

# of dwelling units

Enter total units in the building. Below — sqft + specific loads describe ONE typical unit; the calc multiplies by this count then applies the multifamily demand factor. The Optional Method requires 3+ units. Use the Standard Method per unit for 1-2 unit buildings.

Floor area (ft²)

Voltage

# Receptacle outlets (180 VA per yoke)

4+ separately controlled electric heating units

Toggle on when the dwelling has 4 or more separately controlled electric heating units (e.g., baseboards in different rooms) — this drops the heating demand factor from 65% to 40%.

Existing dwelling service-upgrade check

Use this when checking whether an existing service has capacity for an addition (new EVSE, new HVAC, new appliances, etc.). When enabled, each specific-load row gains an "Existing" checkbox — flag what's already in service. Unflagged loads are treated as new. Requires an existing 120/240 V or 208Y/120 V 3-wire service.

Specific loads (optional)

Add a row for any dedicated appliance, sign, show window, multioutlet assembly, or heavy-duty lampholder beyond general lighting + receptacles.

Show conductor sizing target for continuous loads (3+ hours)

The load total stays at face value. The sizing target is the load with 125% applied to the continuous portion (lighting, signs, show windows, heavy-duty lampholders) — use it to size the conductor and breaker, not the load demand.

Dwellings: two values, two purposes

2 VA/ft² is the actual load demand for a dwelling unit — used for service sizing and branch-circuit load. General-use receptacles, the receptacle outlets specified for laundry/garage/outdoor, and the lighting outlets specified for habitable rooms are all baked into this value. Don't add them separately. 3 VA/ft² applies separately when sizing the minimum number of branch circuits a dwelling needs — that's a circuit-count calc, not a load-demand calc.

Non-dwelling: pick the occupancy

Each non-dwelling occupancy has its own VA/ft² value covering the general lighting load. Receptacle outlets get added on top at 180 VA per yoke (or 90 VA per receptacle for quad+ on a single yoke). Office buildings have an alternate path — the receptacle load is the larger of (180 VA × count) or (1 VA/ft² × area). Sign and outline lighting circuits add 1200 VA each, show windows 200 VA per linear foot, heavy-duty lampholders 600 VA each.

Hotel / motel guest rooms

In guest rooms or guest suites of hotels and motels, general-use receptacle outlets and lighting outlets are already included in the 1.7 VA/ft² unit load — no additional load calc required for those outlets. Specific loads (dryers, HVAC, etc.) still add separately.

Continuous loads — sizing vs. load demand

A continuous load is one expected to run at maximum current for 3 or more hours. The load demand itself is not multiplied by 125% — the load is what it is. Where the 125% factor matters is downstream: the conductor and breaker feeding the circuit must be sized so the noncontinuous load plus 125% of the continuous load fits within the conductor's allowable ampacity. Lighting, signs, show windows, and heavy-duty lampholders are typically continuous; dryers and dedicated appliances usually aren't. Toggle the checkbox to surface the conductor sizing target — it sits separately from the load value so you can see both at once.

Cable Lube

Conduit Type

Trade Size

Run Length (ft)

Fill Percentage (%)

Pull Difficulty

Apprentice tip: good lubricant cuts pulling tension by 30–60% depending on conduit type. That's the difference between hand-feeding and breaking out the tugger — never skimp on lube for a long pull.

Field Guidance

Why lube matters: drops conductor friction 50–80%, protects insulation from chafing through bends, saves your back on long pulls. A dry pull through multiple 90°s can physically damage the jacket — and the cost of a reel of cable dwarfs the cost of a quart of lube.

Rule of thumb: ~1 quart per 100 ft at 40% fill in a 2" conduit. Scales up with conduit size and fill %, and with length.

How to apply: coat the cable heavily at the mouth of the conduit as it feeds in — don't pre-pour into the pipe. On long pulls re-lube the feed every ~50 ft so the cable keeps carrying a fresh coat. On tough pulls do a "basket pass" first — pull a lube-soaked rag or lube-impregnated roping ahead of the cable to pre-coat the interior.

Buy more than you need: running out mid-pull is worse than paying for an extra quart. Leftover keeps for the next job.

Tough pull factors: long runs (>300 ft), 2+ 90° bends, high fill (>40%), stiff or XLPE-jacketed cable, cold weather, dry lube from prior coat — bump 1.5× to 2×.

Estimate based on general cable-pulling-lube guidelines. Always check the lubricant manufacturer's coverage spec for the specific product you're using.

Motors

Motor Full-Load Amps pick HP + voltage/phase

Motor Sizing Wizard PRO

Motor horsepower

Voltage / phase

Motor type

Single-phase motors — use squirrel-cage.

Protective device

Conductor material

Termination temp

Nameplate FLA

Overload class

SF ≥ 1.15 or temp rise ≤ 40°C → 125%; otherwise 115%.

Sizes the branch circuit from the table full-load current — conductor at 125%, protection per the motor-type and device, disconnect at 115%, overload off the nameplate. Estimating tool — verify against the current code and your AHJ.

Multi-Motor Feeder PRO

Branch protective device

Each motor's branch device is sized from this; assumes squirrel-cage induction motors.

Voltage / phase (all motors)

All motors on the feeder use this voltage — most feeders carry motors at one voltage.

Apprentice tip: “largest motor” means largest FLA, not largest HP. A 1Ø motor at the same horsepower draws more amps than its 3Ø equivalent — size off the amps.

Feeder conductor sized at 125% of the largest motor plus the rest at 100%; feeder protection from the largest branch device plus the other motors' full-load current. Estimating tool — verify against the current code and your AHJ.

Locked-Rotor Code Letters

Code letter

Motor horsepower

Voltage / phase

The code letter (A–V) is on the motor nameplate — it sets the locked-rotor kVA-per-HP band. Higher letter, higher inrush. If the nameplate isn't handy, most general squirrel-cage induction motors are Code G or H. Used for sizing instantaneous-trip breakers and checking start-up voltage dip.

Duty-Cycle Conductor Sizing

Duty classification

Motor time rating

Motor nameplate current (A)

Non-continuous-duty motors — hoists, valve actuators, tool heads, drawbridges — can take smaller branch-circuit conductors than the usual continuous-duty 125%. Estimating tool — verify against the current code and your AHJ.

Motor Overload Units

Supply system

The number of overload units (heaters or sensing elements) the motor running-overload device must have, and which conductors they go in. Estimating tool — verify against the current code.

Controller Terminal Bending Space

Conductor size

Wires per terminal

The straight-line space a conductor needs at the controller terminal — drives the controller enclosure size. The table runs 8 AWG through 900 kcmil; for 10 AWG and smaller it specifies no value, and for 3 or more wires per terminal it points to the Wire Bending Space tool. Estimating tool — verify against the current code.

Voltage Drop

System Type

Voltage

Load (Amps)

One-Way Distance (ft)

Power Factor

PF 1.0 for resistive loads (heaters, incandescent) · ~0.85 for motor loads.

Conductor Material

Wire Size

Apprentice tip: 3% drop on a branch circuit, 5% total (feeder + branch) is the rule-of-thumb target. Long runs blow past it fast — that's why you'll see #10 on a 20 A circuit when ampacity alone would let you use #12.

Watch out: motors, sensitive electronics, and some manufacturer specs require tighter than 3% — check the equipment data sheet before you commit to a wire size.

Voltage Drop Guidelines

Branch Circuit

Max 3% voltage drop recommended on branch circuits

Feeder

Max 3% voltage drop recommended on feeders (5% total branch + feeder)

Total (Feeder + Branch)

Max 5% total voltage drop recommended for efficiency

Grounding & Bonding

GEC Sizing PRO

Largest ungrounded service conductor

Service conductor material

Parallel sets (1 = single conductor)

For parallel runs, the GEC is sized on the equivalent area (CM × sets). 2 sets of 4/0 ≈ a single 500 kcmil.

GEC material

Electrode type (drives the exception)

"Sole connection" exceptions: a sole connection to a rod, pipe, or plate caps the GEC at 6 Cu / 4 Al. A sole connection to a Ufer caps it at 4 Cu. For a ground ring it's the larger of the ring conductor and a 2 AWG minimum.

Apprentice tip: GEC = Grounding Electrode Conductor — the wire from the service to the ground rod / Ufer / water pipe. Sized off the service conductor, not the load.

EGC Sizing PRO

OCPD rating ahead of equipment (A)

Use the rating of the overcurrent device protecting the circuit. Table rounds UP to the next standard size — so 110A protects to the 200A row.

EGC material

Apprentice tip: EGC = Equipment Grounding Conductor — the green-or-bare wire running with every branch circuit. Sized off the OCPD (the breaker), not the conductor size.

EGC Voltage-Drop Upsize PRO

Ampacity-minimum phase size

The size that ampacity rules alone (base ampacity + derates + continuous) would have required, BEFORE upsizing for VD or any other reason.

Actually-installed (upsized) phase size

OCPD rating ahead of equipment (A)

Same as the EGC card above. Determines the table-minimum EGC before the upsize is applied.

EGC material

Apprentice tip: when you upsize the ungrounded conductors for voltage drop, the EGC has to grow with them — same proportional bump. Easy code violation to miss.

Bonding Jumpers PRO

Jumper type

MBJ bonds the equipment ground bar to the grounded service conductor AT the service. SSBJ runs on the supply (line) side of the service or SDS disconnect. Both sized against the largest ungrounded supply conductor — the same lookup as the GEC.
EBJ bonds equipment on the LOAD side of OCPD. Sized against the OCPD ahead of the bonded equipment — the same lookup as the EGC.

Largest ungrounded supply conductor

Supply conductor material

Parallel raceways (1 = single)

For SSBJ in parallel raceways: a separate SSBJ in EACH raceway is sized based on the LARGEST phase conductor IN THAT raceway (NOT the equivalent total). Set this to 1 if you're sizing a single jumper; set to N to compute the per-raceway jumper for an N-set parallel run.

Bonding jumper material

Apprentice tip: bonding jumpers tie metal raceways and enclosures together so a fault finds its way back to the source. Different sizing than an EGC because they carry a different kind of current.

Separately Derived Systems PRO

Largest SDS ungrounded phase conductor

The largest ungrounded conductor on the secondary side of the SDS (transformer X1/X2/X3 phase or generator output).

SDS phase conductor material

Parallel sets (for SDS-GEC)

The SDS-GEC considers equivalent CM for parallel sets. The SBJ and SSBJ use a SINGLE conductor per raceway — not the equivalent total.

Electrode type for the SDS

Same electrode exceptions as the service-side GEC card — applies to the SDS-GEC output below.

Output conductor material

Applies to the SBJ, SSBJ, and SDS-GEC. Most installations use copper for all three.

Apprentice tip: a separately derived system has no direct electrical connection to the supply except through grounding — transformers, generators, UPS units. Each needs its own grounding electrode plus a system bonding jumper.

Grounding Electrode Comparator

Bond them all. Every electrode AT the building or structure must be bonded together to form a single grounding electrode system. You don't pick one — you bond every one you've got. The picker below shows you what's available; the install dictates which ones are present.

EGC Types Reference

⚡

Copper or Aluminum wire

The most-encountered EGC. Solid or stranded, insulated or bare. Aluminum permitted in most installs but NOT for direct earth contact and NOT in concrete-encased applications.

🔧

Rigid Metal Conduit (RMC)

The conduit itself serves as the EGC. Heavy steel threaded joints. Common where the install is hardpiped and a separate green wire would be redundant.

🔧

Intermediate Metal Conduit (IMC)

Same EGC role as RMC, lighter wall. Threaded joints carry the bond.

🔧

Electrical Metallic Tubing (EMT)

EMT can serve as EGC with listed set-screw or compression fittings. Most-encountered metal-conduit EGC in commercial. Verify connectors are listed for grounding.

🔄

Flexible Metal Conduit (FMC) — limited

FMC can serve as EGC for circuits ≤ 20 A with fitting+conduit total length ≤ 6 ft. Otherwise pull a separate EGC wire. Common gotcha on fixture whips — long whips need the green wire.

🔄

Liquid-Tight Flexible Metal Conduit (LFMC) — limited

Similar limits to FMC: typically ≤ 20 A circuits at ≤ 6 ft. Pull a separate EGC for longer runs or larger circuits.

📏

Type MC armored cable (with listed armor)

The armor of certain Type MC cables can serve as EGC when listed for that use. Other MC types include a dedicated EGC inside the cable.

📏

Type AC cable (BX)

The bonding strip + armor combination serves as EGC. Common in older residential — verify the cable type label before relying on it.

📦

Metal cable trays (with conditions)

Cable tray can serve as EGC when listed for that purpose AND when properly sized for the fault current. Common on industrial installs where pulling individual EGCs through long tray runs would be excessive.

📦

Other metallic raceways and surface raceways

RMC/IMC/EMT/MC/AC are the big four. A few less-common raceway types also qualify (e.g., specific surface raceways and busways with listed bonding). When in doubt, pull a separate green wire — it's never wrong.

Apprentice rule of thumb: if you're using flex or any raceway with questionable continuity (loose set-screws, painted joints, oversized fittings), pull a separate green wire. The EGC's only job is providing a low-impedance fault-clearing path; never compromise it for cost or convenience.

Apprentice read — GEC vs EGC

GEC (Grounding Electrode Conductor) connects the service to the grounding electrode system (rods, Ufer, ring, building steel, water pipe). Sized by the largest service conductor.

EGC (Equipment Grounding Conductor) bonds equipment back to the source so a fault has a low-impedance path to clear the breaker. Sized by the OCPD ahead of the equipment, not by the circuit conductors.

The two are NOT interchangeable — they answer different questions and use different sizing rules. A 200A service might have a #4 Cu GEC AND a #6 Cu EGC running to a 60A subpanel off it.

Tap Rules

Feeder Tap Conductors PRO

A feeder tap is a conductor connected to a feeder without an OCPD at the connection. Four tap rules say when that's allowed. Describe the tap and SparkRef walks the applicable rule, condition by condition.

Tap type

Location

Feeder OCPD rating (A)

Tap length (ft)

Tap conductor ampacity (A)

Allowable ampacity of the tap conductor — from the Ampacity calc or Wire Explorer.

Calculated load on the tap (A)

Terminating OCPD rating (A)

The single breaker or fuse set the tap lands in.

Apprentice tip: a "tap" is a smaller conductor pulled off a larger feeder without its own breaker at the connection. The 10/25/100-ft rules trade length for safety — short enough that a fault clears upstream before things heat up.

The over-25-ft industrial high-bay tap is the High-bay tap mode above — a qualified-persons-only special case. Transformer secondary conductors are handled in the Transformers section. Estimating tool — verify against the current code and your AHJ.

Fixture-Wire Tap PRO

Fixture wire — smaller than the branch-circuit conductor — may be tapped to a branch circuit within set size and run-length limits. Pick the circuit rating, the fixture-wire size, and the run length.

Branch-circuit rating

Fixture-wire size

Run length (ft)

Length of the fixture-wire run from the tap to the luminaire.

Apprentice tip: fixture wires (the small flexible leads on pendants, drop cords, recessed cans) can be much smaller than the branch circuit when the run is short. The chart covers every standard pairing.

Busway Tap PRO

In an industrial establishment, the overcurrent device may be omitted where a busway is reduced in ampacity — within length and sizing limits. Enter the reduced busway section and the device feeding it.

Reduced busway ampacity (A)

Reduced busway length (ft)

OCPD rating next back on the line (A)

The overcurrent device protecting the larger busway upstream.

Apprentice tip: busways take taps differently than wire feeders because the bus itself has fault-clearing built in. The plug-in tap units handle the local OCPD where the tap leaves the bus.

Motor Tap PRO

In a group installation, the tap conductors to a single motor don't need their own short-circuit/ground-fault device if they meet one of three rules. Pick the rule, then describe the tap.

Tap rule

Tap length (ft)

Tap conductor ampacity (A)

Branch-circuit conductor ampacity (A)

Used by the 25-ft rules (⅓ rule).

Branch-circuit OCPD rating (A)

Used by the 10-ft manual-controller rule (⅒ rule).

Apprentice tip: motor taps are sized off the motor's branch device, not the feeder. The 1/3 rule is the floor — most installs land bigger because conductors are sized for the motor's full-load amps.

Range / Cooking Tap PRO

Branch-circuit tap conductors get a smaller-ampacity allowance for household cooking appliances, and for a few other small loads (lampholders, luminaires, snow-melting cable). Pick the case and describe the tap.

Tap type

Branch-circuit rating (A)

Tap conductor ampacity (A)

Calculated load on the tap (A)

Apprentice tip: range / cooktop taps off a small-appliance branch are the most common residential tap. The 20 A floor on the tap conductor is why you'll never see a #14 pigtail on a range — it has to carry the load.

Transformers & Fault Current

Transformer Full-Load Current PRO

Transformer rating (kVA)

Primary voltage (V)

Secondary voltage (V)

Phase

Apprentice tip: FLC = kVA × 1000 ÷ voltage (× √3 for 3-phase). Primary and secondary FLC are different numbers off the same transformer — one formula, two voltages.

Watch out: if the nameplate FLA differs from what the kVA math gives, use the nameplate — it accounts for actual coil losses.

Transformer OCPD Sizing PRO

Transformer rating (kVA)

Primary voltage (V)

Secondary voltage (V)

Phase

Protection method

Apprentice tip: primary protection sizes off primary current, secondary off secondary. Bigger transformer = bigger inrush — that's why the code allows higher percentage multipliers as kVA goes up.

Transformer Secondary Conductors PRO

Which rule

Secondary conductor length (ft)

Secondary conductor ampacity (A)

Calculated load (A)

Terminating OCPD (A)

Primary OCPD rating (A)

Primary ÷ secondary voltage ratio

480 V → 240 V = 2.0 · 240 V → 120 V = 2.0 · 208 V → 480 V = 0.43

Apprentice tip: four distinct rules — the toggle picks the right one for your install. Pick wrong and the calc fails for the right reason.

Four everyday transformer-secondary rules — the 10-ft run, the industrial 25-ft allowance, outside conductors of unlimited length, and the 25-ft rule. Feeder taps live in the Tap Rules calc. Estimating tool — verify against the current code and your AHJ.

Available Fault Current PRO

Transformer rating (kVA)

Transformer impedance %Z

From the transformer nameplate — usually 2–6%.

Secondary voltage (V)

Phase

Run length to panel (ft)

Secondary terminals to the panel. Leave 0 for the value at the transformer.

Conductor size

Conductor material

Conduit type

Conductors per phase

Parallel sets — 1 for a single run.

Connected motor load (A)

Optional — sum of motor FLAs on the panel.

Apprentice tip: AIC = "Amps Interrupting Capacity" — the most a breaker can clear without becoming part of the fault. Pick AIC ≥ the available fault current at that panel, period.

Available fault current = the transformer let-through (worst case, at the secondary) reduced by the conductor run to the panel, plus first-cycle motor contribution. Compare it against the interrupting rating (AIC) of the breakers it feeds. Estimating tool — verify against the current code and your AHJ.

Electrical Laws

What do you want to find?

Solve using

Tip: values accept k / M / m prefixes — type 5k for 5000 Ω, 2.2M for 2.2 MΩ, 50m for 0.050 A.

Apprentice tip: in the field you really only need four formulas: V = IR, P = VI, P = I²R, and P = V²/R. The wheel below covers the other eight — handy when you only have two of the four quantities.

Ohm's Law / Power Wheel — All 12 Formulas

Voltage · V

V = I × R

V = P ÷ I

V = √(P × R)

Current · I

I = V ÷ R

I = P ÷ V

I = √(P ÷ R)

Resistance · R

R = V ÷ I

R = V² ÷ P

R = P ÷ I²

Power · P

P = V × I

P = I² × R

P = V² ÷ R

Each row = one unknown solved from two knowns. DC only (for AC with power factor, see HP/Power).

The Four Quantities — V · I · R · P

V Voltage (volts)

Electrical pressure. How hard the electrons are being pushed through the wire. Higher voltage = stronger push.

I Current (amps)

Flow rate. How many electrons move past a point per second. Current is what actually does the work — and what sizes the wire.

R Resistance (ohms · Ω)

Opposition to flow. Higher R = less current for the same voltage. Every conductor, connection, and load has some resistance; that's where voltage drop comes from.

P Power (watts)

Work per second. Volts × Amps = watts of real work being done — heat, light, motion. All load ratings (HP, kW, BTU) come back to this.

HP / Power / Amps

Convert From

Value

Apprentice tip: three-phase amps are lower than single-phase by a factor of √3 (about 1.73) for the same kW. That's why commercial gear runs 3-phase — smaller conductors carry the same horsepower.

Field Calculator

Benfield Presets tap to insert · edit the numbers

How to use the Calculator

Entering numbers

14 — whole inches
14.375 — decimal inches
5/8 — a pure fraction (inches)
2 space 3/8 — a mixed number (tap space between the whole and fraction)
1' 2 3/8 — full feet-inches (feet button auto-inserts the space after ')
-5 or tap ± to flip the sign of the current value

Math & order of operations

Standard PEMDAS: 2 + 3 × 4 = 14, use parens to override
All four operators: + − × ÷
Trig in DEGREES: sin, cos, tan, csc (cosec), cot
Square root: √ button inserts sqrt(

Precision toggle (1/8″ ↔ 1/16″)

Changes how the result is rounded for display. 1/8″ matches most tape measures and is the field default. Switch to 1/16″ for finer work (cabinetry, panel layout). Your input is never rounded — only the displayed result.

Tape

Tap Tape to show your last 20 calculations. Tap any entry to pull its result back into the expression for re-use.

Memory slots (M1–M5)

Five memory slots live at the top of the keypad for values you want to reuse. Each slot shows its stored value once filled.

Tap a slot → recalls the stored value into your expression (appends it)
Long-press a slot (hold ½ second) → stores the current result; overwrites if already filled
Tap the × badge on a filled slot → clears just that slot
Toasts confirm every action (M1 ← 1' 4″, M1 cleared, etc.)

Field example: Running a series of 8″ offsets at 30° across a multi-pipe rack. Store the mark spacing (8 × csc(30) = 1′ 4″) in M1. On each pipe, mark the first bend, recall M1, tap +, enter the first mark, hit = — the second mark pops out. M1 stays ready for every pipe in the rack.

Heads up: Tape and Memory both reset when you reload the app. Persistent tape + labeled memory slots are a planned update.

Benfield preset chips

Below the keypad you'll find 4 tappable preset pictograms: an offset zigzag, an offset shrink (zigzag with a small bracket on top), a 3-Pt saddle hump (with a dot showing the obstacle), and a saddle shrink (hump with a bracket on top). These are the formulas from Jack Benfield's manual, pre-filled with placeholder numbers you can change in place. The two Offset pictograms pair together (distance + shrink at any angle); the two Saddle pictograms pair together (distance + shrink at the locked 45°/22.5° field standard).

Workflow:

Tap a preset → the formula loads in the display and the preset parameter bar appears. The first placeholder (e.g. the offset depth) is already highlighted amber — ready for your value.
Tap any number in the display to edit it. Every number is tappable (dashed underline). Tap → it highlights amber → your next digit replaces it. Keep typing to extend (e.g. 5 then 0 gives you 50). A blinking caret shows exactly where your next digit will land.
For angles on the two Offset presets, use the one-tap chips in the bar (10° · 22.5° · 30° · 45° · 60°). Tapping an angle chip rewrites the angle inside the active preset's trig call (csc, cot, etc.) so you can flip angles without retyping. The two Saddle presets lock the angle at 22.5°/45° (the field-standard 3-point saddle) — only the rise is editable.
Tap an operator or function → selection/caret clears and further input appends at the end. Tap an empty part of the expression to deselect manually.
Tap = to lock it in; the tape records it, the result glow brightens to show it's committed, and long-pressing a memory slot saves it.
Made a mistake? Tap Undo (bottom-right of the keypad, same row as space) to reverse the last destructive action — preset load, angle swap, clear, or equals.
Dismiss the preset bar with its × when you're done; it does NOT dismiss on its own so the swap chips stay available while you iterate.

Example: 5″ offset at 22½° bend → tap Offset → the 8 is already selected, type 5 → tap 22.5° in the bar → tap =. Expression becomes 5 × csc(22.5) = 1' 1 1/16″. No backspacing, no re-typing the whole formula.

3-point saddle example: 3″ rise over a 2″ pipe → tap 3-Pt Saddle → the 2 is already selected, type 3 → =. Expression becomes 3 × csc(22.5) ≈ 7 13/16″ (distance from center mark to each side mark). Then tap Saddle Shrink → type 3 → = to get the shrink correction.

For the full math behind each preset (when to use which), expand the Conduit bending math (Benfield) section below.

Chaining calculations

Press = and the result replaces your expression so you can keep going. Example: compute 8 × csc(30) = 1' 4″, press =, then − 2 = to subtract 2″.

Live preview vs. equals

As you type, the amber result updates live — useful for checking your expression before committing. = locks it in and writes to the Tape.

Conduit bending math (Benfield)

Sections below match the preset chip order under the keypad. Each preset loads the corresponding formula with editable parameters.

Offset — 2-bend cosecant method

distance between bends = offset × csc(θ)

Classic Benfield offset. Offset is how far you need to jog the raceway sideways. θ is the bend angle — same on both pulls. Pull the first bend, flip the pipe flat, measure to the second mark, pull the second bend at the same angle.

Example: 8″ offset at 30° → 8 × csc(30) = 1' 4″. Mark the first bend, measure 16″ down the pipe, mark the second bend, pull both at 30°.

How to use this preset: Tap Offset. The 8 is pre-selected — type your offset depth. Then tap an angle chip (10° / 22.5° / 30° / 45° / 60°) in the bar above the keypad to change the bend angle. Hit = when done.

Offset Shrink — what the offset "eats"

shrink = offset × ( csc(θ) − cot(θ) )

Shrink is the pipe length lost to the bend itself — the finished run covers less distance end-to-end than the stick you started with. Pipe comes in 10' lengths, so the practical question isn't "what do I cut?" — it's "how far will my stick actually reach after I put this offset in it?"

Example: 8″ offset at 30° → 8 × (csc(30) - cot(30)) ≈ 2 1/8″ of shrink. A 10' stick bent with that offset covers 9' 9 7/8″ end-to-end (10' minus 2 1/8″). Need the run to reach a full 10'? Put a coupling past the offset, or land the offset where the 9' 9 7/8″ coverage is enough — don't plan on ordering a non-standard pipe length.

How to use this preset: Tap Offset Shrink. The 8 is pre-selected — type your offset depth. Tap an angle chip to match the bend angle you're pulling. Same angle chips as the Offset preset, so the pair works as a quick "distance-then-shrink" check.

3-Point Saddle — over a round obstruction

distance from center mark to each side mark = rise × csc(22.5°) ≈ rise × 2.6

A 3-point saddle clears a round obstacle (a pipe, a beam, a conduit crossing your run) with three bends: a center bend at 45° that goes up and over, and two side bends at 22.5° each that bring the pipe back flat. The center bend is always twice the side angle. The 45°/22.5° combination is the field standard — multipliers and shrink work out clean, and it clears most real-world obstructions (1″–3″ pipes) without a ridiculous rise.

Rise = obstruction diameter + any clearance you want. For a 1″ pipe with 1″ of clearance, use 2″ rise.

Example: 2″ rise over a 1″ pipe → 2 × csc(22.5) ≈ 5 1/4″. Mark the center of your saddle on the obstacle, measure 5 1/4″ outward in each direction for the side marks. Pull the center bend first (45°), then the two side bends (22.5° each) in the opposite direction, keeping the pipe flat between pulls.

How to use this preset: Tap 3-Pt Saddle. The 2 is pre-selected — type your rise. Hit =. The angle is locked at 22.5° (side) / 45° (center) — the standard that matches the 3/16″ shrink rule. For non-standard saddle angles, type the expression manually: rise × csc(your-side-angle).

Saddle Shrink — what the 3-point saddle "eats"

shrink = rise × ( csc(22.5°) − cot(22.5°) ) ≈ rise × 3/16″

The 3-point saddle behaves like a single 22.5° offset for shrink purposes — the center bend pushes the pipe up, the two side bends bring it back, and the net loss of end-to-end coverage equals one 22.5° offset's shrink. Field rule of thumb: about 3/16″ of shrink per inch of rise.

Example: 2″ rise → 2 × (csc(22.5) - cot(22.5)) ≈ 3/8″ of shrink. Shift your center mark 3/8″ toward your reference end so the finished saddle lands dead-center on the obstacle instead of past it.

How to use this preset: Tap Saddle Shrink. The 2 is pre-selected — type your rise (same value you used for the 3-Pt Saddle preset). Hit =. Angle is locked at the 22.5° field standard to match the 3-Pt Saddle preset.

Math source: Jack Benfield's Benfield Conduit Bending Manual. Always verify takeoff and deduct values against your specific bender and conduit type. The app is a reference — your eyes and the pipe are the final authority.

Wire Explorer

Scroll → Smallest (16 AWG) on the left grows to 2000 kcmil on the right.

Ampacity Chart

how amperage scales across the whole wire-size range

60°C = 140°F (TW, rare) · 75°C = 167°F (THW/THWN) · 90°C = 194°F (THHN/XHHW, most common)

Tap any point on a curve for the exact value.

Apprentice read: see how the curves flatten at bigger sizes? Doubling the wire doesn't double the ampacity once you're past 4/0 — you run into heat-dissipation limits. That's why long/high-current feeders often run in parallel instead of one giant conductor.

Voltage-Drop Chart

Voltage Drop Chart

how voltage drop scales with distance, by wire size

Load A

Voltage

Phase

Material

Target 3%

Tap a curve or chip below the chart to see where it crosses your target.

Apprentice read: small wires drop hard, fast. Doubling the distance doubles the drop. Doubling the load doubles the drop too. But going up two wire sizes (e.g. #12 → #8) cuts the drop by more than half because resistance falls off non-linearly. Long runs reward big wire even when ampacity says you don't need it.

Conductor Comparator

Pick up to 4 conductor sizes to see their properties side-by-side. Toggle between copper and aluminum to see how material shifts ampacity and resistance for the same physical size.

Pick up to 4 to compare

Quick pairs:

Apprentice tip: notice how doubling the cross-section roughly halves the resistance but only bumps ampacity by 30–40%. That's why long feeder runs upsize for voltage drop before ampacity — voltage drop is the harsher limit on long pulls.

Junction-Box Sizer

Add every conduit entering the box and pick which wall it's on. SparkRef sizes the box — straight pulls need 8× the largest conduit; angle & U-pulls need 6× the largest plus the sum of the other conduits on that wall. Pull boxes are square, so the bigger of the width and height requirement sets the size — then SparkRef picks the standard box.

Pull type

Conduits entering the box

Check against a box

Leave on "Smallest that fits" for a recommendation, or pick a specific square box you've got to verify it passes.

Pull-box sizing only kicks in at 4 AWG and larger. For 6 AWG and smaller conductors, use the cubic-inch box-fill rules (the Box Fill calc). Standard square pull boxes run 4, 6, 8, 10, 12, 14, 16, 18, 20, 24, 30, 36 inch — typically 4″–6″ deep on the smaller sizes, deeper on the large ones. The 8×/6× rules exist so big stiff conductors get the straight-line room they need — too-small a box bends them tighter than their listed radius and damages the insulation.

Burial Depth

Pick where the run is going and what you're burying. The picker returns the minimum cover depth and draws the cross-section to scale. Vehicular and frost-heave areas drive the deepest cover; raceways generally allow shallower depths than direct-buried cable.

Location

Wiring method

Cover depths are MINIMUMS. Local frost line trumps the table — in cold-climate jurisdictions you may need 36"+ even though the table says 24". Verify with your AHJ. Also: direct-buried conductors emerging from grade must be protected from physical damage by raceway extending at least 8 ft above grade OR to the point of entry to the building, whichever is less.

Overhead Clearances

Pick what's beneath the service drop and at what voltage. The picker returns the minimum vertical clearance and draws the side-view to scale. Truck-traffic areas drive the highest clearances; sloped roofs and side-mounted risers permit reductions.

Scenario (what's under the service drop)

Heights are MINIMUMS measured at the LOWEST point of the drip loop or service-drop conductor. If the conductors sag (and they always do — that's how messenger-supported service drops work), measure to the lowest point of the sag. The point of attachment must keep the conductors above these clearances under all expected loading conditions (wind, ice, temperature contraction).

Working Space

Equipment likely to be examined or serviced while energized needs guaranteed clear space in front of it. Pick the voltage to ground and the condition across the working space — SparkRef returns the minimum required depth and draws the clearance to scale. Condition 1: nothing live or grounded on the far side. Condition 2: grounded parts (concrete, masonry, metal) on the far side. Condition 3: exposed live parts on both sides.

Voltage to ground

Condition across the working space

The working space is for a person — it cannot be used for storage. Depth is measured from the exposed live parts (or the enclosure front if the parts are enclosed). Width is the wider of 30 in or the equipment itself, and the space must allow equipment doors to open at least 90°. Headroom is 6½ ft or the height of the equipment, whichever is greater. Verify against your AHJ.

Wire Bending Space

A conductor landing on a terminal needs room to bend without being kinked. Pick the conductor size, how many wires share the terminal, and whether the conductors enter or leave through the wall opposite the terminal — SparkRef returns the minimum bending space and draws it to scale. Opposite-wall runs need more depth because the conductor makes its bend in line with a straight run.

Conductor entry

Conductor size

Wires per terminal

Measure the bending space in a straight line from the end of the lug or wire connector, perpendicular to the enclosure wall. Removable or lay-in terminals listed for a single wire may permit a small reduction. The 14–10 AWG sizes have no tabulated minimum — general installation practice applies. Verify against your AHJ.

Photovoltaic Systems (PV)

Solar generation — string sizing, voltage vs temperature curve, conductor sizing, OCPD, and rapid shutdown.

PV String Sizing

Max #modules in series before exceeding inverter max input voltage

Module Voc (V)

Voc temp coeff (%/°C)

Typical crystalline silicon: −0.30 to −0.35. Check module datasheet.

Lowest expected temp (°C)

From ASHRAE 2% extreme annual minimum for site location. −15°C ≈ 5°F (common cold-climate baseline).

Inverter max input (V)

Common: 600 V (residential), 1000 V or 1500 V (commercial). Verify nameplate.

Why temp-correct?

PV module open-circuit voltage rises as temperature drops (negative temp coefficient on Voc). On the coldest day of the year at sunrise, Voc can spike 15–25% above STC nameplate. If your string's cold-Voc exceeds the inverter's max input voltage, the inverter trips or damages. Using the lowest expected ambient + the module's temp coefficient to compute corrected Voc is required.

The math: Voc_corrected = Voc_nameplate × (1 + temp_coeff_per_C × (T_min − 25°C)). Coefficient is negative, T_min < 25°C, so the product is positive — Voc goes UP.

Max modules per string = floor(Inverter_max_input / Voc_corrected). Always round DOWN.

String Voltage vs Temperature

Watch cold-weather Voc rise climb toward the inverter's ceiling

Module Voc (V)

Voc temp coeff (%/°C)

Negative — crystalline silicon runs −0.30 to −0.35.

Modules in series

Inverter max input (V)

PV Conductor Sizing

Minimum conductor ampacity for PV source + output circuits — the 1.25 × 1.25 stack

Source-circuit Isc (A)

Short-circuit current per source circuit from module datasheet. For PV output circuits (combiner → inverter), use the sum of paralleled source-circuit Isc values.

Conductor terminal rating

PV wire (USE-2 / PV / THWN-2) is typically rated 90°C, but the inverter/combiner terminals are usually 75°C — and the lowest-rated termination governs.

PV OCPD Sizing

Source-circuit fuse rating + when the OCPD-not-required exception applies

Source-circuit Isc (A)

Parallel source circuits in combiner

How many strings tie into the same combiner / inverter input. 1 string = single-source config (no parallel current to back-feed any individual string on fault). 2+ = parallel config — each string can be back-fed by the others.

Rapid Shutdown

Where required, voltage + distance thresholds, module-level shutdown rules

Where required: PV systems on buildings — one- and two-family dwellings, commercial, industrial. NOT required on ground-mounted arrays that are not on or attached to a building (verify with AHJ; some jurisdictions extend it).

Boundary thresholds:

Outside the array boundary (more than 1 ft from the array on all sides) → controlled conductors must drop to ≤ 30 V within 30 seconds of rapid-shutdown initiation.
Inside the array boundary (within 1 ft of the array) → controlled conductors must drop to ≤ 80 V within 30 seconds. Drives the module-level shutdown requirement — string voltage in a 12-module string at 80 V/module = 960 V; can't get to ≤ 80 V at the array without per-module devices.

Module-level rapid shutdown: required on all such systems installed after 2019. MLPE (microinverters or DC optimizers with shutdown firmware) and integrated shutdown PV modules both satisfy this.

Initiation device: the rapid-shutdown switch must be labeled and accessible from outside the building. Typically co-located with the service disconnect or marked at every service entrance.

Labeling: permanent, reflective, contrasting label at every service entrance reading "SOLAR PV SYSTEM EQUIPPED WITH RAPID SHUTDOWN" — exact wording depends on whether rapid shutdown is initiated outside the array boundary or at the array.

Apprentice tip: the rapid-shutdown switch is a life-safety device for first responders pulling the front-line knock-out on a structure fire. Wrong wiring of the initiation circuit, missing label, or no module-level device = open inspection failure AND a real first-responder hazard. Treat it like a fire-alarm circuit — install, label, and TEST.

Why 1.56× the array Isc?

Two stacked 1.25 factors — multiplied together = 1.5625× — are what the code demands on every conductor that carries PV source or output current.

First 1.25 — the irradiance factor. Module nameplate Isc is measured at Standard Test Conditions (STC) — 1000 W/m² irradiance, 25°C, AM 1.5 air mass. Real-world conditions routinely exceed STC: high-altitude clear days deliver 1100–1300 W/m²; cloud-edge enhancement can briefly push 1400+ W/m² as a cloud reflects extra light onto the array. The 1.25 factor wraps that overshoot.

Second 1.25 — the continuous-load factor. A continuous load is one "expected to flow for 3 hours or more." Midday PV is continuous by definition. Any continuous load already has to be sized with a 1.25 conductor factor — PV inherits this rule.

Stacked: 1.25 × 1.25 = 1.5625. The conductor ampacity must be at least 1.5625 × Isc before any temperature or bundling correction. After applying derates, the conductor must still carry ≥ 1.25 × Isc continuously (the OCPD protecting it is sized to 1.25 × Isc).

Apprentice tip: if you remember one number for PV wire sizing, it's 1.56. Take the array Isc, multiply by 1.56, pick the conductor with at least that ampacity at the lowest-rated termination temperature. That gets you to a code-compliant conductor for 95% of residential and small-commercial PV.

Energy Storage Systems (ESS)

Battery storage — disconnect & ampacity, aggregate kWh limits, runtime & depth-of-discharge, working space, ground-fault protection.

ESS Disconnect + Ampacity

Battery disconnect + conductor ampacity — 125% continuous

Max continuous current (A)

From battery / inverter nameplate. Use the LARGER of continuous discharge or continuous charge rating.

System voltage

ESS-only circuit (disconnect at battery)

Check when this is a dedicated ESS feeder. Toggle off for combined PV+ESS feeders (different disconnect-grouping rules apply).

Why 125% continuous?

Conductors supplying ESS DC circuits shall be sized for at least 125% of the rated max continuous current — same rule pattern as PV and EVSE since batteries can charge/discharge continuously for hours during peak demand or solar-to-storage events. The 25% margin gives thermal headroom for the conductor under sustained current.

The math: conductor_amp = max_current × 1.25. Round UP to the next standard OCPD/conductor size. The disconnect is then sized to match or exceed the conductor.

Aggregate ESS Capacity

Multi-cabinet kWh totals + location-based residential limits

Number of ESS cabinets

Rated kWh per cabinet

Typical residential lithium ESS: Tesla Powerwall 13.5 kWh · Enphase IQ 10/40 · LG RESU 10/16.

Installation location

Residential limits assume listed + thermal-runaway-tested cabinets. Non-listed lead-acid > 50 V is capped at 9.6 kWh and is not covered by this calc — consult the applicable fire-safety standard directly.

ESS Runtime & Depth of Discharge

How long backup actually lasts — total kWh × DoD × round-trip efficiency ÷ load

Rated capacity (kWh)

Nameplate rated capacity — e.g. 2 × 13.5 kWh Powerwalls = 27 kWh, 1 × Enphase IQ Battery 10 = 10.5 kWh.

Average continuous load (kW)

Steady-state load during backup. Residential whole-home backup avg: 2-4 kW. Critical-loads-only backup: 0.8-1.5 kW. Cooking / EV charging spikes pull more.

Depth of discharge limit (%)

How much of rated kWh the system will actually deliver. Tesla Powerwall: 100% usable. Enphase IQ Battery 10: ~96%. Older lead-acid: 50% (cycle-life protection).

Round-trip efficiency (%)

DC-to-AC conversion + battery internal losses. Lithium-ion: 90-95%. Hybrid inverter: ~90%. Lead-acid + standalone inverter: 75-85%.

Runtime vs Load

How fast backup hours collapse as the load climbs

Rated capacity (kWh)

Operating load (kW)

Marks your operating point on the curve.

Depth of discharge (%)

Round-trip efficiency (%)

Working Space Envelope

Clearance + access requirements around every ESS cabinet

Every stationary ESS over 50 V nominal needs general working space plus ESS-specific clearances. The cabinet's listing usually calls out additional ventilation and thermal clearances on top of the electrical-code minimum.

The working-space envelope:

Depth in front of the ESS: 3 ft (914 mm) minimum for 0–150 V to ground (Condition 1). Deeper if the opposite wall has live exposed parts.
Width: at least the width of the ESS cabinet, but never less than 30 in (762 mm). The disconnect handle must swing freely in both open and closed positions.
Height: minimum 6.5 ft (2 m) headroom above the floor in the working space.

Additional spacing between adjacent ESS cabinets: 3 ft (914 mm) separation between units in residential installs unless the listing permits closer spacing (some listings allow 0 in side-by-side mounting for thermal-runaway-tested cabinets — check the listing).

Egress & access requirements:

ESS shall not block an egress path — install where the working space doesn't overlap an exit corridor.
Disconnect shall be readily accessible — handle reachable without using a ladder or moving objects.
Lockable in the open position for service access.

Apprentice tip: the 3 ft front + 3 ft side gives every ESS a roughly 6 ft × 6 ft floor footprint when you draw the working-space envelope. Two Powerwalls side-by-side in a garage = ~12 ft of clear wall. Plan the layout BEFORE the customer picks the spot — "we'll just stack them in the corner" is the most-flagged callback on ESS installs.

Ground-Fault Protection

DC GFCI / GFPE on ESS circuits — what the standard requires

Every ESS DC circuit shall include ground-fault detection and interruption — but the implementation is almost always integrated into the ESS itself, not a field-installed device.

What's required:

Detection: the ESS shall include a means to detect a ground fault on the DC side
Interruption: a ground fault shall cause the affected ungrounded conductor(s) to be automatically disconnected from the source
Indication: the system shall provide visual or audible indication of a ground-fault condition

How it's typically met: the listing for the ESS cabinet includes internal ground-fault detection circuitry on every DC string. Tesla Powerwall, Enphase IQ Battery, LG RESU all embed this — the installer does not add a separate DC GFPE device. Your job is to verify the listing and install per the manufacturer's instructions (which usually means following torque specs on the DC bus and grounding electrode connections).

AC-side GFCI? Not generally required for the ESS itself, but the inverter feeding the ESS may need GFCI/GFPE on its AC output per the inverter's listing or per local AHJ amendments. Verify with the inverter spec sheet.

Apprentice tip: a "ground-fault detected" alert on a residential ESS is almost always a moist conductor or a corroded ring terminal at the DC disconnect. Open the cover with the system OFF, inspect every torqued connection, look for green-blue corrosion on copper, and re-torque to the manufacturer's spec. If clean and tight, it's an internal cell-balance issue — call the manufacturer; do NOT open the battery cabinet.

Why ESS has kWh location limits

Lithium-ion ESS doesn't burn the way a wood-frame house burns. A cell in thermal runaway expels gas at 800–1200°F, the gas re-ignites in the room, and the fire is self-sustaining without external oxygen because the cells provide their own. Fire crews can't extinguish a runaway-stage ESS fire — they protect the perimeter and let it burn out, which can take 12–48 hours per cabinet.

That's the reason the fire-safety standard caps residential ESS:

Living space: 20 kWh per cabinet, 40 kWh aggregate inside the dwelling — the most stringent, because occupants are at higher fire-progression risk.
Garage / utility room with rated thermal barrier: 40 kWh per cabinet, 80 kWh aggregate — the standard residential install. A 1-hour rated wall (5/8" Type X gypsum on the dwelling side) buys time for occupants to evacuate before runaway breaches.
Outdoor / detached structure: no fixed aggregate limit in the residential scope (commercial rules apply at large scale). Cabinet spacing still enforced — 3 ft inter-unit per the listing.

The thermal-runaway propagation test is what differentiates a "listed" ESS that gets these limits from a non-listed system capped at 9.6 kWh. That test intentionally triggers thermal runaway in a single cell and measures whether the cabinet contains the propagation. ESS that pass get the higher kWh allowances; ESS that don't pass are residential utility-room-only at the 9.6 kWh cap.

Apprentice tip: when a homeowner asks "can I put 3 Powerwalls in my finished basement?" — the answer is NO without an upgraded thermal barrier construction and AHJ buy-in, because that exceeds the 40 kWh living-space aggregate. The conversation is "let's put them in the garage" or "let's add a rated mechanical room." Don't make this someone else's problem at inspection.

Electric Vehicle Supply Equipment (EVSE)

EV chargers — branch-circuit sizing, multi-EVSE service load with EMS, panel-load impact, disconnect, GFCI, hardwired vs cord-and-plug.

EVSE Branch-Circuit Sizing

Continuous load at 125% · EMS reduction

Charging level

EVSE current rating (A)

From EVSE nameplate continuous rating. L1: 12–16 A · L2: 16–80 A · DC Fast: 60–500 A.

Voltage (auto from level)

Energy Management System limits this EVSE

Multi-EVSE Service Load

Multiple EVSEs on one service — with or without an Energy Management System

Number of EVSEs

Per-EVSE continuous current (A)

Assume identical EVSEs. If mixed ratings, run this calc once per group and sum results.

Site voltage (V)

EMS limits site total current

Panel-Load Impact

Does the EVSE fit the service — or does it need an upgrade or load management?

Service rating (A)

Existing demand load (A)

The home's current calculated demand load, before the EVSE.

EVSE nameplate current (A)

Single EVSE nameplate, or the aggregate from the Multi-EVSE card above. The gauge applies the 1.25 continuous factor for you.

EVSE Disconnect Requirements

When the disconnect is required, where it lives, what it has to do

When required: EVSE rated > 60 A or > 150 V to ground. Below those thresholds the branch-circuit breaker is permitted to serve as the disconnect.

What it does: simultaneously disconnects all ungrounded supply conductors. Single-pole + isolation switch combinations do not qualify.

Lockable in the open position — either an integral lock or a portable lockout / hasp that stays with the disconnect.

Location: readily accessible. If not within sight of the EVSE, the disconnect must be lockable so a technician working on the EVSE can confirm power is dead and lock it out before working on it.

Apprentice tip: the >60 A threshold catches every L2 charger 80 A and above (common commercial spec) plus all DC fast chargers. Residential 40/48 A L2 chargers fall below 60 A and don't need a separate disconnect, but most municipalities still require one within sight as a local amendment — check with the AHJ.

GFCI & Receptacle Rules

When GFCI is required on the supply side vs. when the EVSE's CCID covers it

Receptacle-supplied EVSE (cord-and-plug): the receptacle outlet feeding the EVSE shall be GFCI-protected. Applies to 125 V / 250 V single-phase receptacles on dedicated EVSE branch circuits. Standard residential 14-50 / 6-50 receptacle EVSE installs need GFCI ahead of the receptacle.

Hardwired EVSE: that GFCI rule does not apply to hardwired EVSE — the EVSE's listed CCID (Charge Circuit Interrupting Device) provides equivalent personnel protection on the cord/handle side.

The CCID is not a GFCI. Common apprentice confusion: GFCI trips on 4–6 mA imbalance for general-use receptacles (Class A); the CCID inside the EVSE is rated for the higher charging currents and trips on charge-cable specific fault patterns. They protect different parts of the circuit and are not interchangeable.

Apprentice tip: hardwired EVSE on a GFCI breaker often nuisance-trips because the EVSE's CCID and the GFCI both monitor the same supply conductors — the CCID's normal operating leakage looks like a fault to the GFCI. You don't need to provide GFCI ahead of a hardwired EVSE; if you did anyway, expect callbacks.

Hardwired vs Cord-and-Plug

Quick decision: can this EVSE plug into a receptacle, or does it need to be hardwired?

EVSE current rating (A)

EVSE nameplate continuous current rating. The cord-and-plug ceiling is 50 A.

EVSE voltage (V)

120 / 208 / 240 / 277 / 480. Cord-and-plug ceiling is 250 V single-phase.

Phase

Why EVSE is always continuous

A continuous load is one "where the maximum current is expected to continue for 3 hours or more." The code nails the EVSE case directly: EVSE shall be considered a continuous load. Not "if" continuous — always continuous.

Why the rule is hard-coded: a level-2 charger pulling 48 A on a 60 A circuit charges a typical EV battery for 4–8 hours per session. A DC fast charger pulling 200 A delivers 80% in 30–45 minutes, but commercial sites run charger after charger for the entire workday. Either way you're sitting on continuous current — the 3-hour test is met without controversy.

The 125% factor applies to the conductor and OCPD on both branch circuits and feeders. EMS-throttled current is still continuous; the EMS just changes the magnitude, not the duration. Multiply EMS-limited current by 1.25 the same way you'd multiply nameplate current by 1.25.

Apprentice tip: when you size an EVSE feeder, the math is EVSE_amps × 1.25 → next breaker size. A 48 A EVSE: 48 × 1.25 = 60 A — feed with 60 A OCPD and #6 Cu. A 40 A EVSE: 40 × 1.25 = 50 A — feed with 50 A OCPD and #8 Cu. A 32 A EVSE: 32 × 1.25 = 40 A — feed with 40 A OCPD and #8 Cu. Anything else than these standard pairings, you're either over-sizing or under-protecting — re-check the math.

Lightning Protection & Surge Devices (LPS / SPD)

Lightning + surge — SPD type selector, air-terminal spacing, rolling-sphere & multi-sphere envelope, sphere walk, cone of protection, isolated masts, down-conductor count, material sizing.

SPD Type Selector

Pick the right Type of Surge Protective Device for your install location + system voltage

Install location

System voltage

SPD Type cheat sheet

Type 1 — service entrance, line side of main disconnect. Highest joule rating, takes the direct strike from utility. Can be installed line- OR load-side of main.
Type 2 — load side of main, panelboard mount. Standard residential/commercial panel-level surge protector.
Type 3 — point-of-use, < 10 m from equipment. The strip-style protectors at desk/AV rack.
Type 4 — component, installed inside an assembly (listed component). Not a field-installed device.

Air Terminal Spacing

Where to place the rods on a roof — edge spacing + flat-roof interior grid

Roof shape

Terminal height

Terminals > 24 in tall may be spaced up to 25 ft apart; standard terminals stay on 20 ft max centers.

Longest edge / ridge length (ft)

For pitched roofs, this is the ridge/eave/hip length. For flat roofs, the longer of the two roof dimensions.

Roof width (ft, perpendicular)

Flat roofs > 50 ft in any dimension need an interior grid in addition to perimeter terminals.

Where the rods go — apprentice tour

Air terminals are the strike-receiving rods. Four constraints decide where each one goes:

1. Edge and ridge spacing. Along every ridge, eave, and roof edge: terminals at intervals not exceeding 20 ft (standard 10–24 in rods) or 25 ft (rods taller than 24 in). The taller rod earns the relaxation because a taller rod's cone of protection reaches farther sideways.

2. Corner offset. A terminal lives within 24 in of every ridge end, gable end, or outside corner. Lightning prefers points and edges — corners are the most-likely strike location, so the rod near the corner takes the hit instead of the building.

3. Flat-roof interior grid. If a flat or gently sloping roof exceeds 50 ft in any direction, the perimeter rods aren't enough — a flat-roof strike can land in the middle of the roof, far from any edge. Add interior terminals at intervals not exceeding 50 ft so no point on the roof is more than ~50 ft from a rod.

4. Height above the protected object. Every terminal projects at least 10 in above whatever it's protecting (parapet, ridge cap, roof-mounted equipment). The 10 in is what creates the geometric cone of protection — shorter and the geometry doesn't carry.

Apprentice tip: walk the perimeter first, mark corners (within 24 in), then fill in at 20-ft (or 25-ft if tall) intervals between them. THEN if you're on a flat roof bigger than 50 ft any direction, walk the interior and place the grid. Pitched roofs almost never need the interior grid because the ridge already runs through the center.

Rolling Sphere — Zone of Protection

Two-building protected-distance solver

Rolling sphere class

The sphere RADIUS in the rolling-sphere method. Class I (150 ft) is the current standard for ordinary structures. Class II (100 ft) is permitted for some low-risk applications.

Higher roof / structure height h₁ (ft)

Lower roof / object top height h₂ (ft)

Use h₂ = 0 to check protection at ground level around a single tower / mast.

Actual horizontal distance between roof edges (ft)

From the protected-side eave of the higher structure to the eave of the lower object.

How the rolling sphere works

Imagine rolling a sphere across your buildings — wherever the sphere CAN touch is exposed to a direct lightning strike. Wherever it CAN'T touch (because a taller structure or air terminal blocks it) is in the zone of protection.

The math: for two structures of heights h₁ > h₂ separated by horizontal distance, the maximum protected distance is:
d = √[h₁(2R − h₁)] − √[h₂(2R − h₂)]
where R = sphere striking-distance radius (150 ft Class I, 100 ft Class II). Some methods state the geometry with diameter D — the math is identical since D = 2R.

Apprentice tip: if your actual distance is < d, the lower structure is protected by the higher one. If > d, you need an air terminal on the lower structure (or extend the higher structure's protection via a mast/cable).

Validity constraint: the difference in heights between the upper and lower roofs (or earth) shall be ≤ R (the striking distance). Beyond that, additional strike termination devices are required on the lower structure.

Multi-Sphere Envelope

Three-peak roofline — rolling sphere tangent to each adjacent pair, valleys checked

Sphere class

Peak 1 height (ft)

Peak 2 height (ft)

The middle structure — a dormer, lower wing, valley peak, or a rooftop unit.

Peak 3 height (ft)

Gap 1→2 (ft)

Gap 2→3 (ft)

Reading the multi-sphere envelope

The two-building Rolling Sphere card above handles a single saddle. Real roofs have multiple peaks — a main ridge, a dormer, a lower wing, a chimney near a valley. The multi-sphere method: roll the sphere across the entire roofline and the protected zone is the union of every position the sphere can't reach.

How this card models it: three peaks, two gaps. The sphere sits tangent to peaks 1 & 2 (one saddle) and tangent to peaks 2 & 3 (the other saddle). Each saddle gives a protected height for the valley below it — the deepest the sphere can dip into that gap. Anything in the valley below that height is in the zone of protection; anything above it is exposed and needs its own strike termination device.

Why the middle peak matters: if peak 2 is tall, it acts as its own strike-termination point and shrinks both valleys. If peak 2 is short (a low dormer), the sphere bridges peaks 1 and 3 over the top of it — and whether peak 2 is protected depends on whether it pokes above the sphere arc spanning 1→3. This card flags when the middle peak is the exposed one.

Apprentice tip: the multi-sphere envelope is why you can't just protect the tallest point and call it done. Every peak the sphere touches is a strike point. Every valley has its own protected height. Walk the whole roofline with the sphere — that's the method certified-system inspectors actually use.

Sphere Walk — Multi-Structure Profile

Roll the sphere across a row of peaks — every peak the sphere touches is a strike point

Sphere class

Structure 1 height (ft)

Structure 2 height (ft)

Structure 3 height (ft)

Structure 4 height (ft)

Structure 5 height (ft)

Spacing between adjacent (ft)

Cone of Protection

Simple-geometry alternative to rolling sphere for short isolated structures

Terminal height above protected object (ft)

Vertical distance from the top of whatever you're protecting (chimney, fixture, equipment) up to the air-terminal apex.

Cone method

1:2 method gives more relative protection per foot of terminal height — only allowed for shorter terminals because short rods attract more efficiently per unit of mass.

Isolated Masts & Overhead Ground Wires

Separately-mounted air terminals — telecom towers, antenna masts, equipment shields

Isolated masts and overhead ground wires are an alternative to mounting air terminals on the protected structure. Use when the structure can't carry strike termination devices directly — flammable storage, sensitive equipment, telecom towers, antenna farms, museum / archive buildings where roof penetrations are unacceptable.

How they work: a separately-installed mast (or overhead grounding catenary) intercepts the strike outside the structure. The protected zone is determined using either the rolling sphere method or the cone of protection — whichever is more restrictive for the geometry. The mast handles the strike current; the structure remains untouched.

Grounding rules:

Each isolated mast needs its own grounding electrode system — usually a single driven ground rod ≥ 8 ft, or a buried plate / ring per local soil conditions.
The mast's ground electrode shall be bonded to the structure's electrical grounding electrode system to prevent ground-potential rise (GPR) from blowing up nearby electronics during a strike.
Bonding distance triggers (the side-flash rule) — if the mast is within the side-flash distance d of the structure, additional bonding is required to equalize potential and prevent arc-over.

Overhead ground wires (OHGW): the catenary-style protection used on transmission lines and outdoor substations. A grounded cable runs horizontally above the protected zone; lightning hits the cable preferentially over the protected equipment. The protected zone under an OHGW is calculated the same way as for a mast — rolling sphere or cone of protection from the cable's height.

Apprentice tip: isolated masts are common on rural radio sites, fire-tower antennas, and high-voltage substations. Residential installs almost never use them — the air terminals just sit on the roof. But when you DO see an isolated mast in the field, look for: (a) its dedicated ground electrode, (b) the bonding jumper to the structure's GEC, and (c) the protected zone overlap with the structure. Missing any one of those is a code violation.

Rolling sphere vs cone vs isolated mast — when to use what

There are three geometric methods for laying out air terminals — pick the right one for the structure shape and the terminal arrangement.

Rolling sphere: the modern, universally-applicable method. Works for any structure shape. Use this for anything residential or commercial with multiple roof planes, valleys, dormers, parapets, rooftop equipment, or adjacent structures of different heights. The visualizer card above handles the two-building case. This is what's officially required for systems certified under the master-label program.

Cone of protection: the older simple-geometry method. Allowed only for simple isolated structures with a single terminal — masts, chimneys, ventilation stacks, isolated equipment. Faster math, no SVG drawing needed in the field. Use the cone calc above when the structure is a single tall vertical element ≤ 50 ft and there's no adjacent structure complicating the geometry.

Isolated mast: not really a "method" but a placement strategy. The air terminal lives on a SEPARATE structure (a dedicated mast or an overhead ground wire) instead of being mounted on the protected building. Use when the protected structure can't carry roof penetrations (museums, archives, flammable storage, telecom equipment). Calculate the protected zone using either rolling sphere or cone from the mast's apex.

Decision tree:

Single isolated mast ≤ 50 ft, simple geometry: cone of protection (faster).
Building with roof planes / dormers / equipment: rolling sphere (universal).
Structure can't carry roof penetrations: isolated mast + rolling sphere from the mast.
Master-label certified system: rolling sphere (required for certification).

Apprentice tip: the cone of protection is what your grandfather's lightning code used exclusively before the rolling sphere was added in the 1980s. It still works for the simple cases. But the moment you have any complexity — a chimney near a roof valley, a heat pump near the parapet, two structures of different heights — the rolling sphere is the only method that gives you a defensible AHJ answer.

Down-Conductor Count

Minimum down-conductor count + spacing for the structure perimeter

Structure perimeter (ft)

For flat / low-slope roofs, measure the roof perimeter. For pitched roofs, measure the building footprint (vertical projection).

Roof type

Minimum Material Sizing

Class I (≤ 75 ft) vs Class II (> 75 ft) · copper or aluminum

Structure class

Conductor material

Class I = structures ≤ 75 ft tall · Class II = structures > 75 ft tall (heavier conductors required for taller buildings due to longer return path + more energy). Air terminals are the strike-receiving devices (the "rods"); main conductors connect terminals to ground; bonding conductors equalize potential between LPS components and grounded metal. Solid-strip is permitted in lieu of cable for shorter Class I runs.

Standby & Emergency Power

Generators + automatic transfer switches — genset sizing, load-shed cascade, transfer-switch ampacity, service-entrance ATS, standby-class reference, fuel-source comparison.

Standby Generator Sizing

Sum of running loads + motor starting current uplift

Continuous running load (kW)

Sum of lighting, receptacles, water heater, fridge, electronics, fans, etc. — the steady-state load when nothing's starting.

Largest motor load (HP)

Biggest single motor on the generator (typically the A/C compressor or well pump). 0 = no motor.

Motor start multiplier

Standby system type

Managed-load transfer switch (load shedding active)

A managed-load transfer switch automatically sheds water heater / A/C / dryer / range when the generator can't carry full house. Reduces required genset size. Toggle off for traditional whole-house transfer.

Why the motor starting uplift?

A motor's INRUSH at startup is 3-6× its running FLA — for 0.1-3 seconds the generator has to deliver that surge or the motor stalls and the breaker trips. So genset sizing is: steady-state load + (largest motor's inrush minus its running FLA). Once the motor reaches running speed, the surge drops back to FLA and the generator runs comfortably.

The math:
Motor running kW ≈ HP × 0.746 (assuming 100% load factor; real-world is lower).
Motor starting surge = running kW × multiplier (3-6 depending on motor design).
Peak demand = continuous load + (starting surge − running kW).
Recommended size = peak demand × 1.25 safety margin, rounded up to next common rung.

Managed-load transfer can drop the required size 20–40% by automatically shedding water heater / A/C / dryer / range when the genset can't carry. Pairs with whole-home transfer at 70-80% of peak demand.

Load-Shed Cascade

Drag the genset size — watch a managed-load transfer switch decide what stays on

Generator size — 14 kW

2 kWportable ↔ whole-home40 kW

Transfer Switch Ampacity

Sizing the ATS to safely carry the transferred load

Generator output (kW)

From the recommended-size output above, or your spec'd genset nameplate. ATS must carry the genset's full output capability.

System voltage

Phase

Power factor (3Ø only)

For genset kW → amps on 3Ø systems. Typical residential standby: 0.85–0.90. Ignored on 1Ø calcs (PF assumed 1.0 for kW-rated genset nameplates).

Service-Entrance Transfer Switch?

When the ATS is the first means of disconnect — service-entrance rated required

Where does the ATS sit in the system?

Verdict —

Transfer switch type required —

Key requirements —

Standby Class Reference

Three standby system categories + the Type / Class system for runtime & start time

Emergency Systems. Life-safety loads required by code: egress lighting, fire alarm, smoke control, hospital critical-care, elevator recall. Type 10 default — generator on-line within 10 seconds of utility loss. Most stringent — listing for emergency service required.

Legally Required Standby Systems. Loads the AHJ designates as critical but not life-safety: sewage pumps, industrial-process safety, communications. Type 60 — generator on-line within 60 seconds.

Optional Standby Systems. Owner's choice: home backup, server rooms, refrigeration. No mandatory start time — typical Type 120 (2 minutes) or longer is acceptable. Residential standby is almost always optional.

Class — runtime at rated load:

Class 0.083 — 5 minutes (very short, transit / spot)
Class 0.25 — 15 minutes
Class 2 — 2 hours (typical residential propane setup)
Class 6 — 6 hours (common commercial)
Class 48 — 48 hours (hospital / data center)

Apprentice tip: Class is FUEL-LIMITED for residential gensets — a Type 120 / Class 6 on a 250-gal propane tank works to roughly 35-50 hours at full load, but the spec says "rated load" so figure 6 hours sustained at full peak demand. Natural-gas gensets are unlimited Class as long as utility pressure holds, which is exactly when the storm knocks it out — Class rating doesn't capture that risk.

Fuel-Source Comparison

Trade-offs across natural gas, propane, diesel, gasoline

Natural gas: Unlimited runtime if utility pressure holds. No on-site storage. Best fit when local NG is reliable in storms. Watch out: ~2% kW derate vs gasoline rating, and earthquakes / floods / utility-side failures cut the supply right when you need it. Most residential standbys ship NG-or-propane dual-fuel.

Propane: Long shelf life (10+ years stored), ~10% derate vs gasoline rating. On-site tank required — 250 gal residential is common, sized for ~24–48 hr at typical load. Vapor draw cold-weather limit ≈ −20°F before the tank can't gasify fast enough. Watch out: tank size + recoverable fuel volume — at 20% reserve, a 250-gal tank yields ~200 gal usable.

Diesel: Highest power density, longest runtime per gallon. Best for ≥ 25 kW commercial / industrial. Fuel cleanliness matters — water and microbial growth foul filters; watch out: fuel stabilizer + annual polishing recommended on standby diesel that doesn't see weekly run. Storage 24–72 hr typical for commercial; permanent on-site tank requires a bunded enclosure + spill-containment per local fire code.

Gasoline: Portable gensets only — not for permanent standby. Shelf life 6–12 months even with stabilizer. Fire hazard at the tank fill. Watch out: the kind of unit your customer wheels out of the garage during a storm — useful for 5–8 kW emergency loads but not for whole-home transfer.

Bi-fuel / tri-fuel: Adds NG capability to an LP or diesel base. Useful when site has both pipeline + on-site tank. Slight first-cost premium; complexity penalty on annual service.

Apprentice tip: For a typical 14–26 kW residential standby, the answer is almost always NG/LP dual-fuel — pipeline when it works, propane backup when it doesn't. Diesel makes sense above 30 kW and on sites with on-site fuel infrastructure already.

Utility Interconnection

Source-side & load-side connections — the 120% busbar rule, supply-side vs load-side wiring, power control systems.

120% Busbar Rule

Load-side connection check for back-fed PV / ESS / standby on existing service panel

New back-fed source OCPD (A)

Breaker protecting the new source — PV inverter output, ESS inverter output, or standby genset back-feed.

Service main breaker (A)

The breaker between the utility and the panel busbar. Standard residential: 100 / 125 / 150 / 200 / 225 A.

Busbar rating (A)

From panelboard nameplate — the busbar's continuous current rating. Often (but not always) equals the main breaker rating; upgraded panels sometimes have 225 A busbar with a 200 A main.

Other back-fed sources on bus (A)

Sum of any OTHER back-fed source OCPD already on this busbar (existing PV inverter, existing ESS). 0 if this is the first interconnect.

Supply-side vs Load-side

Where to tie in — and which connection type has size limits

Load-side connection: the new source ties into the existing service panel busbar via its own OCPD. The above 120% rule applies. Most common for residential PV (≤ 11 kW AC) on a 200 A panel.

Cheapest install — uses existing panel + meter
120% rule caps the size of the back-fed source
Back-fed breaker must be at the END of the busbar opposite the main (older "center-fed" panels have a different rule)

Supply-side connection: the new source ties between the utility transformer and the main service disconnect — effectively a second utility feed. Used when the back-fed source is too big for the 120% rule.

No 120% rule — the source is treated as another utility supply
Requires a service-rated fused disconnect or breaker box, often outside next to the meter
More expensive — additional service-rated equipment + utility coordination
Only allowed where the sum of utility + source AC capacity is within the service drop / conductor ratings

Apprentice tip: when a customer wants 15 kW of PV on a 200 A panel and the 120% calc fails — the answer is usually "upgrade to a supply-side tap with a separate service-rated disconnect outside." Quote ~$2,000–$4,000 of extra labor + materials. Don't try to make load-side work by downgrading the main below the calculated service load — that creates a different code problem.

Power Control Systems

Listed PCS lets you exceed 120% — when the source can throttle itself

A listed Power Control System (PCS) is permitted to bypass the 120% busbar rule when the system is listed and configured to limit the source's output dynamically. The PCS monitors the busbar load in real time and throttles the inverter so the combined utility + source current never exceeds the busbar rating.

How it works in practice:

The PCS includes a current-measuring CT clamp on the busbar
When loads spike, the PCS throttles the inverter output proportionally
The "effective" back-fed current is the PCS-managed throttled value, not the inverter's nameplate
The listing covers the PCS as a complete system

When to use it: commercial PV/ESS installs where the 120% rule would otherwise force a costly service upgrade or supply-side tap. Tesla Powerwall with Backup Gateway, Enphase IQ System Controller, and SolarEdge EnergyHub all include PCS functionality in their listings — check the spec sheet for PCS-compliant language.

Apprentice tip: if the customer already has a Tesla Gateway or Enphase Controller, the PCS path is the cleanest way to add PV without re-servicing. But the AHJ has to accept the listed PCS — some jurisdictions still require a load-side OR supply-side connection because their inspectors don't have PCS training. Verify before quoting.

Why 120%? The busbar safety math

A residential panelboard's busbar is rated for a continuous-current limit (usually equal to the main breaker — 200 A for a 200 A panel). The OCPDs feeding the busbar — utility main + any back-fed source — sit on opposite ends. The busbar in between has to carry whatever combined current the loads on the panel demand.

Why the rule isn't 100%: the worst-case current at any single point of the busbar is bounded by the OCPD CLOSEST to that point. The back-fed source can only push current down one direction; the utility main can only push it the other direction. With back-fed source at one end and main at the other, no single point sees the FULL sum of both OCPDs simultaneously — there's a thermal margin.

Why the rule isn't 200%: the 20% margin (120% total) accounts for real-world load distribution. If all the panel's heavy loads happen to be in the middle of the busbar, BOTH sources are pushing current toward that midpoint. The 20% gives thermal headroom for the worst-case mid-bus loading.

The center-fed exception: if the panel main is mid-busbar (not at the end), the geometry breaks down. Those panels need different treatment — main + source ≤ 100% of busbar (not 120%). Less common but check the panel layout BEFORE quoting the calc above.

Apprentice tip: the 120% rule arithmetic is simple — main + source + others ≤ busbar × 1.20. The hard part is identifying the actual busbar rating (read the panel nameplate, not the main breaker label) and checking whether the panel is end-fed or center-fed. End-fed = 120% rule applies. Center-fed = 100% rule applies. When in doubt, photograph the nameplate and bring it back to your supplier for verification.

EV Charger — Job Details

Answer these and SparkRef plans the whole circuit. Every output is checkable in the individual calcs.

EVSE nameplate current (A)

The continuous output current from the charger's nameplate — commonly 32, 40, or 48 A for Level 2.

System voltage

One-way run length, panel → charger (ft)

Drives the voltage-drop check. Long runs get the wire upsized automatically.

Conductor material

Termination temp rating

75°C covers most modern breakers and lugs. Check the equipment listing if unsure.

Existing service rating

Existing calculated load on the service (A)

From a load calc. Leave blank and the planner skips the panel-fit check.

How the Job Planner works

The Planner doesn't replace the individual calcs — it runs them in sequence the way a journeyman thinks through a job: continuous load → breaker → wire → voltage drop → ground → conduit → termination → panel fit.

Every number it gives you is checkable in the dedicated calc (Ampacity, V-Drop, Grounding, Conduit Fill). The Planner is the fast first pass; the calcs are the verification.

Estimating tool — verify against the current code and your AHJ before final installation.

First time here? Quick start

Pick the path that matches what you're doing on the job right now:

Sizing a feeder or branch circuit? Calculations → Ampacity. Add a row per wire size in the bundle and watch derated ampacity update live. Tap "Verify fill in a gutter or wireway" or "Find minimum conduit size" to bounce sideways without re-entering anything.
Pulling conductors through a raceway or gutter? Calculations → Conduit Fill for conduit (53/31/40 fill rule, plus 60% for ≤24" nipples), Aux Gutter for sheet-metal/nonmetallic gutters and wireways (20% fill, 30-conductor derate trigger on metal aux gutter only), or Cable Tray for any of the 6 tray types (Ladder / Vent Trough / Solid Bottom / Wire Mesh / Vent Channel / Solid Channel). All three share the multi-row conductor entry pattern and bounce sideways with one tap.
Bending conduit? Calculations → Calculator. Tap the Offset preset (or Offset Shrink / 3-Pt Saddle / Saddle Shrink), tap the highlighted number to type your value, tap an angle chip. For quick-lookup take-up and shrink values, see Info → Conduit Bender Reference.
Sizing a box? Calculations → Box Fill. Type the stamped cubic-inch volume of any UL-listed plastic box in Custom Box Volume, add the volume of any mud ring or box extension below, then enter your conductor count + grounds + devices. The boxes-that-fit grid filters down to enclosures that clear the required fill.
Sizing the load on a branch circuit? Calculations → Load Calc. Pick the occupancy (dwelling / non-dwelling / hotel-motel guest room), enter floor area, add any specific loads (dryer, sign, show window, multioutlet assembly, heavy-duty lampholders), toggle the continuous-load checkbox if any load runs 3+ hours. Get back total VA, amps at your chosen voltage, and minimum 15 A and 20 A circuits required.
Picking the right enclosure for the environment? Info → NEMA Enclosure Types. Each type's pictogram and "what breaks it" line tells you what it'll survive — and what bumps you up to the next rating.
Learning the trade? Visuals tab is the teaching star. The Ampacity Chart shows how amps scale across every wire size and temp rating; the VD Chart shows how voltage drop climbs with distance per wire size; the Wire Explorer draws each conductor at true relative scale.
Every calc gives a number — always cross-check. SparkRef is a field calculator, not a code substitute. Read the Disclaimer card and keep your codebook (or Ugly's) on the truck for the official word.

Tap the summary above to collapse this once you've got your bearings.

Electrical Symbols (ANSI/IEEE)

Duplex Outlet

GFCI Outlet

Weatherproof

Quad Outlet

240V Outlet

Switch (SP)

3-Way Switch

4-Way Switch

Dimmer

Ceiling Light

Wall Light

Recessed Can

Fluorescent

Exit Sign

Smoke Detector

Thermostat

Panel

Junction Box

Motor

Ground

Standard ANSI/IEEE architectural electrical symbols. Symbols may vary by specification and region.

NEMA Receptacle Configurations

2-Pole, 2-Wire — Ungrounded (Legacy)

1-15R

125V 15A
Legacy 2-prong

2-20R

250V 20A
Legacy ungrounded

2-30R

250V 30A
Legacy ungrounded

Ungrounded outlets are no longer code-compliant for new installations in most jurisdictions. Existing installations may remain, but replacements should be a 5-15R (grounded) or a GFCI device with proper "No Equipment Ground" labeling.

Straight-Blade (Non-Locking)

5-15R

125V 15A
Standard outlet

5-20R

125V 20A
T-slot (20A only)

5-30R

125V 30A
RV, commercial

5-50R

125V 50A
High-amp 120V

6-15R

250V 15A
240V appliance

6-20R

250V 20A
A/C, compressors

6-30R

250V 30A
A/C, large loads

6-50R

250V 50A
Welder, shop

7-15R

277V 15A
Commercial lighting

7-20R

277V 20A
Commercial HID

7-30R

277V 30A
Commercial HVAC

TT-30R

125V 30A
RV shore power

Twist-Lock (NEMA L-Series)

L5-15R

125V 15A Locking
Light commercial

L5-20R

125V 20A Locking
Twist-lock

L5-30R

125V 30A Locking
Stage, data center

L6-15R

250V 15A Locking
240V twist-lock

L6-20R

250V 20A Locking
Industrial 240V

L6-30R

250V 30A Locking
Industrial 240V

L5-50R

125V 50A Locking
Industrial / stage

L6-50R

250V 50A Locking
Heavy 240V industrial

L7-15R

277V 15A Locking
Commercial lighting

L7-20R

277V 20A Locking
Commercial lighting

L7-30R

277V 30A Locking
Commercial HVAC

L8-20R

480V 20A Locking
Heavy industrial

L9-20R

600V 20A Locking
High-voltage industrial

Straight-Blade Split-Phase (Legacy 125/250V)

10-30R

125/250V 30A
Legacy dryer

10-50R

125/250V 50A
Legacy range

Straight-Blade 3-Phase (250V, no ground, no neutral)

11-15R

250V 3ø 15A
3 hots, no ground

11-20R

250V 3ø 20A
3 hots, no ground

11-30R

250V 3ø 30A
3 hots, no ground

Twist-Lock

L10-30R

125/250V 30A Locking
Legacy split-phase

L11-20R

250V 3ø 20A Locking
3 hots, no ground

L11-30R

250V 3ø 30A Locking
3 hots, no ground

L12-20R

480V 3ø 20A Locking
Heavy industrial

L12-30R

480V 3ø 30A Locking
Heavy industrial

L13-30R

600V 3ø 30A Locking
High-voltage industrial

3P 3W includes both legacy split-phase (10-series, 125/250V, no ground) and 3-phase without neutral (11, L11-L13). All lack a separate equipment ground — replaced by 3P 4W (14/15-series + L14-L17) in modern installations.

Straight-Blade Split-Phase (125/250V)

14-20R

125/250V 20A
Shop / commercial

14-30R

125/250V 30A
Modern dryer

14-50R

125/250V 50A
Range / EV charger

14-60R

125/250V 60A
Large range

Straight-Blade 3-Phase (250V, 3 hots + ground)

15-20R

250V 3ø 20A
3 hots + ground

15-30R

250V 3ø 30A
3 hots + ground

15-50R

250V 3ø 50A
3 hots + ground

15-60R

250V 3ø 60A
3 hots + ground

Twist-Lock

L14-20R

125/250V 20A Locking
Portable equipment

L14-30R

125/250V 30A Locking
Generators, RVs

L14-60R

125/250V 60A Locking
Large generator

L15-20R

250V 3ø 20A Locking
3 hots + ground

L15-30R

250V 3ø 30A Locking
3 hots + ground

L15-60R

250V 3ø 60A Locking
Large 3-phase

L16-20R

480V 3ø 20A Locking
Industrial

L16-30R

480V 3ø 30A Locking
Industrial

L17-30R

600V 3ø 30A Locking
High-voltage industrial

Straight-Blade (Legacy, 3 hots + neutral, no ground)

18-15R

120/208V 3ø 15A
Legacy industrial

18-20R

120/208V 3ø 20A
Legacy industrial

18-30R

120/208V 3ø 30A
Legacy industrial

18-50R

120/208V 3ø 50A
Legacy industrial

18-60R

120/208V 3ø 60A
Legacy industrial

Twist-Lock

L18-20R

120/208V 3ø 20A Locking
Legacy industrial

L18-30R

120/208V 3ø 30A Locking
Legacy industrial

L19-20R

277/480V 3ø 20A
Heavy industrial (no ground)

L20-20R

347/600V 3ø 20A
High-voltage (no ground)

Legacy 3-phase configurations with neutral but no separate equipment ground. Largely replaced by NEMA 21/22/23-series (4P 5W) for all modern installations. If encountered, plan for conversion on retrofit.

Straight-Blade 208Y/120V 3-Phase

21-20R

208Y/120V 3ø 20A
3 hots + N + ground

21-30R

208Y/120V 3ø 30A
Commercial 3-phase

21-50R

208Y/120V 3ø 50A
Heavy commercial

21-60R

208Y/120V 3ø 60A
Large 3-phase

Straight-Blade 480Y/277V & 600Y/347V

22-20R

480Y/277V 3ø 20A
Heavy industrial

22-30R

480Y/277V 3ø 30A
Industrial

23-30R

600Y/347V 3ø 30A
High-voltage industrial

Twist-Lock 208Y/120V

L21-20R

208Y/120V 20A 3-Phase
Data center, IT

L21-30R

208Y/120V 30A 3-Phase
Server racks, PDUs

Twist-Lock 480Y/277V & 600Y/347V

L22-20R

480Y/277V 3ø 20A
Heavy industrial

L22-30R

480Y/277V 3ø 30A
Heavy industrial

L23-20R

600Y/347V 3ø 20A
High-voltage

L23-30R

600Y/347V 3ø 30A
High-voltage

AC Charging Connectors (Level 1 / Level 2)

J1772

SAE J1772 (Type 1)
L1: 120V 12-16A
L2: 240V up to 80A

NACS

NACS / J3400
Tesla + Ford/GM/Rivian
AC + DC combined

Type 2

IEC 62196 (Mennekes)
European Type 2
AC up to 22kW

DC Fast Charging Connectors

CCS1

CCS Type 1
Combined AC+DC
DC: 50–360 kW

CCS2

CCS Type 2 (Combo 2)
European DC fast
DC: 50–350 kW

CHAdeMO

CHAdeMO
Nissan Leaf, Mitsubishi
DC: up to 100 kW

GB/T

GB/T 20234 (China)
DC: up to 250 kW
Rare in NA

Common EVSE-Side NEMA Receptacles

These are the wall receptacles electricians most often install for Level 2 EV charging. Tap to jump to the full NEMA spec.

14-50R

Level 2 most common
240V 50A → 40A charge

14-30R

Dryer outlet reuse
240V 30A → 24A charge

6-50R

Welder outlet reuse
240V 50A → 40A charge

TT-30R

RV park / camper
120V 30A → 24A L1

L14-30R

Portable EVSE locking
240V 30A → 24A charge

Install tip: EVSE circuits should be sized at 125% of continuous load per standard code requirements. A 48A EVSE needs a 60A circuit. Most residential Level 2 installations land on a 40A circuit (50A breaker) with a 14-50R, feeding a 32A EVSE. GFCI is required for all Level 1/2 EVSE branch circuits.

Classification by poles (current-carrying blades) and total wires (including ground). First number = NEMA voltage/phase code, second = amp rating. "L" prefix = locking (twist-lock). R = receptacle, P = plug.

Circuit # → Phase Color

Circuit #

Panel Type

Black

PHASE A

Standard US 3-phase panel. Odd circuits = left side, even = right. Phases rotate A/B/C every two circuits. Verify against your panel's legend label — some manufacturers differ.

Electrical Formulas

Ohm's Law

V = I × R

Volts = Amps × Ohms · I = V/R · R = V/I

Power (DC / Resistive)

P = V × I

Watts = Volts × Amps · P = I²R = V²/R

Power (Single-Phase AC)

P = V × I × PF

PF = power factor (0–1). For resistive loads PF = 1.

Power (Three-Phase AC)

P = √3 × V_L × I × PF

V_L = line-to-line voltage. √3 ≈ 1.732.

Power Factor Relationship

kW = kVA × PF

PF = kW ÷ kVA — the ratio of real power (work) to apparent power (what the utility sees). Resistive loads: PF = 1, so kW = kVA. Motors and lighting ballasts drop PF below 1.

Voltage Drop (Single-Phase)

VD = 2 × K × I × L / CM

K = 12.9 (Cu) or 21.2 (Al) · L = one-way ft · CM = circular mils. Recommended: ≤3% branch, ≤5% total (branch + feeder).

Voltage Drop (Three-Phase)

VD = √3 × K × I × L / CM

Same constants as single-phase; √3 factor replaces the 2.

Motor Horsepower → Amps (estimate)

A ≈ (HP × 746) / (V × Eff × PF)

Approximate — check the motor nameplate or a motor-sizing reference for the published FLA. Eff and PF default to ~0.85 if unknown.

Amps → Horsepower (estimate)

HP ≈ (V × I × Eff × PF) / 746

Reverse of HP→Amps — useful for estimating motor size from running current. Cross-check with the nameplate HP when available.

kVA → Amps (Single-Phase)

I = (kVA × 1000) / V

Line current for a 1Ø load rated in kVA. Example: 10 kVA at 240 V = 41.7 A.

kVA → Amps (Three-Phase)

I = (kVA × 1000) / (V × √3)

Line current for a 3Ø load rated in kVA. Example: 75 kVA at 480 V = 90.2 A.

Transformer Full-Load Current (1Ø)

I_FL = (kVA × 1000) / V

Same math as kVA→Amps, framed for transformer primary/secondary sizing. Use the rated voltage of the side you're sizing — primary and secondary currents are computed independently.

Transformer Full-Load Current (3Ø)

I_FL = (kVA × 1000) / (V × √3)

3Ø transformer full-load current. Primary amps and secondary amps scale inversely with their respective voltages (higher V side carries less current).

Short-Circuit at Transformer Secondary (3Ø)

I_SC = (kVA × 100,000) / (V × √3 × %Z)

Available fault current at the transformer's secondary terminals. %Z is the transformer's per-unit impedance (typically 2–6%, printed on the nameplate). Actual fault current downstream is lower due to wire resistance — useful for sizing breaker interrupt ratings (AIC).

Pythagorean / Rolling Offset

c = √(a² + b²)

Right-triangle hypotenuse. For a rolling (3D) offset: true offset = √(rise² + set²). Also the go-to formula for squaring layouts, diagonal measurements, and any right-angle geometry in the field.

Offset Bend Multipliers

Mark Distance = Offset × Multiplier

10°: 6 · 22.5°: 2.6 · 30°: 2 · 45°: 1.4 · 60°: 1.15
Shrink per inch of offset · 10°: 1/16" · 22.5°: 3/16" · 30°: 1/4" · 45°: 3/8" · 60°: 1/2"

Watts ↔ BTU/hr

BTU/hr = W × 3.413

Convert electrical power to heat output. 1 kW ≈ 3,413 BTU/hr. Useful for panel-heat load, resistive heaters, or sizing enclosure cooling.

Cooling Tons ↔ BTU/hr

1 ton = 12,000 BTU/hr

HVAC unit of cooling capacity — the energy needed to melt 1 ton of ice over 24 hours. A 5-ton AC removes 60,000 BTU/hr.

Wire Resistance Temperature Correction

R_hot ≈ R_cold × (1 + α × ΔT)

α = 0.00393/°C (0.00218/°F) for copper, 0.00403/°C (0.00224/°F) for aluminum. ΔT = temperature rise. Wire resistance rises with temperature — measurable on long feeders running at full load, especially in hot spaces. Common terminal ratings: 60°C (140°F), 75°C (167°F), 90°C (194°F).

Conduit Fill %

Fill % = (Σ Wire Area ÷ Conduit Area) × 100

Max fill: 1 conductor = 53% · 2 conductors = 31% · 3+ conductors = 40%. Use the wire's insulated cross-section area, not bare-conductor area.

How to Bend Conduit

Key Terms

Deduction — the distance a bender "eats" at the bend. Subtract from your measured length before marking.
Take-up (Stub-up) — for a 90°, distance from end of conduit to the back of the bend. EMT rule of thumb: ½"→5", ¾"→6", 1"→8", 1¼"→11".
Gain — distance saved by the curve of a bend vs. two straight sections meeting at a square corner. Matters on back-to-backs.
Developed length — total length of conduit consumed by a bend, measured along the centerline arc.
Centerline radius (CLR) — radius measured to the centerline of the conduit, not the inside or outside.
Shrink — how much shorter the run becomes after the bend (especially offsets and saddles). Always measure after accounting for shrink.
Spring-back — conduit relaxes slightly after the pressure comes off; over-bend by 2°–5° on tight bends to compensate.
Arrow — the bender's primary bend-alignment mark. Your pencil mark goes here for most pulls (stub-ups, offsets, saddles).
Star — back-of-bend indicator on many benders; used on back-to-back 90°s so the back of the finished bend lands on your mark.
Back-to-back — two 90°s on the same stick where the backs of both bends face each other. Three shapes depending on how you orient the pipe between pulls: a U (both stubs pointing the same direction — the classic stub-up / run / stub-up), a Z/S in one plane (one stub up, one stub down, both in the same flat layout), or a perpendicular Z/S (stubs in different planes — pipe rotated 90° between the pulls so the second stub comes out sideways relative to the first). Either way, the second mark goes on the star (back-of-bend) so the inside-to-inside dimension between the two bends equals what you measured. Miss this and the pipe lands one deduction long.
Dog leg — when the two bends of an offset aren't in the same plane. The pipe twists out of square between pulls, so the finished offset doesn't lay flat against a wall, strut, or floor — it rocks or stands off the surface. Usually an apprentice mistake: pipe rotated in the shoe between the first and second bend, or the bender wasn't held square on the second pull. Catch it by laying the pipe flat after both bends — if it rocks or one end lifts, you've got a dog leg.
Rim notch — center reference on some shoes, used for the middle bend of a 3-point saddle.
Multiplier — a fixed number per offset angle that converts offset depth → distance between bend marks. Mathematically, multiplier = cosecant of the angle.
Rise / Set — in a rolling offset, "rise" is the vertical component of the jog and "set" is the horizontal component.
Cosecant (csc) — the trig function behind the offset multipliers; csc(30°) = 2, csc(45°) ≈ 1.41, csc(22.5°) ≈ 2.61, csc(60°) ≈ 1.15.
Reference end — the end you measure from. Pick one at the start of the job and don't flip it mid-layout.

Bender Anatomy & Markings

Arrow — primary alignment mark on the shoe. Place your pencil mark on the arrow for stub-ups, offsets, saddle end-bends, and most simple pulls.
Star — back-of-bend reference, used mainly on the second bend of a back-to-back 90°. Not every brand has one; learn your bender.
Rim notch (or center mark) — center indicator on some shoes. Pairs with the middle mark on a 3-point saddle.
Deduct stamp — many benders have the take-up value stamped on the shoe for the size they're rated for (e.g. "5"" on a ½" EMT shoe). Factory value, trade-verified.
Handle — the leverage arm. Longer handles let you pull slower and smoother; jerky pulls twist the pipe out of plane. Keep your pull straight and steady.
Foot pedal / hook — where your weight goes. Step directly over the shoe, not off to the side, or the pipe rolls off-plane as you bend.
Shoe size — a ½" EMT shoe doesn't bend ¾" EMT without kinking the pipe. Check the size stamp on the shoe before you pull. Same goes for EMT vs. Rigid — the take-up is different.
Bender orientation — "arrow facing you" vs. "arrow facing away" flips the pull direction. If a measurement looks wrong, check which way you're pulling first.

Bend Types

90° Stub-Up

Mark = Stub Height − Take-up

Measure the height you want from the end of the conduit to the back of the bend. Subtract the take-up for your size and bender. Place the arrow of the bender on the mark; pull a full 90°.

Standard take-up values (hand bender):
· ½" EMT → 5"
· ¾" EMT → 6"
· 1" EMT → 8"
· 1¼" EMT → 11"

Rigid / IMC shoes cut the same sizes with slightly different take-ups (typically ~1" over EMT). Always verify against the stamp on your shoe — Klein, Ideal, Greenlee, and Gardner Bender can round differently.

Example: ½" EMT, stub height 14". Take-up is 5". Mark at 14 − 5 = 9" from the end. Arrow on the 9" mark; pull a full 90°.

Watch out: Using the take-up for the wrong size shoe; measuring stub from the wrong end of the pipe; under-pulling (the pipe springs back 2°–5° — pull a hair past 90°).

Back-to-Back

Mark 2 = Mark 1 + Distance Between Backs

First 90° is marked the normal way. For the second bend, measure from the back of the first bend to where you want the back of the second, and place the star (back-of-bend indicator) on that mark. Reverse the bender on the conduit for the second pull.

Example: ½" EMT, 36" back-to-back. Pull the first stub-up. From the back of that finished 90°, measure 36" and mark. Flip the bender (arrow facing the first bend now), align the star on the 36" mark, pull a full 90°.

Watch out: Forgetting to flip the bender on the second pull; using the arrow instead of the star (puts the back of the second bend in the wrong place); measuring second bend from the end of the pipe instead of the back of the first bend.

Offset Bend

Distance Between Marks = Offset × Multiplier

Used to jog around an obstacle. Pick an angle (common: 22.5°, 30°, 45°, 60°), multiply the offset depth by the multiplier for that angle, mark the two bend points that distance apart.

Multipliers — 10°: 6 · 22.5°: 2.6 · 30°: 2 · 45°: 1.4 · 60°: 1.15
Shrink per inch of offset — 10°: 1/16" · 22.5°: 3/16" · 30°: 1/4" · 45°: 3/8" · 60°: 1/2"

Shallower angles = less shrink + smoother pulls. Steeper angles = tighter jog in less space.

Example: 3" offset at 30°. Distance between marks = 3 × 2 = 6". Shrink = 3 × ¼" = ¾" — add to overall run length so the finished piece still lands where you need it. Pull 30° at the first mark, rotate the pipe flat to keep the second bend in the same plane, pull 30° at the second mark.

Watch out: Ignoring shrink — finished run comes out short. Rotating the pipe between bends (not keeping it flat) → offset comes out twisted. Using inconsistent angles on the two pulls → legs not parallel.

3-Point Saddle

Center Bend = 2 × End Bend Angle

Goes over a round obstruction (pipe, beam, conduit). Middle bend is double the angle of the two outer bends — most common combo is 45° center with 22.5° outers.

Distance from center to each end mark = Rise × multiplier for the end-bend angle (e.g. 2.6 for 22.5° outers).
Center mark = sits on the center of the obstacle + add the shrink.
Shrink ≈ Rise × 3/16" for a 45°/22.5° saddle.

Mark all three points on the conduit first, then bend center first, ends second — keeps the three bends in plane.

Example: 2" rise over a 1"-dia pipe, 45° center / 22.5° outers. End distances = 2 × 2.6 = 5.2" on each side of center. Shrink = 2 × 3/16 ≈ ⅜" — shift the center mark ⅜" toward your reference end so the saddle lands on the pipe.

Watch out: Bending the ends before the center → twisted saddle, out of plane. Skipping the shrink correction → saddle lands past the obstacle. Using a 30°/15° combo without recalculating the multipliers.

4-Point Saddle

= Offset In + Offset Out

Goes over a square-ish obstruction (a box, a beam web, another raceway). Treat as two offset bends back-to-back — an "up" offset followed by a matching "down" offset. Distance between the two inner bends = obstacle width + a small clearance. Each offset uses its own multiplier and contributes its own shrink.

Example: 2" rise over a 4" beam flange, 30° angles both sides. Each offset distance = 2 × 2 = 4" between its own marks. Inner marks (between the two offsets) = 4" flange + 1" clearance = 5" apart. Total shrink ≈ 2 × ¼" × 2 offsets = 1".

Watch out: Both offsets must be the same angle, or the finished run comes off non-parallel. Skipping the clearance margin → the saddle rests on top of the obstacle instead of clearing it. Forgetting to keep the pipe flat between the four pulls.

Rolling Offset (3D)

True Offset = √(Rise² + Set²)

For when a run has to jog both sideways and up/down between two points (common on parallel tray-to-panel drops). Treat the two jogs as legs of a right triangle:

True offset (hypotenuse) = √(Rise² + Set²)
Roll angle = arctan(Rise ÷ Set) — the angle you rotate the conduit in the bender so the offset comes out tilted the right way

Once you have the true offset, compute the bend marks exactly like a normal offset bend. Dry-fit before you cut to length.

Example: rise 6", set 8". True offset = √(36 + 64) = √100 = 10". Roll angle = arctan(6 ÷ 8) ≈ 36.87°. Treat 10" as the offset depth: at 30°, marks are 20" apart. Rotate the pipe 36.87° in the shoe before the second pull so the offset tilts into the rise/set direction you want.

Watch out: Getting the roll direction backward → pipe lands on the opposite side of where you need it. Measuring rise and set off wrong reference planes. Always dry-fit on the floor with tape before cutting to length.

Choosing Your Offset Angle

10° — nudge bends only. Marks are 6× the offset apart (a 3" offset = 18" between marks). Rarely useful on short runs; shines when you have real estate and just need a barely-there correction.
22.5° — parallel-set workhorse. Lowest shrink of the common angles (3/16" per inch), smoothest pulls, easiest on wire fill. Use on long pipe racks, low-ceiling runs, and any parallel set where shrink stacks.
30° — the default. Balanced distance-between-marks (2× offset), moderate shrink (¼" per inch), easy math. Use unless you have a reason not to.
45° — tight quarters. Distance between marks = 1.4× offset, shrink ⅜" per inch. Good when you need a quick jog in short space.
60° — very tight clearance, e.g. between stud bays, around compact junction boxes. Max shrink (½" per inch), steepest pull, highest strain on the pipe. Use only when space forces your hand.

Field decision shortcut: start at 30°. Go shallower (22.5° or 10°) if you have room and are worried about shrink or pull friction. Go steeper (45°/60°) only when the wall, rack, or obstacle forces you.

Parallel Runs & Same-Plane Bending

When multiple conduits run side-by-side on parallel centerlines and all need the same bend, the outer pipe travels a longer arc than the inner pipe. Mark every pipe identically and the outer ones come out short. Key idea: each successive pipe needs a slightly larger take-up — or a slightly larger distance between offset marks — to stay parallel after the bend.

Parallel 90° stub-ups ("wheeling") — when you want a row of stubs to land flush at the same face, each outer pipe's take-up grows by the center-to-center spacing. Example: ½" EMT parallel set, 4" center-to-center spacing. Inner pipe take-up 5". Next pipe out: 5" + 4" = 9". Next: 5" + 8" = 13". And so on. Mark each pipe with its own take-up, then bend.

Parallel offsets — when a group of conduits offsets together over the same obstacle, the outer pipes need their bend marks spaced slightly farther apart to stay parallel in the finished run. Rough rule: add (spacing × cos of the offset angle) to the distance between marks on each successive outer pipe. Always lay out on the floor first and confirm the geometry.

Parallel 3-point saddles — same idea, harder. Each pipe needs its own calculated center + end marks. Many journeymen build a simple layout template — one master pipe laid out perfectly, then transfer marks pipe-to-pipe with the spacing correction baked in. Saves re-doing the math six times in a 6-pipe rack.

Dry-fit everything. Parallel work rewards planning. Lay the run out on the floor, mark in chalk, and confirm before you cut or bend a single pipe. Real industrial pipe racks can have 8+ conduits going around the same obstacle — one misread turns into 8× the rework.

Same-plane rule: "same-plane" means all the bends stay in one flat plane — no twist. To hold plane on any multi-bend pipe, set the pipe flat on the floor after each pull and check that it lies without rocking. If it rocks, the last bend went off-plane.

Pro Tips (Field Wisdom)

Placeholder — Remy's 20-year journeyman voice goes here.

Good candidates for this section: which side of the bender arrow to put your mark on (and when that flips), how to square bends against a known-flat reference before the second pull, bending technique for tight quarters, which sizes cheat which way on deduction, cold-weather bending, PVC heating/forming, and the single field trick you wish your apprentice had learned first.

NEMA Enclosure Types

Pick the right enclosure for the environment. The label tells you what it'll survive — and what bumps it up to the next rating. Listed by what apprentices encounter most often, not by NEMA sequence.

Type 1 · General-purpose indoor

Conditioned indoor spaces — panelboards, interior switchgear, junction boxes in finished walls. Drips and incidental dust OK.

Bumps up to: any liquid spray, hose-down, dusty/oily environments, anything outdoor.

Type 3R · Outdoor rain-tight

Most common outdoor rating. Residential service equipment, exterior disconnects, meter mains. Falling rain and snow OK; built to drain water that gets in.

Bumps up to: blowing dust, hose-down, sustained driving rain at angles.

Type 4 · Indoor/outdoor hose-down

Industrial environments with washdown. Splashing water, hose-directed water, blowing dust, sleet, snow OK.

Bumps up to: prolonged immersion (use 6/6P), salt or chemical exposure (use 4X).

Type 4X · Hose-down + corrosion-resistant

Type 4 with stainless or non-metallic body. Coastal, food processing, chemical plants, wastewater. Adds resistance to salt, mild acids, cleaning chemicals.

Bumps up to: prolonged immersion (use 6P), high-pressure washdown beyond rating.

Type 12 · Indoor industrial dust + drip

Manufacturing floors, industrial control panels. Falling dust, dirt, lint, fibers, dripping non-corrosive liquids OK. Gasketed door but not hose-rated.

Bumps up to: outdoor exposure, hose-down water, oil spray (use 13).

Type 13 · Indoor industrial oil/coolant

Machine tool environments — CNC, mills, lathes. Resists oil seepage, lubricant spray, non-corrosive coolants.

Bumps up to: outdoor, hose-down, water immersion.

Type 7 · Hazardous Class I Div 1 (gas/vapor)

Hazardous locations where flammable gases or vapors may be present in normal operation — gas stations, fuel handling, paint booths, refineries, chemical plants. Built to contain an internal arc before it ignites the external atmosphere.

Bumps up to: dust environments (use Type 9), water immersion.

Type 9 · Hazardous Class II Div 1 (dust)

Hazardous locations with combustible dust — grain elevators, flour mills, coal handling, wood-product plants. Sealed against dust ingress AND keeps internal arcs from heating the outside enough to ignite surrounding dust.

Bumps up to: gas/vapor environments (use Type 7), water exposure.

Type 6 · Submersible (occasional)

Brief, limited-depth water immersion. Pump pits, vault installations. Survives temporary submersion to manufacturer's stated depth.

Bumps up to: prolonged submersion (use 6P), depth beyond rating.

Type 6P · Submersible + prolonged + corrosion

Like Type 6 plus extended submersion and corrosion-resistance. Marine, sustained-flooding pits, harsh chemical immersion.

Bumps up to: depth or pressure beyond manufacturer rating.

Enclosure type designations are defined by NEMA 250. Descriptions above are field-oriented summaries in the author's own words — consult NEMA 250 or your local AHJ for authoritative definitions. Always verify the equipment's listed rating against the install environment before accepting it.

Pro Tips (Remy's voice)

Placeholder — Remy's journeyman field notes on enclosure selection go here.

Good candidates: when "outdoor" really means 4X (coastal salt, agricultural chemicals), enclosure-painting traps that downgrade the rating, gasket compression and over-tightening, conduit-entry drilling without compromising the rating, when to spec a junction box vs. extend the run.

Conduit Bender Reference

Quick-lookup field reference for hand benders. The "how to" lives in the How to Bend Conduit card — this is the at-a-glance numbers.

90° Stub-Up Take-Up (EMT)

Trade Size	Take-Up
1/2"	5"
3/4"	6"
1"	8"
1-1/4"	11"
1-1/2"	14"
2"	16"

Rigid/IMC take-up runs roughly +1" larger per size. Verify against your bender's stamping.

Offset Multiplier (distance between marks)

Angle	Multiplier
10°	×6
22.5°	×2.6
30°	×2
45°	×1.4
60°	×1.2

Distance between marks = rise × multiplier. Run the math live in the Calculator → Offset preset.

Offset Shrink (per inch of rise)

Angle	Shrink
10°	1/16"
22.5°	3/16"
30°	1/4"
45°	3/8"
60°	1/2"

Total shrink = rise × this value. Add to your starting measurement before marking.

3-Point Saddle (45° / 22.5° / 22.5°)

Distance between marks	rise × 2.5
Shrink (per inch of obstruction)	3/16"

Common Residential Stub-Up Heights

Application	Height (centerline)
Receptacles (general)	15"
Switches	46–48"
Counter-top receptacles	43–46"
Disconnect / panel	varies (verify spec)

Verify against the project plans, ADA requirements, and local amendments. These are common rule-of-thumb values, not code mandates.

Take-up and shrink values vary slightly between bender brands (Klein, Ideal, Greenlee, Gardner Bender). Always verify against the stamping on your specific bender — it's the authoritative number for your tool.

Pro Tips (Remy's voice)

Placeholder — Remy's journeyman bending tips go here.

Good candidates: brand-specific deduction quirks, spring-back compensation by material (EMT vs IMC vs RMC), how to feel the bender's sweet spot for repeatable bends, when to over-bend by ½° vs trust the marker, and tricks for hitting a stub-up height first try without measuring twice.

Fuses / Inverse-Time Circuit Breakers

The standard ampere ratings for fuses and inverse-time circuit breakers. When a conductor's ampacity doesn't land on a standard rating, the code lets you round up to the next standard size — for circuits rated 800 A or less.

152025303540455060708090100110125150175200225250300350400450500600700800100012001600200025003000400050006000

All values in amperes. Adjustable, restricted, or otherwise non-standard ratings are separately permitted — the list above is the standard set, the ratings a breaker or fuse is normally manufactured and stocked in.

Protective Devices (AFCI · GFCI · TR · SPD)

Where shock, arc-fault, and surge protection are required in a dwelling unit. Look up an area and read across — most locations need more than one type.

Location

AFCI

GFCI

Kitchen

✓

Laundry area

✓

Bedroom

✓

—

✓

Living / family / dining room

✓

—

✓

Hallway

✓

—

✓

Closet

✓

—

✓

Sunroom / rec room

✓

—

✓

Bathroom

—

✓

Garage

—

✓

Outdoors

—

✓

Basement (unfinished)

—

✓

Crawl space

—

✓

AFCI — arc-fault protection for 120-volt, single-phase, 10/15/20-amp branch circuits feeding outlets and devices in the rooms checked above. Met with an AFCI breaker or an outlet-branch-circuit AFCI device. A finished basement living or rec area follows the living-room row.

GFCI — ground-fault protection for 125- to 250-volt receptacles in the areas checked above. Also required for any receptacle within 6 ft of the outside edge of a sink, and within 6 ft of a bathtub or shower stall. Indoor damp and wet locations are included.

TR — tamper-resistant receptacles for all 15- and 20-amp, 125/250-volt non-locking receptacles throughout a dwelling and its garage and accessory buildings. Common exceptions: receptacles more than 5½ ft above the floor, those that are part of a listed appliance or luminaire, and a receptacle in a dedicated space for an appliance that isn't easily moved.

A dash means not required by that location on its own — a circuit can still need protection for another reason. Non-dwelling occupancies carry their own GFCI rules; this card covers dwelling units.

Surge Protection (SPD)

A Type 1 or Type 2 surge protective device is required on the service of every dwelling unit — on new services and whenever the service equipment is replaced. It mounts in or right beside the service equipment, or at the first downstream panel. One SPD protects the home's whole wiring system and connected electronics from voltage spikes off the utility and from nearby lightning. The same protection now extends to dormitories, guest rooms, and similar sleeping quarters.

Requirements reflect the current code cycle. Local amendments vary — always confirm with your AHJ before final installation.

What's New

Recent updates — most recent first. SparkRef ships continuously; this card surfaces what's changed since the version you last walked.

v253 🔗 "See this in Visuals" jumps from V-Drop, Ampacity, and Controller Bending Space — one tap to the visual counterpart.
v252 📋 Intro headers on all 9 Visuals sub-tabs — every abbreviated chip now opens with a clarifying title + topic description.
v251 📐 NEC Chapter 9 Table 5 — insulated conductor dimensions added to Wire Explorer + Conductor Comparator. 6 insulation types (THHN, XHHW-2, RHH ±cover, TW, USE), 14 AWG → 2000 kcmil.
v250 🔌 Wire Bending Space — side-exit cable routing only (bottom is vents), FMC at the transformer transitioning to EMT, primary opposite secondary.
v249 🏭 Wire Bending Space graphic rebuilt from real-photo references — windings in the back, terminal bus + lugs in front, separate ground bar with X0 SDS bonding jumper.
v248 🏷️ Visuals chip rename: "Bend Space" → "Wire Bending Space" for consistency with the card title.
v247 🌩️ Renew tab graphics — centered cross-sections, cloud banks across the top, multiple lightning strikes with LPS dissipation paths.
v246 ⚡ Electrical Laws calc → live-update (no Calculate button, every input change runs the math).
v245 ⚡ HP / Power / Amps converter → live-update.
v244 ⚡ Conduit Jam Ratio calc → live-update.
v243 🎯 "Field next step" hints — Conduit Fill warns when fill is close to the limit; Ampacity warns when bundling derate is heavy.
v242 🎓 5 apprentice tips on the Grounding calcs (GEC, EGC, EGC V-drop upsize, bonding jumpers, SDS) + 3 watch-outs.
v241 ⚠️ "Watch out" callouts on the trickiest calcs (Ampacity parallel rule, Conduit Fill nipples, V-Drop sensitive loads, Transformer FLC nameplate).
v240 📋 17 Calcs sub-tab intro headers — every chip now has a clarifying title + topic line.
v239 🎓 11 more apprentice tips — Transformers (4), Tap Rules (5), Smart Wiring Wizard, Electrical Laws.
v238 🎓 First wave of apprentice tips — short field-useful insights on the top calc cards.
v237 🏠 Renew tab graphics rebuilt with illustrated scenes — gable houses with lightning rods, ground strips with grass, storm clouds.

Settings & Defaults

Pro Status

App Defaults

These set the default values that calc forms will use on first render. Changing them doesn't affect calcs already on screen — open a calc fresh to see the new defaults. Calcs that don't read these defaults yet will be updated in future versions; the storage is in place from v188.

Conductor material

Insulation temp rating

Common voltage

Phase

Data Management

SparkRef stores your data locally on this device — nothing is sent to a server. The summary below shows what's stored. Use the buttons to back up, restore, or clear data.

About

App SparkRef Version — Code target 2026 edition Cache — Author Remy B., IBEW (WA)

About SparkRef

SparkRef is an electrician's field calculator built by a union journeyman, for working electricians.

Ampacity, conduit fill, jam ratio, box fill, voltage drop, motor HP, Ohm's law, cable lube estimation, and a fractions-and-Benfield field calculator — all offline, all in your pocket, no ads, no tracking, no sign-in.

Built with the belief that the best tools on your ladder should match the best tools in your pouch.

Disclaimer

SparkRef is a field calculator — not a substitute for official electrical code, AHJ review, or licensed-electrician judgment.

All calculations and formulas should be independently verified against authoritative code references and applicable local amendments before being used on installations.

"National Electrical Code" and "NEC" are registered trademarks of the National Fire Protection Association (NFPA). SparkRef is not affiliated with, endorsed by, or certified by NFPA.

Use of this app is at your own risk. The author assumes no liability for errors, omissions, or damages arising from use of SparkRef.

SparkRef Pro

A one-time unlock for working journeymen who want the deep features. No subscriptions — own it forever.

What's unlocked with Pro:

Jobs — named project containers, save-to-job from every calc, PDF / JSON / CSV export, import jobs shared by other SparkRef users
Load Calc — Tier 2 + Tier 3 — full service-load with HVAC, EVSE, multi-motor sizing; Optional Method including multifamily, schools, restaurants
Renewables & Resilience — PV string & conductor sizing, ESS aggregate kWh, EVSE branch sizing, lightning protection (rolling sphere + cone + air-terminal + sphere walk), standby genset + ATS + load-shed cascade, 120% interconnect
Grounding — GEC + EGC sizing, VD upsize, bonding jumpers (main + supply-side + equipment), separately derived systems, grounding electrode comparator
Smart Wiring Wizard — load + distance + target VD% → minimum wire in one pass, runs every binding rule
Feeder Tap Conductors — tap rules walked condition by condition: 10-ft, 25-ft, transformer feeder tap, and outside taps
Transformers — full-load current, overcurrent-protection sizing, and secondary conductor sizing

Check the Settings & Defaults card above to see your current Pro status and upgrade if you haven't yet.

Support the Developer

Built on my own time by a union journeyman. If SparkRef saves you time on the job and you want to throw a coffee or a tool-truck lunch my way, I'd appreciate it.

Buy me a coffee on Ko-fi

Every bit helps fund future features. Thanks for the support.

Feedback & Contact

Found a bug, a wrong value in a table, or have a feature request?

feedback@sparkrefapp.com

Please include the tab, table, or calculator you're referencing so we can track it down quickly.

Tier Free

SparkRef v1.0.0

Updated 2026-05-08

parkRef

Entering numbers

Math & order of operations

Precision toggle (1/8″ ↔ 1/16″)

Tape

Memory slots (M1–M5)

Benfield preset chips

Chaining calculations

Live preview vs. equals

Offset — 2-bend cosecant method

Offset Shrink — what the offset "eats"

3-Point Saddle — over a round obstruction

Saddle Shrink — what the 3-point saddle "eats"

Renewables & Resilience

The Renewables & Resilience edition

Job Planner

Info