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Electrical Trade calculators
Electrical Trade calculators: Size conduit, wire, transformers and boxes using standard NEC reference calculations.
Conductor ampacity, conduit fill, box fill and power conversion have separate requirements. Use the actual insulation, raceway, conductor count, temperature and adopted code edition.
What is the difference between real power and apparent power in a three-phase circuit?
Real power (kW) is the power actually doing work. Apparent power (KVA) also includes the reactive power the circuit draws without doing useful work. Power factor is the ratio between them.
A lower power factor means more current is needed to deliver the same real power
Conduit Fill Calculator
Confirm a conduit run stays within the NEC Chapter 9 fill limit before pulling wire.
Inputs: Conduit internal area, Total conductor area, Number of conductors
Open calculator →02Wire Ampacity Sizing Calculator
Check a conductor's derated capacity against the connected load.
Inputs: Conductor, Combined derating factor
Open calculator →03Transformer Sizing Calculator
Size feeder conductors and overcurrent protection from the calculated current.
Inputs: Transformer rating (KVA), Primary voltage, Secondary voltage, Phase
Open calculator →04Three-Phase Power Calculator
Compare a load's real power draw against its apparent power for billing or sizing.
Inputs: Line-to-line voltage, Line current, Power factor
Open calculator →05Electrical Box Fill Calculator
Confirm a device box is large enough before roughing in.
Inputs: #14 AWG conductors, #12 AWG conductors, Devices (switches/receptacles), Grounding conductors present, Internal cable clamp present, Box volume
Open calculator →Electrical Trade calculator results and examples
| Calculator | What it answers | Headline output | Worked example |
|---|---|---|---|
| Conduit Fill Calculator | Confirm a conduit run stays within the NEC Chapter 9 fill limit before pulling wire. | Conduit fill | 7.5% |
| Wire Ampacity Sizing Calculator | Check a conductor's derated capacity against the connected load. | Base table ampacity (amps) | 25 |
| Transformer Sizing Calculator | Size feeder conductors and overcurrent protection from the calculated current. | Primary full-load current (amps) | 90.2 |
| Three-Phase Power Calculator | Compare a load's real power draw against its apparent power for billing or sizing. | Real power (kW) | 37.4 |
| Electrical Box Fill Calculator | Confirm a device box is large enough before roughing in. | Required box volume (cu in) | 16 |
Where people go wrong with electrical trade numbers
For an assumed balanced three-phase load at 400 V line-to-line, 10 A and power factor 0.8, real power is √3 × 400 × 10 × 0.8 = about 5.543 kW. This does not size protection or conductors.
Electrical Trade: related calculator categories
Common electrical trade questions
Why does the NEC cap conduit fill at 40% for three or more conductors?
The fill limit leaves enough clearance to pull wire without damaging insulation and lets heat generated by the conductors dissipate. A conduit packed tighter than the table allows both raises the risk of insulation damage during the pull and lets heat build up inside the conduit
Why does a conductor's ampacity change with the number of current-carrying conductors in the same conduit?
Multiple current-carrying conductors bundled together each add heat to their shared surroundings, and NEC 310.15(B) requires a derating adjustment as the count rises so a conductor's real-world operating temperature stays within its insulation rating, even though the base table ampacity assumes only a few conductors together
Should I size a transformer by KVA or by amps?
KVA is the nameplate rating, but the current that actually flows depends on voltage as well, the same KVA transformer produces different primary and secondary amperage depending on the voltages involved, which is why the feeder and overcurrent protection are sized from the calculated current, not the KVA number directly
Why does a switch or receptacle count as two conductors in a box-fill calculation?
NEC 314.16(B) treats each device as requiring the same volume allowance as two conductors of the largest gauge entering the box, since a device and its associated wiring take up meaningfully more physical space inside the box than a single conductor does
Why do grounding conductors only count once in a box-fill calculation, no matter how many there are?
All the grounding conductors together are treated as a single conductor at the volume allowance of the largest gauge grounding conductor present, because they are typically pigtailed together rather than each needing separate clearance inside the box
Does a longer wire run change what ampacity I need?
Ampacity itself doesn't change with distance, but voltage drop does, and a long run at high current can need a larger conductor than ampacity alone would suggest just to keep voltage drop within an acceptable range at the load. Check both figures separately before finalizing a wire size
Why do local amendments sometimes override the National Electrical Code?
The NEC is a model code that individual states and municipalities adopt, often with local amendments for regional conditions or stricter requirements. Always confirm the exact code edition and any local amendments your jurisdiction has adopted before relying on a specific table value
What is the maximum conduit fill?
The NEC limits fill to 40% of the conduit area for three or more conductors, 31% for two and 53% for one
How do I calculate three-phase power?
Multiply the square root of 3 by the line voltage, the line current and the power factor to get real power in watts
Why derate wire ampacity?
Heat builds up when many conductors share a conduit or the ambient temperature is high, so the safe current is lower than the table value