Wire Ampacity Calculator
Determine allowable ampacity for copper and aluminum conductors according to NEC tables, temperature correction, and conduit bundling deratings.
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How NEC Wire Ampacity is Calculated
Conductor ampacity is the maximum current in amperes that an insulated conductor can carry continuously under operating conditions without exceeding its insulation temperature rating. The National Electrical Code (NEC Table 310.16) provides baseline ampacity figures at a 30°C (86°F) ambient baseline with not more than 3 current-carrying conductors in a raceway or cable.
Core Formula
Derated Ampacity = Base Ampacity × Temp Correction Factor × Conduit Fill Factor
Small Conductor Overcurrent Rules (NEC 240.4(D))
Regardless of the insulation rating (even 90°C THHN), copper branch circuits must follow fixed maximum overcurrent protection limits:
- 14 AWG Copper: 15 Amp maximum breaker
- 12 AWG Copper: 20 Amp maximum breaker
- 10 AWG Copper: 30 Amp maximum breaker
Frequently Asked Questions
Why can 12 AWG THHN have a 30A base ampacity but only a 20A breaker?
NEC 240.4(D) enforces small conductor rules limiting 12 AWG copper to 20A overcurrent protection. The higher 90°C rating (30A) is used as the starting point for derating due to high ambient temperatures or conduit bundling.
What is the continuous load rule?
NEC Section 210.19 requires branch conductors serving continuous loads (running 3 hours or more) to be sized for 125% of the continuous load current.
Why does aluminum require a larger wire size than copper?
Aluminum has higher electrical resistivity than copper (approx. 61% the conductivity of copper). An aluminum conductor typically needs to be two AWG sizes larger to carry the same current safely.