NEC Wire Size Calculator

The NEC wire size calculator applies the National Electrical Code methodology to determine the minimum conductor size for a circuit, considering both the NEC 125% continuous load rule for ampacity sizing and the voltage drop criterion. It outputs the minimum AWG by each criterion separately, then presents the governing (more conservative) recommendation.

NEC Section 210.19(A)(1) requires that branch circuit conductors have an ampacity no less than the maximum load served, and for continuous loads (loads operating continuously for 3 hours or more), the conductor ampacity must be at least 125% of the continuous load current. This means a 16A continuous load requires a conductor rated for at least 16 × 1.25 = 20A — AWG 12 copper (25A rated at 75°C per NEC Table 310.12).

The 125% rule exists because conductor ampacity tables assume steady-state thermal equilibrium. A continuous load keeps the conductor at maximum temperature continuously, whereas a non-continuous load allows the conductor to cool between operations. The 125% factor provides a thermal safety margin ensuring the conductor doesn't slowly overheat during extended operation.

The voltage drop criterion is technically advisory (NEC informational notes recommend 3% for branch circuits), but it is widely adopted as a minimum design standard. This calculator uses 3% as the threshold for the voltage drop calculation. For sensitive loads or long runs, a more conservative 2% limit may be appropriate.

The recommended AWG is the smaller of the two minimum AWG values (the one with the lower number, meaning larger wire) — the governing criterion. Often for short runs at normal current levels, ampacity governs. For long runs or low-voltage systems, voltage drop governs.