Breaker Sizing Calculator — NEC 125% Rule + Wire Gauge
Size a circuit breaker in seconds. This free breaker sizing calculator applies the NEC 125% continuous-load rule (NEC 210.20(A) / 215.3), snaps the result to the next standard breaker size from NEC 240.6(A), and shows the minimum copper wire gauge. Includes a solar backfeed mode with a 120% rule check (NEC 705.12) for inverter output circuits.
Enter the load that runs for 3 hours or more (EV charger, heater, compressor, lighting). The calculator multiplies by 125% per NEC 210.20(A).
How the NEC 125% rule works
A continuous load (3+ hours) must not exceed 80% of the breaker's rating, so the breaker is sized at 125% of the load current and rounded up to the next standard rating in NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200… amps. Wire sizes shown assume copper conductors with 75°C insulation (THHN/THWN-2) and the NEC 240.4(D) small-conductor limits: 14 AWG max 15 A, 12 AWG max 20 A, 10 AWG max 30 A.
Always verify against the equipment nameplate and local code. Aluminum conductors, long runs (voltage drop), high ambient temperatures, and more than three current-carrying conductors in a raceway all require upsizing. When in doubt, consult a licensed electrician.