Breaker Size Calculator
Get the right breaker size and minimum wire for any load — entered in amps or watts — with the NEC continuous-load rule applied.
Calculations are built on the 2023 National Electrical Code (NEC) and cross-checked against the published code tables. How we verify. Last reviewed June 2026.
How breaker sizing works
First find the load current: for a resistive/known load, amps = watts ÷ volts. If the load is continuous (runs 3+ hours), multiply by 1.25. Then round up to the next standard breaker size from NEC 240.6 (15, 20, 25, 30, 35, 40, 45, 50, 60A …).
The conductor’s ampacity must be at least the breaker rating (NEC 240.4), and small conductors are capped by 240.4(D): 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A. Motors, HVAC and welders follow special rules (NEC 430/440/630) and are sized from the nameplate, not this general method.