Circuit Breaker Calculator: Required Amps & Breaker Size
Apply the NEC's 125% continuous-load rule to your load in amps, then round up to the nearest standard breaker size — verify with a licensed electrician.
next standard NEC 240.6(A) size = 20 A
How it works
A breaker protects the wire, so its size follows the load and the code rules, not the other way around. This calculator applies NEC 210.20(A): the overcurrent device must be rated for 100% of a noncontinuous load plus 125% of a continuous load. A continuous load is one expected to run at its maximum for three hours or more, per NEC Article 100 — think of lighting, EV charging, or a long-running motor. It then rounds up to the next standard breaker size. Treat the result as a planning estimate and have a licensed electrician confirm it against the full circuit.
The 125% factor exists because breakers and conductors heat up under a sustained load, so the code builds in a 25% margin. Enter the load current and whether it is continuous, and the calculator multiplies by 1.25 when needed to get the minimum device rating. A 16 A continuous load, for instance, needs a device rated for at least 20 A.
Breakers only come in standard sizes, listed in NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 amps and up. The calculator picks the smallest standard size that meets or exceeds your requirement. This is where round numbers surprise people — there is no 65 A breaker, so a 62.5 A requirement rounds up to 70 A. The breaker must also match the conductor's ampacity, which is why wire size and breaker size are always chosen together.
required = 50 A × 1.25 = 62.5 A
standard sizes near 62.5: 60 A (too small), 70 A
result = 70 A breaker (there is no 65 A standard size)