Breaker Size Calculator
Size the circuit breaker for a load from its watts and voltage, applying the NEC 125% continuous-load rule and rounding up to a standard breaker.
How it works
load amps = watts ÷ volts · required amps = load amps × 1.25 if continuous · breaker = next standard size ≥ required amps
A breaker protects the wire, not the appliance, so it is sized just above the expected current and matched to a conductor that can carry that current. The NEC treats loads that run three hours or more (lighting, EV charging, heaters) as continuous and requires the overcurrent device and conductors to be rated at 125% of that load. The result is rounded up to a standard breaker value, since breakers only come in fixed sizes.
Worked example
A 1,500 W continuous load at 120 V draws 1,500 ÷ 120 = 12.5 A. At 125% that is 15.6 A, so the next standard breaker is 20 A, protected by 12 AWG copper (20 A ampacity). A non-continuous 12.5 A load would land on a 15 A breaker instead.
Frequently asked questions
What counts as a continuous load?
Any load expected to run at its maximum for three hours or more — store lighting, baseboard heat, EV chargers, pool pumps. The 125% factor gives the breaker and wire margin so they do not run at their thermal limit for hours on end.
Can I put a bigger breaker on to stop nuisance trips?
No. The breaker must match the wire’s ampacity — oversizing it removes the wire’s protection and is a fire hazard. If a circuit keeps tripping, reduce the load or run a properly sized new circuit.
Why round up to a standard size?
Breakers are manufactured only in fixed ratings (15, 20, 30, 40 A…). You always choose the next size at or above the required current, then confirm the conductor’s ampacity supports that breaker.
Related calculators
Educational estimates only — electrical work must follow the National Electrical Code (NEC) and local amendments, and be done or inspected by a licensed electrician.