Breaker Size Calculator
Size a breaker from load current with the 125% continuous-duty factor.
Breaker selection must also match conductor ampacity, terminal ratings and equipment listings. Never fit a breaker larger than the wire it protects. Have all work verified by a licensed electrician.
How this calculator works
required amps = watts ÷ volts × load factor
Round up to the next standard size: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100. The breaker protects the conductor, so the wire must be rated for the breaker you fit — never the other way around.
Worked example
With load 1800 , voltage 120 , load factor 1.25 , this calculator returns 18.75 .
Frequently asked questions
Why multiply the load by 125%?
The NEC defines a continuous load as one running three hours or more, and requires the overcurrent device to be rated at 125% of it. That is why a 20 amp breaker may only carry 16 amps continuously.
What size breaker for 1800 watts?
At 120 V that is 15 amps of load. As a continuous load it needs a 20 amp breaker with 12 AWG wire; as an intermittent load a 15 amp breaker on 14 AWG is acceptable.
Can I fit a bigger breaker if it keeps tripping?
No. The breaker protects the wire, and fitting a larger one lets the conductor overheat before the breaker trips. A breaker that trips repeatedly means the circuit is overloaded or faulty.
What are the standard breaker sizes?
Fifteen, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 and 100 amps in residential panels. Always round up to the next standard size, then confirm the conductor is rated for it.