CalcPit
Electrical

Breaker Size Calculator

Size a breaker from load current with the 125% continuous-duty factor.

1.25 for continuous loads running 3 hours or more. 1.0 for intermittent loads.
Minimum breaker rating —
Actual load current —
Max continuous load on a 20 A breaker —

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.

Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.

Where these numbers come from

Check the edition. Codes are amended every few years, and the one your inspector works from may not be the one named above.

Worked example

With load 1800 , voltage 120 , load factor 1.25 , this calculator returns 18.75 .

Breaker size for common continuous loads

Every row here opens the calculator already filled in. Copy that link and the numbers travel with it.

JobSizeMinimum breaker rating
Baseboard heater 1500 × 240 × 1.25 7.81
Bathroom receptacle circuit 1800 × 120 × 1.25 18.75
EV charger, level 2 7680 × 240 × 1.25 40.00
Electric water heater 4500 × 240 × 1.25 23.44
Well pump 1500 × 240 × 1.25 7.81

Round up to the next standard size, then confirm the conductor is rated for that breaker — never the other way round.

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.

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