Breaker Size Calculator

How the calculator sizes a breaker

Start with the current the load draws, then split it into two parts: the continuous part, which runs at full current for 3 hours or more, and everything else. The rule in NEC 210.20(A) says the breaker must be rated at least the non-continuous load plus 125% of the continuous load. The calculator works out that design current and picks the next standard breaker size at or above it.

It never rounds down. If the design current lands exactly on a standard size, that size is the answer. If it is even a little above, the answer is the next size up. The result also names the size just below and shows why it is too small, so you can follow the working.

The example on this page is a 7,200 W heater on 240 V single phase that runs for hours at a time. It draws 30.0 A. Counted at 125%, that is 37.5 A. A 35 A breaker is below that, so the answer is a 40 A breaker, and the heater uses 75.0% of it.

Continuous and non-continuous loads

A continuous load is one expected to run at its maximum current for 3 hours or more. Water heaters, space heaters, EV chargers and lighting that stays on all evening usually count. A load that runs for a few minutes at a time, such as a kettle or a power tool, does not.

The 125% goes on the continuous part only. Put the long-running load in the first box and the rest in the second. A 20 A continuous load with 10 A of short loads gives 10 + 1.25 × 20 = 35 A, so a 35 A breaker. Multiplying the whole 30 A by 1.25 would ask for 40 A, one size more than the rule needs.

The 80% figure

People often call this the 80% rule. It is the same math seen from the other side: 1 ÷ 1.25 = 0.8, so a continuous load should stay at or under 80% of the breaker. A 20 A breaker carries up to 16 A of continuous load, which is 1,920 W at 120 V or 3,840 W at 240 V. The NEC writes the rule in its 125% form.

From watts to amps

If you know the watts, the calculator turns them into amps first:

• Single phase: amps = watts ÷ (volts × power factor)
• Three phase: amps = watts ÷ (1.732 × volts × power factor), with the voltage measured line to line

Heaters, ovens and incandescent lamps have a power factor of 1. A motor draws more current than its watts suggest, but motors follow their own breaker rules anyway (see below). For three phase the answer is the current in each line, and the calculator offers only the line-to-line voltages 208, 240 and 480 V. The watts to amps calculator shows the same conversion on its own.

The breaker protects the wire

A breaker is there to protect the wire behind it. It is sized for the load, and the wire is then chosen so it can carry at least the breaker’s rating. Small copper wire has fixed limits: 14 AWG on a 15 A breaker, 12 AWG on 20 A and 10 AWG on 30 A (NEC 240.4(D)). Never fit a bigger breaker on the same wire to stop it tripping.

Under the result the calculator shows the typical copper wire for the breaker, from the same figures as the wire size calculator: for a 40 A breaker, 8 AWG at its 60 °C rating. Long runs lose voltage and may need a bigger wire, so the link opens the wire calculator with your breaker and circuit filled in. Above 100 A it shows no wire.

When this does not apply

• Motors, compressors, air conditioners and heat pumps follow their own rules. Use the maximum breaker size printed on the nameplate.
• EV chargers count as continuous: enter the charger’s amps in the 3 hours or more box, and follow the charger’s manual.
• Breakers listed for 100% of their rating skip the 125%. They come with a matching enclosure and are rare in homes.
• Above 200 A (services and large feeders) the calculator stops and gives no size: ask an electrician.
• Heat and bundling are not applied. Hot attics and bundled cables lower what a wire can carry.

Common mistakes

• Putting the whole load at 125%. Only the part that runs 3 hours or more gets the extra 25%.
• Using the appliance’s amps as the breaker size. A 30 A continuous load needs more than a 30 A breaker.
• Fitting a bigger breaker because the old one trips. Find out why it trips, or run a heavier circuit.
• Mixing up line-to-neutral and line-to-line volts. On three phase, use 208, 240 or 480 V, never 120 or 277 V.

This is a typical result from the 125% rule. Your code edition, the equipment’s instructions and your local inspector (AHJ) have the final say.

Worked example

1. Inputs: 7,200 W that runs 3 hours or more, 0 W that does not, single phase AC at 240 V, power factor 1.
2. Continuous amps = watts ÷ (volts × power factor) = 7,200 W ÷ (240 V × 1) = 30.0 A
3. Design current = non-continuous + 1.25 × continuous = 0 A + 1.25 × 30.0 A = 37.5 A
4. Breaker = the next standard size at or above 37.5 A = 40 A
5. Load on breaker = (30.0 A + 0 A) ÷ 40 A = 75.0%
6. Typical copper wire for a 40 A breaker: 8 AWG (60 °C rating 40 A). Long runs need bigger wire.

Questions people ask

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