How this calculator works
A breaker protects the wire, and must not be too small for the load or too big for the wire. For a continuous load (one that runs three hours or more) the breaker and the wire are sized for 125% of the load, which is the same as saying the load is limited to 80% of the breaker. The result is rounded up to the next standard breaker size: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 amps and so on.
Worked example: a 3,600 W continuous load on 240 V draws 15.0 A, which is 18.8 A at 125%, so it needs a 20 A breaker and 12 AWG copper. A 1,500 W heater on 120 V draws 12.5 A and needs a 20 A breaker. A 9,600 W EV charger on 240 V draws 40 A and needs a 50 A breaker. A 30 A load that is not continuous fits a 30 A breaker.
Breaker size by load (240 V, continuous)
The load in watts, the current and the breaker with a 125% allowance.
| Load | Current | Breaker | Copper wire |
|---|---|---|---|
| 1,500 W | 6.3 A | 15 A | 14 AWG copper |
| 3,000 W | 12.5 A | 20 A | 12 AWG copper |
| 4,800 W | 20.0 A | 25 A | 10 AWG copper |
| 7,200 W | 30.0 A | 40 A | 8 AWG copper |
| 9,600 W | 40.0 A | 50 A | 8 AWG copper |
| 12,000 W | 50.0 A | 70 A | 4 AWG copper |
| 19,200 W | 80.0 A | 100 A | 3 AWG copper |
Tips before you buy
- A circuit breaker sized to the wire protects the wire from overheating. Never put a larger breaker on a circuit to stop it tripping.
- A continuous load, one that runs for three hours or more, is limited to 80% of the breaker rating. Examples are EV chargers, space heaters on timers, and lighting.
- A 240 V load has half the current of the same watts at 120 V, so a smaller breaker and wire.
- Check the equipment label: it often states the maximum overcurrent protection, which is the largest breaker you may use.
- Garage and outdoor circuits usually need GFCI protection, and bedrooms and living areas AFCI, under current codes.
Tools & materials for this project
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Frequently asked questions
What size breaker do I need for 3,600 watts?
20 amps at 240 volts for a continuous load, because 3,600 W is 15 A and 125% of that is 19 A. On 120 V it is 40 amps.
What size breaker for a 1,500 watt heater?
20 amps at 120 volts, since the heater draws 12.5 A and a continuous load needs 125%. A 1,500 W heater on a 15 amp breaker is over the 80% limit.
What size breaker for an EV charger?
A 9,600 W (40 A) charger on 240 V needs a 50 amp breaker, at 125% of its current.
What is the 80% rule for breakers?
A breaker should carry no more than 80% of its rating for three hours or more. A 20 amp breaker carries 16 amps, which is 1,920 watts at 120 volts.
Can I put a bigger breaker on a circuit that trips?
No. The breaker protects the wire, so a bigger one lets the wire overheat before it trips. Reduce the load, add a circuit, or have the wire upgraded.
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