A breaker must be at least as large as the load, and 125% of the load if it runs three hours or more. Then round up to the next standard size. A 24 amp EV charger runs for hours, so it needs 24 × 1.25 = 30 amps: a 30 amp breaker on 10 AWG copper. A 12.5 amp heater used now and then fits a 15 amp breaker; left on all evening it needs 15.6 amps, so a 20 amp circuit.
How the breaker size is calculated
Continuous or not. The NEC calls a load continuous when its maximum current is expected to last three hours or more: EV chargers, water heaters, space heaters, pool pumps, lighting in a shop that runs all day. EC&M sums up the rule in 210.19(A)(1), 210.20(A) and 215.3: branch-circuit conductors and overcurrent protection are sized at 125% for continuous loads. The rest of the load counts at 100%.
Breaker ≥ noncontinuous amps + 1.25 × continuous amps, rounded up to a 240.6(A) size
Standard sizes. Table 240.6(A) lists the standard ratings: 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 amps and up. The calculator picks the first one at or above the amps needed.
Wire for the breaker. The breaker protects the wire, so the wire's ampacity in Table 310.16 must be at least the breaker rating, in the temperature column the wiring allows. Southwire's data for Romex NM-B limits it to the 60 °C column. On top of that, 240.4(D) caps 14 AWG copper at 15 A, 12 AWG at 20 A and 10 AWG at 30 A.
Worked example: a 24 A EV charger
The calculator opens on it: 24 A, continuous, copper, NM-B.
- 24 A × 1.25 = 30 A needed
- 30 A is a standard size, so a 30 A breaker
- Copper at 60 °C: 12 AWG carries 20 A, 10 AWG carries 30 A, and 240.4(D) allows 30 A on 10 AWG: 10 AWG
Enter it as watts instead and you get the same: 5,760 W ÷ 240 V = 24 A. A 32 A charger needs 40 A and 8 AWG; a 40 A charger needs a 50 A breaker.
Breaker and wire size chart
The smallest copper wire each breaker may protect, from Table 310.16 and 240.4(D):
| Breaker | Copper, NM-B (60 °C) | Copper, 75 °C | Aluminum, 75 °C |
|---|---|---|---|
| 15 A | 14 AWG | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 12 AWG | 10 AWG |
| 30 A | 10 AWG | 10 AWG | 8 AWG |
| 40 A | 8 AWG | 8 AWG | 8 AWG |
| 50 A | 6 AWG | 8 AWG | 6 AWG |
| 60 A | 4 AWG | 6 AWG | 4 AWG |
The 75 °C column applies only when the wire, the breaker and the equipment terminals are all rated 75 °C, typically individual THHN wires in conduit. A 50 A circuit in NM-B cable needs 6 AWG because 8 AWG is only 40 A at 60 °C.
What changes the answer
- Long runs. Voltage drop can call for a bigger wire than the breaker does, never a smaller one. Check with the voltage drop calculator.
- Several wires together. More than three current-carrying wires in a conduit lose ampacity (Table 310.15(C)(1)); the wire size calculator applies it.
- Motors and air conditioners. Their nameplates list a minimum circuit ampacity and a maximum breaker, which take precedence over this general method.
- A breaker that trips. A breaker tripping on a circuit whose wire matches it is doing its job. Moving the load to its own circuit is the fix; a larger breaker on the same wire is a fire risk and a code violation.
From watts to breaker
If the nameplate gives watts, divide by the voltage: a 4,500 W water heater at 240 V draws 18.75 A. Counted as a continuous load, 18.75 × 1.25 = 23.4 A, which rounds up to a 25 A breaker, and 10 AWG copper because 12 AWG is capped at 20 A. The amp calculator converts watts, kVA and horsepower, including three-phase.
Next steps
Planning a panel upgrade or a new service? Add up every circuit with the electrical load calculator. All electrical tools are on the electrical calculators page.