Wiring
Breaker size calculator
Convert the load to amps (watts ÷ volts). If it runs 3 hours or more, multiply by 1.25. Then pick the next standard breaker size up. A 1,500 W heater at 120 V draws 12.5 A, which becomes 15.6 A, so it needs a 20 A breaker and 12 AWG wire.
20 Abreaker at 120 V
- Load current
- 12.5 A (1,500 W)
- Sizing current
- 15.6 A = 125% for a continuous load
- Breaker loaded to
- 63%
- Copper NM-B (Romex)
- 12 AWG
- Copper THHN, 75 °C
- 12 AWG
Plug-in appliances on a shared circuit: one appliance on a cord should not exceed 80% of a 15 or 20 A circuit (NEC 210.23).
The rule
amps = watts ÷ volts sizing amps = amps × 1.25 (continuous load, 3 h or more) breaker = next standard size ≥ sizing amps (NEC 240.6(A))
A breaker protects the wire, not the appliance. It has to be big enough not to trip under normal load, and small enough that the wire behind it can never carry more than it is rated for. Standard breaker sizes are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 and 100 A, then larger steps.
The 125% rule exists because breakers and terminals heat up under sustained load. A load that runs three hours or more must not exceed 80% of the breaker's rating — which is the same as sizing the breaker at 125% of the load (NEC 210.20(A)). Water heaters (422.13), EV chargers (625.41) and fixed electric space heating (424.4) are treated as continuous by rule.
Common household loads
| Load | Watts | Volts | Amps | Continuous | Breaker | Copper NM-B |
|---|---|---|---|---|---|---|
| Space heater | 1,500 | 120 V | 12.5 | yes | 20 A | 12 AWG |
| Microwave | 1,200 | 120 V | 10.0 | no | 15 A | 14 AWG |
| Window AC 12,000 BTU | 1,100 | 120 V | 9.2 | no | 15 A | 14 AWG |
| Water heater 4,500 W | 4,500 | 240 V | 18.8 | yes | 25 A | 10 AWG |
| Water heater 5,500 W | 5,500 | 240 V | 22.9 | yes | 30 A | 10 AWG |
| Electric dryer | 5,400 | 240 V | 22.5 | no | 25 A | 10 AWG |
| Electric range | 9,600 | 240 V | 40.0 | no | 40 A | 8 AWG |
| Baseboard heat 2,500 W | 2,500 | 240 V | 10.4 | yes | 15 A | 14 AWG |
| Hot tub | 7,200 | 240 V | 30.0 | yes | 40 A | 8 AWG |
| Mini split 18,000 BTU | 1,900 | 240 V | 7.9 | no | 15 A | 14 AWG |
Always use the nameplate and the installation manual when you have them. Many appliances state a "maximum overcurrent protection" (MOP) or minimum circuit ampacity (MCA) — air conditioners and heat pumps in particular — and that number overrides any calculation.
Can I just put in a bigger breaker?
No. If a 15 A circuit keeps tripping, swapping in a 20 A breaker on 14 AWG wire removes the protection that keeps the wire from overheating inside your walls. Either reduce the load on the circuit or run a new circuit sized for it.
Safety. Working inside a panel is dangerous even with the main breaker off: the service conductors above it stay live. Leave panel work to a licensed electrician unless you are trained for it.
Common questions
What size breaker for a 1,500 W space heater?
20 A at 120 V. It draws 12.5 A; as a continuous load that becomes 15.6 A, over a 15 A breaker's limit.
What size breaker for a 4,500 W water heater?
30 A double-pole at 240 V with 10 AWG copper. 4,500 W ÷ 240 V = 18.75 A × 1.25 = 23.4 A, rounded up to 25 A minimum; 30 A is the usual choice with 10 AWG.
How many watts can a 20 amp breaker handle?
2,400 W at 120 V for short loads, 1,920 W continuous (80%). At 240 V, 4,800 W and 3,840 W.
Guides that use this calculator
- What Size Breaker for a Water Heater?
- What Size Wire for a 30 Amp Breaker?
- 10 vs 12 Gauge Wire for a 20 Amp Circuit