Circuit Breaker Size Calculator
Enter the load and voltage, tell it whether the load runs continuously — and get the standard breaker size plus the copper wire gauge that goes with it.
Applies the code-required 125% sizing factor.
Motors briefly draw several times their running current at startup.
Recommended breaker
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Planning guidance only — not electrical advice. Breaker and wire sizing must follow your local electrical code and be verified by a licensed electrician. The breaker protects the wire: never upsize a breaker without upsizing the wire.
How breaker sizing works
Three steps:
- Find the load current: amps = watts ÷ volts.
- Apply the 125% rule if the load is continuous (3+ hours): sized amps = load amps × 1.25. Put differently, a breaker should carry no more than 80% of its rating continuously.
- Round up to the next standard size: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A…
Standard breaker-to-copper-wire pairings:
| Breaker | Copper wire (AWG) | Typical uses |
|---|---|---|
| 15 A | 14 AWG | Lighting, general outlets |
| 20 A | 12 AWG | Kitchen/bathroom outlets, space heaters |
| 30 A | 10 AWG | Dryers, water heaters, RV hookups |
| 40 A | 8 AWG | Ranges, heat pumps, 32 A EV charging |
| 50 A | 6 AWG | Ranges, hot tubs, 40 A EV charging |
| 60 A | 4 AWG | Large AC, subpanels, 48 A EV charging |
| 100 A | 1 AWG | Subpanels |
Long cable runs may need a size (or two) thicker to keep voltage drop under 3% — check with the voltage drop calculator.
Breaker sizing FAQ
How do I calculate what size breaker I need?
Amps = watts ÷ volts, ×1.25 if continuous, then round up to a standard size. A 1,500 W heater at 120 V → 12.5 A → 15.6 A continuous → 20 A breaker.
What is the 125% rule?
Code requires circuits for continuous loads (3+ hours) to be sized at 125% of load current — equivalently, breakers should only carry 80% of their rating continuously.
What wire size goes with each breaker?
Copper: 15 A → 14 AWG, 20 A → 12, 30 A → 10, 40 A → 8, 50 A → 6, 60 A → 4. Aluminum needs 1–2 sizes larger; long runs need extra for voltage drop.
Can I install a bigger breaker if mine keeps tripping?
No — the breaker protects the wire. Upsizing the breaker without upsizing the wire is a fire hazard. Reduce the load or have an electrician run a new circuit.