How It Works
First convert the load to amps: A = W ÷ V. If the load is continuous (3+ hours), multiply by 1.25 per NEC. Then pick the smallest standard breaker that covers the result.
Calculation Explanation
Example: 1,500W heater at 120V → 12.5A. Continuous → 15.6A required → 20A breaker with #12 wire. A 15A breaker would trip after about 2.9 hours.
Verify the wire can handle the breaker with the Wire Size Calculator and estimate your panel load with the Electrical Load Calculator.
Breaker & Wire Quick Reference
| Breaker | Max Continuous Load | Min Copper Wire |
|---|---|---|
| 15A | 1,440 W @ 120V | #14 |
| 20A | 1,920 W @ 120V | #12 |
| 30A | 5,760 W @ 240V | #10 |
| 40A | 7,680 W @ 240V | #8 |
| 50A | 9,600 W @ 240V | #6 |
| 60A | 11,520 W @ 240V | #6 (or #4 for long runs) |