Breaker Size for Load
Required breaker amps = load watts × 1.25 / volts. NEC 80% derate rule.
Result
Required breaker
20 A
Load draws 12.5 A; need ≥ 15.6 A (NEC 125%).
- Load1500 W
- Voltage120 V
- Actual current12.50 A
- Required (×1.25)15.63 A
- Next standard breaker20 A
Step-by-step
- Current = W / V = 1500 / 120 = 12.50 A.
- Required = current × 1.25 (NEC 80% rule for continuous loads) = 15.63 A.
- Next standard breaker ≥ required = 20 A.
How to use this calculator
- Enter continuous load watts.
- Pick voltage (120/240).
About this calculator
NEC 80% derate rule: continuous loads (>3 hours) cannot exceed 80% of breaker rating. Equivalently, breaker must be ≥ load × 1.25. A 1500 W heater on 120 V draws 12.5 A; needs 15 A breaker (12.5 × 1.25 = 15.6 A → next size 20 A). Standard breaker sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, etc. Always use the next size up.
Frequently asked
NEC 210.19/210.20: continuous loads can't exceed 80% of breaker rating. Inverse of 0.8 = 1.25.
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