Breaker Size for Load

Required breaker amps = load watts × 1.25 / volts. NEC 80% derate rule.

Inputs

Result

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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

Why 1.25 multiplier?+
NEC 210.19/210.20: continuous loads can't exceed 80% of breaker rating. Inverse of 0.8 = 1.25.
What's "continuous"?+
Operates ≥3 hours at max load. Examples: heaters, lighting, motors. Inrush peaks don't count.
What about non-continuous?+
Use breaker ≥ actual amps. No 1.25 multiplier. Hair dryer (1875W) on 15A: actual 15.6A — but used for minutes, not 3 hr.
Wire size with breaker?+
Wire must match breaker AT LEAST. 15A: 14 AWG. 20A: 12 AWG. 30A: 10 AWG. 50A: 8 AWG.
GFCI / AFCI required?+
NEC 210.8 / 210.12: bathrooms, kitchens, garages, outdoor: GFCI. Bedrooms, living rooms: AFCI. Combo for new construction.

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