Circuit Breaker Sizing Calculator

Determine the correct circuit breaker size for your electrical load per NEC guidelines. Accounts for continuous load derating and motor starting current to prevent nuisance tripping and ensure code compliance.

System Voltage

Total Circuit Load (watts)

Continuous Load? (runs 3+ hours)

Motor Load? (high starting current)

Pricing (edit to match local rates)

15A Breaker ($)

20A Breaker ($)

30A Breaker ($)

40A Breaker ($)

50A Breaker ($)

60A Breaker ($)

100A Breaker ($)

Circuit Amperage

15.0 A

1800W / 120V

Adjusted Amps

18.8 A

125% continuous derating

Recommended Breaker

20A

12 AWG wire · $9.00

Load on Breaker

94%

Near breaker capacity

Breaker Size (Wire)	Load %	Price
15A Breaker (14 AWG)	Undersized	$8.00
20A Breaker (12 AWG)Best value	94% loaded	$9.00
30A Breaker (10 AWG)	63% loaded	$12.00
40A Breaker (8 AWG)	47% loaded	$18.00
50A Breaker (6 AWG)	38% loaded	$25.00
60A Breaker (4 AWG)	31% loaded	$35.00
100A Breaker (1 AWG)	19% loaded	$75.00

Recommended: 20A breaker with 12 AWG wireA 20A breaker handles the 18.8A adjusted load at 94% capacity. This is the most economical adequate breaker at $9.00. Use 12 AWG wire per NEC Table 310.16.

Continuous Load Derating AppliedNEC 210.20 requires that continuous loads (running 3+ hours) not exceed 80% of the breaker rating. The adjusted amperage (18.8A) includes this 125% safety factor. Examples of continuous loads include lighting, HVAC, and electric water heaters.

This is a simplified sizing estimate based on general NEC guidelines. Actual breaker sizing depends on wire length (voltage drop), ambient temperature, conduit fill, and local code amendments. Always consult a licensed electrician for final circuit design.