Breaker Size Calculator

Find the smallest standard breaker for a load, sized at 125% for continuous loads (3 hours or more).

What Do You Want to Find?
Values from the equipment nameplate.
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Breaker size
Load current
Minimum rating
Wire check
Standard sizes nearby
Work

Saved Calculations

Items you add are saved in this browser.

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How to Size a Breaker

Minimum rating = non-continuous load + 125% × continuous load (NEC 210.20(A), 215.3)
Amps = watts ÷ volts (single-phase)   watts ÷ (volts × √3) (three-phase)

Round up to the next standard rating in NEC 240.6(A): 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A and up.

Worked Example

Tips

Motors, HVAC equipment and appliances with nameplate "maximum overcurrent protection" follow their own rules; use the nameplate. Have a licensed electrician confirm sizing and wiring against your local code.