BREAKER SIZE
CIRCUIT BREAKER SIZE
breaker = next std ≥ load × (1.25 if continuous)
V
RESULT
—
FILL IN ABOVE
NEC 240.6 standard sizes + 210.20(A) 125% continuous multiplier. Motor circuits flagged for 430.52 sizing. Estimate only — verify with a licensed electrician and local code/inspector before purchase or installation. Not a substitute for engineered drawings.
About this calculator
This circuit breaker calculator picks the next standard breaker size (NEC 240.6) for a given load. Enter wattage or amperage and the calculator converts watts to amps using your system voltage, applies the NEC 210.20(A) 125% multiplier when the load is continuous (anything operating ≥3 hours — lighting, EV chargers, electric heat), and rounds up to the nearest standard breaker (15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A). Motor loads use NEC 430.52 sizing (250% inverse-time breaker for typical service factor 1.15 motors) and are flagged separately. ESTIMATE ONLY — final breaker selection must be verified by a licensed electrician and the local AHJ before any work goes in.
Common questions
How are standard breaker sizes set?
NEC 240.6(A) lists the standard sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250 A and up. Anything else is non-standard and rarely stocked. The calculator always rounds up to the next standard size.
What is the 80% / 125% rule?
Two ways to say the same thing: a continuous load (≥3 hr operation) can be no more than 80% of breaker rating, OR breaker must be sized to 125% of the continuous load. NEC 210.20(A). A 16 A continuous EV charger needs a 20 A breaker; a 20 A non-continuous appliance also lands on a 20 A breaker.
Why are motor breakers oversized?
NEC 430.52 lets motor inverse-time breakers run up to 250% of full-load current to ride out the inrush at startup without nuisance tripping. The motor itself is protected by the overload relay, not the breaker. This is why a 12 A motor can be on a 30 A breaker and still be code-compliant.