Circuit Breaker Sizing Calculator
Size a circuit breaker correctly using NEC rules.
Enter load wattage, voltage, and whether the load is continuous to get the required breaker amperage.
Circuit breakers protect wiring from overheating. A breaker rated for 20 amps will trip before the wire it protects gets hot enough to damage insulation. Sizing one too small causes nuisance trips. Sizing one too large defeats the protection entirely.
The NEC rules
For non-continuous loads (running less than 3 hours continuously):
Breaker size >= load amps
Round up to the next standard size.
For continuous loads (running 3 hours or more — lighting, HVAC, EV chargers):
NEC Section 210.20(A) requires the breaker to be sized at 125% of the continuous load.
Breaker size >= load amps x 1.25
Standard breaker sizes in amperes: 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200.
120V vs 240V
Household outlets and most lighting are 120V. Large appliances (dryer, range, central AC, EV charger, water heater) are 240V. For 240V loads, the amps are half what they would be at 120V for the same wattage — and the breaker is a double-pole that takes two spaces in the panel.
Wire must match the breaker
A 20A breaker requires minimum 12 AWG wire. A 15A breaker requires minimum 14 AWG. A 30A circuit requires 10 AWG. Oversizing a breaker on undersized wire is a code violation and a fire hazard. Wire and breaker must be matched — upgrading the breaker without upgrading the wire changes nothing except risk.
AFCI and GFCI requirements
NEC 2020 requires arc-fault circuit interrupters (AFCI) in most living areas and ground-fault protection (GFCI) in bathrooms, kitchens, garages, and outdoors. When replacing or installing breakers in these areas, select the appropriate AFCI or GFCI breaker type.