Circuit Breaker Sizing Calculator

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

15A needs 14 AWG, 20A needs 12 AWG, 30A needs 10 AWG, 40A needs 8 AWG and 50A needs 6 AWG. Note that last one: plenty of sources say 8 AWG is fine for 50 amps, and it is, if the cable is THHN in conduit rated to 75°C. NM-B cable (Romex) is capped at the 60°C column by NEC 334.80 no matter what its conductors say, and there 8 AWG is only good for 40 A. Since almost every house is wired in Romex, this calculator uses the 60°C figures.

Putting a bigger breaker on existing wire is the classic mistake. It does not give the circuit more capacity; it just removes the thing that was protecting the wire from melting.

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.