A circuit breaker or fuse protects the conductor from overload and short circuit. Its rating must be high enough to carry the load without nuisance tripping, but no higher than the wire can safely handle. The governing rule for ordinary loads is the 125% continuous-load factor: the device and conductor are rated for the non-continuous load at 100% plus the continuous load at 125%. After applying the factor you round up to the next standard size in NEC 240.6.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: NEC (NFPA 70) Articles 210.20, 215.3, 240.6.
Safety notice. Overcurrent protection is life-safety equipment. This tool covers general lighting and appliance branch circuits and feeders; motors (NEC 430), HVAC (NEC 440) and other special loads have their own rules. Final design must follow your local code and be verified by a licensed electrician. See our disclaimer.
How breaker sizing works
Three steps cover the vast majority of branch circuits:
A "continuous load" is one expected to run at its maximum for three hours or more — lighting, EV charging, fixed heating. The 25% margin prevents the breaker and the lugs from running hot during long, steady operation.
Worked example 1 — EV charger
Scenario: A 40 A continuous EV charging load on a 240 V circuit.
50 A is itself a standard size, so use a 50 A breaker with conductor rated at least 50 A (typically 6 AWG copper at 75 °C). This is exactly why a 40 A charger is installed on a 50 A circuit.
Worked example 2 — sizing from power
Scenario: A 3-phase, 415 V, 11 kW continuous heater at power factor 1.0.
Use the electrical power calculator to get the current, then size the cable with the cable-sizing calculator so its ampacity is at least 20 A.
Frequently Asked Questions
Take the load current, multiply by 1.25 if it is continuous (3+ hours), then choose the next standard breaker at or above that. 16 A continuous → 20 A breaker. The conductor must be rated for at least the breaker size.
NEC 210.20(A) and 215.3 require the device and conductor to be rated for at least 125% of a continuous load. The 25% margin prevents overheating during long steady operation. Multiplying by 1.25 equals dividing by 0.8 (the 80% rule).
NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 A and up. Round up to the next standard size.
No — the breaker protects the wire, so conductor ampacity must be at least the breaker rating (with limited exceptions like motor circuits and the small-conductor next-size-up rule). Size the cable to match the breaker.
Convert power to current first: I = P ÷ V (DC/1-phase) or I = P ÷ (√3 × V × PF) (3-phase). Then apply the 125% continuous factor and round up to the next standard size.