Circuit Breaker Sizing Calculator — NEC 125% Rule | WireStandard

Enter load current (A) and load type — continuous (on for 3+ hours per NEC Art.100) or non-continuous — to get the NEC-compliant breaker size. The calculator applies the NEC 210.20(A) and 215.3 rule: breaker must be rated at least 125% of continuous load, then rounds up to the next standard size per NEC 240.6(A). Used by electricians sizing branch-circuit and feeder overcurrent protective devices.

Formula

I_breaker ≥ I_noncont + 1.25 × I_cont. For purely continuous loads: I_breaker ≥ 1.25 × I_load. Round UP to the next standard size from NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 A. NEC 240.4(B) permits rounding to the next standard size when the calculated value falls between sizes for conductors protected at 800 A or less.

Worked Example

32 A continuous load (e.g., Level 2 EV charger). I_breaker = 1.25 × 32 = 40 A. NEC 240.6(A) lists 40 A, so breaker = 40 A. Size conductor per NEC 310.16 at 40 A (AWG 8 at 75 °C). If load were 33 A: 1.25 × 33 = 41.25 A → next standard up = 45 A breaker; conductor must handle 45 A (AWG 6 minimum). The 125% rule prevents nuisance trips on circuits that run at full rated current for extended periods.

Frequently Asked Questions

What counts as a continuous load?

NEC Art.100 defines a continuous load as one where maximum current is expected to continue for 3 hours or more. Typical examples: commercial lighting, Level 2 EV chargers, HVAC compressors, and industrial process equipment. Receptacle circuits used intermittently are generally non-continuous.

Can I use a smaller breaker than 125% if the equipment nameplate specifies one?

Only if both the breaker and its enclosure are listed for 100% continuous duty per NEC 210.20(A) Exception. This exception is rare in residential installations but more common in large commercial panelboards and switchgear rated for 100% loading.

Why must the conductor ampacity match the breaker rating, not just the load current?

NEC 240.4(A) requires conductor ampacity to equal or exceed the OCPD rating. If the breaker is 45 A, the wire must handle 45 A — not just the 41.25 A calculated load — so the insulation is not damaged if a fault holds current below the trip threshold for an extended period.