Circuit Breaker Sizing for a Continuous Load
Circuit Breaker Sizing for a Continuous Load
Electrical codes require circuit breakers supplying loads that run continuously for three hours or more to be sized at 125% of the continuous load current, not the bare load current itself. This margin exists because breakers are only rated for their full nameplate current on non-continuous duty.Sizing a breaker at exactly the load current for a continuous load like site lighting or HVAC equipment can cause nuisance tripping from thermal buildup over hours of steady operation, which is why the 125% factor is a mandatory, not optional, design rule.
The minimum breaker rating is I_breaker = 1.25·I_continuous, the continuous load current increased by 25%. where I_continuous is the steady-state continuous load current.
Inputs
I_cont := 32 A=32 A
Applying the mandatory 125% factor for continuous loads gives the minimum standard breaker rating that will not be considered overloaded by code.
I_brk := 1.25·I_cont=40 A
Results
A minimum breaker rating of 40 A for a 32 A continuous load is a clean, standard result, and the designer would then select the next standard breaker size at or above 40 A. Sizing any lower than this — even if the breaker technically carries 32 A fine — violates the continuous-load derating rule and is a common code compliance error on inspections.
Assumptions & limitations
- Continuous load defined as operating at maximum current for three hours or more.
- Breaker must be selected at the next standard size at or above the calculated minimum.
- Conductor ampacity must independently meet or exceed the same 125% requirement.