Continuous-Load OCPD and Conductor at 125% (NEC 210.20 / 215.3)
The single most reused NEC sizing step, standalone: the breaker and minimum conductor for a mixed load are 125% of the continuous part (on 3 hours or more) plus 100% of the rest, rounded up to a 240.6(A) standard rating. 40 A continuous + 20 A noncontinuous -> 70 A minimum -> a 70 A device; a listed 100%-rated assembly drops the factor and one device size. Before ampacity adjustment; the motor, HVAC, and welder articles carry their own percentages. A design aid; the AHJ governs.
Formula and source
mult = rated_100 ? 1.00 : 1.25; A_min = mult x l_cont_A + l_noncont_A; ocpd_A = smallest NEC 240.6(A) standard rating >= A_min.
The continuous-load overcurrent and conductor rule of NEC 2023 210.20(A) (branch) / 215.3 (feeder) and 210.19(A) / 215.2(A) -- 125% of the continuous plus 100% of the noncontinuous load, device from the 240.6(A) standard ratings, with the 100%-rated-assembly exception -- by name. A computational aid; the AHJ-adopted NEC edition governs.
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