Resistance / Spot-Welder Branch-Circuit Conductor and OCPD (NEC 630.31)
Sizes a resistance (spot / seam / projection) welder branch circuit: because the welder fires in brief pulses, the conductor carries the primary current times the square root of the duty cycle (NEC 630.31(A)(2)), the same duty-derating as an arc welder. A 100 A primary, 50%-duty spot welder needs conductors rated 70.7 A (a #4 Cu at 75 C). But the overcurrent device may run up to 300% of the rated primary (NEC 630.32(A)) -- higher than the 200% for arc welders -- so the pulses do not nuisance-trip it: up to a 300 A device on that welder. Use the nameplate rated primary current and duty cycle; the AHJ, the welder nameplate, and the adopted NEC edition govern. Arc welders use the separate 630.11/630.12 (200%) method.
Formula and source
duty_multiplier = sqrt(duty_pct / 100) (NEC 630.31(A)(2)); conductor_current_a = primary_current_a x duty_multiplier; ocpd_max_a = 3.0 x primary_current_a (630.32(A)).
NEC 630.31 (conductors) and 630.32 (overcurrent protection) for resistance welding equipment, by name; the welder nameplate and the adopted NEC edition govern.
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