Resistance / Spot-Welder Branch-Circuit Conductor and OCPD (NEC 630.31)

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.

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.

The duty-cycle conductor derating and the 300% OCPD limit are public NEC (many jurisdictions post the adopted NEC); the rated primary current and duty come from the welder nameplate.

Estimate. AHJ and licensed professional govern.

Field names used by the API: primary_current_a, duty_pct, conductor_current_a, ocpd_max_a

• Duty multiplier conductor at I_primary x sqrt(duty) per 630.31(A)(2) for a specific nonrepetitive welderNEC 630.31
• OCPD 300% resistance-welder overcurrent device up to 300% of the rated primary (630.32(A)); arc welders are 200%NEC 630.32