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

ScopeBecause the welder fires in brief pulses, the conductor carries the primary current times the Table 630.31(A)(2) duty multiplier (the printed value at a listed duty, 0.71 at 50%; the square root of the duty between rows; 0.22 at 5% or less), the same duty-derating idea as an arc welder. A 100 A primary, 50%-duty spot welder needs conductors rated 71 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 or tableduty_multiplier = the Table 630.31(A)(2) value at a listed duty cycle (50% 0.71, 40% 0.63, 30% 0.55, 25% 0.50, 20% 0.45, 15% 0.39, 10% 0.32, 7.5% 0.27, 5% or less 0.22), else sqrt(duty_pct / 100), the relation the table rounds; conductor_current_a = primary_current_a x duty_multiplier; ocpd_max_a = 3.0 x primary_current_a (630.32(A)).

Source and editionNEC 630.31 (conductors) and 630.32 (overcurrent protection) for resistance welding equipment, by name; the welder nameplate and the adopted NEC edition govern.

Free accessThe 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.

What governsEstimate. AHJ and licensed professional govern.

API field namesprimary_current_a, duty_pct, conductor_current_a, ocpd_max_a

• Duty multiplier conductor at I_primary x the Table 630.31(A)(2) value at a listed duty, sqrt(duty) between rows, 0.22 at 5% or less, 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