Arc-Welder Branch-Circuit Conductor and OCPD (NEC 630.11)
Sizes an arc-welder branch circuit off the nameplate, not the running current: the conductor carries an EFFECTIVE current I_eff = primary current x the NEC Table 630.11(A) duty-cycle multiplier (the square root of the duty cycle), because a low-duty welder heats the wire less. A 40 A primary, 50%-duty welder needs conductors rated 28.3 A (a #10 Cu at 60 C). The overcurrent device may run up to 200% of the rated primary (NEC 630.12(A)), so up to an 80 A breaker on that welder. Use the nameplate rated primary current and duty cycle; the AHJ, the welder nameplate, and the adopted NEC edition govern. Resistance (spot) welders use the separate 630.31/630.32 method.
Formula and source
duty_multiplier = sqrt(duty_pct / 100) (NEC Table 630.11(A)); effective_current_a = primary_current_a x duty_multiplier; ocpd_max_a = 2.0 x primary_current_a (630.12(A)).
NEC 630.11 (conductors) and 630.12 (overcurrent protection) for arc-welding equipment, with the Table 630.11(A) duty-cycle multipliers, by name; the welder nameplate and the adopted NEC edition govern.
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