Arc-Welder Branch-Circuit Conductor and OCPD (NEC 630.11)
Sizes an arc-welder branch circuit off the nameplate, not the running current.
Example
You enter
- Nameplate primary current (A) 40
- Duty cycle (%) 50
You get
- Effective current (size conductor to) 28.284
- Max overcurrent device 80
Details, formula, and sources
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.
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.
The duty-cycle multiplier (square root of the duty) and the 200% 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, effective_current_a, ocpd_max_a
- Duty multiplier conductor at I_primary x sqrt(duty) per Table 630.11(A); a low-duty welder heats the wire lessNEC 630.11(A)
- OCPD 200% arc-welder overcurrent device up to 200% of the rated primary (630.12(A)); resistance welders use 630.31/32 (300%)NEC 630.12