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) 28.4
- Max overcurrent device 80
Checked against published worked examples from: EC&M (Mike Holt).
Details, formula, and sources
ScopeThe conductor carries an EFFECTIVE current I_eff = primary current x the NEC Table 630.11(A) duty-cycle multiplier from the column for the welder type (transformer/rectifier: the printed rounding of the square root of the duty cycle, 0.45 at or below 20%; motor-generator: a higher column, 0.55 at or below 20%), because a low-duty welder heats the wire less. A 40 A primary, 50%-duty transformer welder needs conductors rated 28.4 A (a #10 Cu at 60 C); a motor-generator welder, 30 A. 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 or tableduty_multiplier from NEC Table 630.11(A): transformer / rectifier column = the printed rows (0.71 at 50%), sqrt(duty_pct / 100) between them, 0.45 at 20% or less; motor-generator column 1.00 / 0.96 / 0.91 / 0.86 / 0.81 / 0.75 / 0.69 / 0.62 at 100 to 30%, 0.55 at 20% or less, linear between rows; effective_current_a = primary_current_a x duty_multiplier; ocpd_max_a = 2.0 x primary_current_a (630.12(A)).
Source and editionNEC 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.
Free accessThe duty-cycle multipliers 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.
What governsEstimate. AHJ and licensed professional govern.
API field namesprimary_current_a, duty_pct, effective_current_a, ocpd_max_a
- Duty multiplier conductor at I_primary x the Table 630.11(A) multiplier for the welder type: transformer / rectifier (sqrt(duty) as printed, 0.45 at 20% or less) or motor-generator (the higher column, 0.55 at 20% or less); 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