Q. How do I size the conductors for an arc welder?

A. The supply conductors for arc welders must have an ampacity not less than the welder nameplate rating. If the nameplate rating isn't available, the supply conductors must have an ampacity not less than the rated primary current as adjusted by the multiplier in Table 630.11(A), based on the duty cycle of the welder.

Let's work an example problem to help hit this point home.

A non-motor/generator arc welder has a primary current rating of 40A with a duty cycle of 50%.
Calculated Load = Primary Rating x Multiplier [Table 630.11(A)]
Calculated Load = 40A x 0.71 = 28.40A

Therefore, the branch-circuit conductor for the welder must not be sized less than 30A. As per Table 310.15(B)(16), a 10 AWG conductor has an ampacity of 35A at 75ºC.