Electric Welders — Conductor Sizing by Duty Cycle

A welder hammers hard for a few seconds, then rests while the operator repositions. The duty-cycle multiplier accounts for that intermittent heating, so the conductor is sized for the real thermal load, not the instantaneous peak.

This is one of those articles that looks strange until you understand welders never run 100 percent. Use the nameplate duty cycle and the math comes out honest.

When it applies

Article 630 addresses supply conductors for electric welders. Because a welder only draws its heavy current for part of the cycle, the duty cycle, you size the conductors using a factor tied to that rating rather than treating the peak draw as continuous. A machine that welds intermittently needs smaller supply conductors than its instantaneous current would suggest.

Common mistakes

• Sizing conductors to the maximum welding current as if it ran continuously, oversizing the feeder.
• Ignoring the duty-cycle multiplier and getting the ampacity wrong in the other direction on a high-duty machine.
• Overlooking overcurrent protection sized specifically for the welder circuit.

In the field

In a fab shop I read the nameplate duty cycle first. Two welders with the same peak current can call for different supply conductors, and the duty rating is the number that settles it.