What Size Wire for a Welder?

For a typical 240V home welder, the simple robust answer is a 50A circuit with #6 copper on a NEMA 6-50 receptacle, which handles essentially any hobby stick, MIG, or small multiprocess machine. If you want the code minimum, NEC 630.11 sizes the conductor from the welder's rated primary current times a duty-cycle factor (Table 630.11(A)): a welder that only runs part of the time draws a smaller effective load, so the wire can be smaller. NEC 630.12 then allows the breaker to be as large as 200% of the rated primary current, so the breaker is often bigger than the wire's normal rating, on purpose.

A welder's duty cycle is the fraction of a 10-minute period it can weld at full output; a 60% duty cycle means 6 minutes welding, 4 minutes cooling. Because it is not welding continuously, NEC 630.11 lets you multiply the nameplate rated primary current by a factor from Table 630.11(A) to get the conductor's required ampacity: 100% duty = 1.00, 80% = 0.89, 60% = 0.78, 50% = 0.71, 40% = 0.63, 30% = 0.55, 20% or less = 0.45. Example: a 50A-primary welder at 60% duty needs a conductor rated for 50 x 0.78 = 39A, which is #8 copper.

Because NEC 630.12 sizes the overcurrent device for the welder, not just the conductor. It permits an overcurrent device up to 200% of the rated primary current (using the next lower standard size if 200% is not a standard rating). The duty-cycle-sized conductor from 630.11 is protected for the intermittent welding load, and the larger breaker keeps it from nuisance-tripping on the heavy momentary draw when an arc strikes. This is similar to a motor circuit, where the wire and the breaker are sized by different rules.

Yes, that is the most common home setup. A 50A / 240V circuit with #6 copper on a NEMA 6-50 receptacle covers the great majority of 240V hobby and light-industrial welders, because their rated primary current at usable duty cycles falls at or below what a 50A circuit supplies. Check the welder's nameplate rated primary amps; if it is 40A or below, a 50A circuit is comfortable. Larger machines that call out a higher primary current need a circuit sized to their nameplate.

The most common 240V welder plug is the NEMA 6-50 (50A), the same style used for many stick and MIG welders, paired with a 50A circuit and #6 copper. Smaller 240V welders may use a NEMA 6-30 (30A, #10 copper) or 6-20 (20A, #12). Match the receptacle, breaker, and wire to each other and to the welder's rated primary current; the machine's cord cap tells you which receptacle the manufacturer expects.