Welders get their own section in the NEC. It's worth looking up.
The nameplate will list a "duty cycle." This bit of info in turd is used to select your breaker and wire size. In theory, one could run #12, on a 100 amp breaker, if the duty cycle was low enough.
DOing so would probably be foolish, though. Replace the welder with another machine, and you're likely to have to re-do the entire circuit.
