I installed a welding circuit with a 60a breaker in the lower part of a split buss residential panel and put a 70a breaker (max size according to the panel specs) feeding the lower part of a the split buss. The the welder draws 92 amps max. I didn't want to go over the 70a split buss feeder breaker, so I told the customer he probably couldn't run his welder at maximum without possibly tripping the breaker. Everything is installed up to code, but is this a good idea even being legal?
(This is the same panel I mentioned in the General discussion area about the ("no main disconnect".)
The Golden Rule - "The man with the gold makes the rule"
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.