Usually welders like this are on a 50 amp circuit. The trip curve of a typical 50 amp breaker pretty closely matches the duty cycle of the welder.
I've seen plenty of them on 30s though. That's what a typical temporary power box on a construction job will have. A 30 will trip if you weld for a long time at a high current setting, or if you stick the rod for more than about 5 seconds.
Here's what the code has to say; (Article 630).
1) Wire size; for a welder like you have, the minimum wire size is the rated input current multiplied by the value given in the duty cycle table. (20%=.45, 30%=.55, 40&=.63, 50%=.71, 60%=.78.) Most buzz-boxes are 50 amps and a 20% duty cycle.
2) Breaker size. There's no minimum, but the maximum cannot be more than 200% of the rated current. If there's a cord and plug, the breaker size cannot be more than the rating of the receptacle. The breaker cannot exceed 200% of the wire size in any case.
This is one of the few times you can have a larger breaker than normally allowed, but only if the welder is hard-wired.
Rob
P.S. If I were you, I'd put it on a 30. That way, if it does trip, there's a good chance that the main at the house will stay on, and thus the lights will also stay on.
