The 250 MP I am using can draw up to 50 amps. I ran a 30A service and separate box to my garage years ago. I also added a 50A plug, but again I have a 30A line. For the kind of welding you will do on the car, I cannot imagine ever drawing 50 amps to even worry about it. I am not an electrician, but I can tell you i've never tripped the circuit breaker on the 30A fuse ever.
My 110V unit (Clarke 130) never did either. That unit can draw up to 23 amps if I recall.
