I have been given a mig welder to supply power for. Rated input is 230volt 20 amp single phase, but plug supplied is 50 amp 250 volt - I guess that although oversized this is just the standard welding plug, but does one wire the receptacle for the demand of the welder or the 50 amp plug supplied?
Welders are pretty hard on a power supply.
Is this in a residential or commercial workshop?
One thing I would note for a start is the nameplate on the welder itself, especially the Duty Cycle.
Welders with high Duty Cycles (of 60-70%) require heavier supply wiring, this is due to the weldor being able to use the welder for longer periods at a time and therefore increasing the amount of heating in the supply cabling.
If however, the welder only has a Duty Cycle of say 30-40% you can pretty much get away with a circuit that matches the current draw of the welder.
Another thing, welders and circuit breakers don't tend to go together that well, unless the CB has magnetic protection only, using a CB with thermal protection added in, will end in tears.
I've seen CB's bypassed by those that don't know any better in the past, because the wrong type of CB was installed in the first place.