The only info on my welder is references to supply amperage. A 20amp circuit is the heaviest called for and the unit only supplies 85amps welding from a 20 amp circuit. I'd assume if there were more amps available from a 30 amp (10ga) circuit, they would have said so. I might step it up to a 12ga cord but I cannot see the need for a 10ga, not for this little welder. I understand that a 10ga ext cord would lessen resistance from the outlet to the power supply, but if the power cord is currently 14ga, then a 12ga replacement cord would do the same, to a lesser degree, but I doubt this welder (it or me) would notice a difference.
I already have a 100ft 12ga ext cord hanging around but do not want to have to keep dragging it all over, plus it's WAY too long for my needs. Even giving the extra 10ft fudge factor suggested, I think 25ft would be long enough.
If I want more I will step up to a 220v welder and a 40amp circuit. I have some #8 wire standing by just for this purpose.
All the dedicated circuits ARE 12ga wiring with a max run length of 45ft on the one and 30ft on the other. Both have 20amp outlets, and each circuit has one, 2 outlet receptical, on the entire 20 amp circuit. There is one, 2 outlet receptical to a 20 amp breaker.
As for cost, based on what I've seen around here an ext cord is cheaper than making up a power cord. If so, I'll get an ext cord and cut one end off and hardwire it in the unit.