Louis, You forgot to add and the power to run it. . 100 amps for most transformers.
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So what? I just checked the Lincoln and Miller web sites for the owner's manuals. The Idealarc and the Dialarc both draw a max of 73 amps...not a huge difference. Lincoln recommends wiring with 8 gauge conductor and Miller recommends wiring with 6 gauge conductor. Breakers are cheap, it is the wire that makes a 240 volt circuit expensive. In either case, 8 gauge or 6 gauge will not break the bank. Most dryer circuits are wired with 10 gauge, and most range circuits are wired with 8 gauge. Either would get him by, he just couldn't crank it up to 250 amps, which he probably doesn't need to do for his intended purpose.
Everyone seems to think you have to use 1 gauge or 2 gauge wire for a 100 amp circuit, but that isn't the case for a dedicated welder circuit. The electric code allows for overrating the breaker size relative to the conductor size depending on the duty cycle of the welder. Plus, if you don't max out the welder, its probably not an issue anyway. I ran my Syncrowave on a 50 amp breaker, up to ~200 amps for a long time with no issues.