Here's the rating table from the 2011 NEC book:
So, for my MM211, rated at 24.3A max, and rated at 15% duty cycle at 210A
(from the bottom of page 2 of this PDF spec sheet:
http://www.millerwelds.com/pdf/spec_sheets/DC12-54.pdf )
means that I have to rate the wire gauge for 24.3A * 0.45, or 10.9A.
So, 14ga (15A) is actually technically OK, but I wouldn't go less than 12ga for voltage drop.
For my MM252, rated at 42A max, rated at 30% duty cycle at 300A
(from the bottom of page 2 on this PDF spec sheet:
http://www.millerwelds.com/pdf/spec_sheets/DC12-49.pdf )
means that I have to rate the wire gauge for 42A * 0.55, or 23.1A
However, it says on the back panel that if it pulls down to 200V, it'll pull 48A, so to be safe, 48A * .55 = 26.4A
So, 10ga (30A) is OK for this welder, but I'd prefer 8ga for less voltage drop.
Now, those calculations don't mean there won't be any voltage drop, but they mean that the wire won't melt in the wall or conduit. Having said that, my MM211 didn't noticeably change output between plugged directly into the wall and at the end of a 75' 12ga 3-conductor J-cord. My MM252 doesn't change in any notable way between plugged right into the wall and plugged into the 10ga, 75' 3-conductor J-cord.
Mike