Can a 4 prong welder run off a 3 prong outlet?
According to the millermatic 350 this welder is a 240v hot,hot ground connection the red connection is not used. On page 23 of the manual I found online.
This makes sense since it is also a 3 phase 480 machine the voltage to ground would be 277 so any control voltage is made from the source voltage.
L1,L2 black , white and ground green.