4 prong (neutral, 120vac ph1, 120vac ph2, and return - 2 phase plug
3 prong (neutral, 120vac, and return) - single phase
3 prong (neutral, 208vac, and return) - 2 phases tied together in a 3 prong plug. These are usually electric ranges, dryers, heavy household items, sometimes arc welders (Lincoln comes to mind)
In other words, it depends.
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I did buy a used Century arc welder (Century has been bought out by Lincoln and has no info on the welder I bought) and the plug has 3 prongs, 1 horizonal, 1 vertical and 1 ground. when I went to Lowes to buy a outlet, they had 2 kinds, 1) that said single phase with a silver screw, a brass screw and a ground screw 2) that said 2 phase with a ground screw and 2 brass screws. I'm assuming that the welder would use the outlet with 2 brass screws ( power to each prong) but I didn't want to wire the wrong outlet with out consulting someone that knew more about welders.
Thanks for all the help Kris
get a voltage tester and stick it into the vertical slits, if its ~220 its double phase, if its ~120 its single...