How do you hook up a three phase welder?

Easy as one,two and three. There are four primary wires coming from you service panel.(provided you have three phase service) you can check by looking at the weather head on the roof. if it has three wires you have single phase but if it has four then you have three phase. the hot wires will be black,red and brown. the neutral will be white or green. the welder should have a manual that will give you the lead phase wire(most likely black to black) the other two hot wires can hook to any other hot wire. The neutral goes to the white or green and posts to the panel. If the welder has a switching power supply then you must have a three phase converter.