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how to convert my 3 wire Millermatic to 4 wire

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  • how to convert my 3 wire Millermatic to 4 wire

    Sorry if a relevant topic already exists but I've spent quite a bit of time searching and can't find it.

    Just bought a 1989 Millermatic 200. It has a 3-wire 240v plug. I have no 240v outlet in my home. By code and for safety reasons, if I install a new 240v outlet, it should be 4-wire. How can I convert my welder to a 4 wire plug? Common sense says I need to replace the 3-wire cord and plug with a 4-wire so the real question is how do I connect the white neutral '4th wire' to the welder?

    I've never welded before and I'm anxious to start but at my age, I'm very safety conscious.


  • #2
    I am by no means an electrician,but none of my 240 volt machines have 4 wires.
    2- XMT's 350 cc/cv
    1- Blue star 185
    1- BOBCAT 250
    1- TRAILBLAZER 302
    1- DYNASTY 200 DX
    1- DYNASTY 280 DX
    1- MAXSTAR 150 STL
    1- HF-251 BOX
    1- S-74D
    1- S-75DXA
    2- 12-RC SUITCASES
    1- 8-VS SUITCASE
    2- 30 A SPOOLGUNS


    • #3
      what he said.
      you can wire it with 4 wires, the 4th wire in the plug is a ground hooked the to box or frame of the outlet, only 3 will go from the plug to the welder. they are just not set up to use an extra ground.
      you are thinking
      red, black as hot leads
      white as a neutral, and green or bare as a ground.
      you can do that and the green or ground will end at the plug box. the welder dose not get the extra ground wire. the plug will only have 3 prongs.
      hope that helps a bit.
      thanks for the help
      hope i helped
      feel free to shoot me an e-mail direct i have time to chat.
      summer is here, plant a tree. if you don't have space or time to plant one sponsor some one else to plant one for you. a tree is an investment in our planet, help it out.


      • #4
        You need three wires: two phase legs (the hot legs) and one ground

        The fourth wire is the groundED (neutral) current carrying conductor, which this machine does not need. It is used only in applications where both 120 and 240V are used by the appliance, and there is no internal transformer to provide the 120V (some electric clothes driers, for example)

        The outlet and plug you need is NEMA 6-50 (50A circuit), based on the manual. The wire to the outlet and the circuit protection (breaker or fuse) must be appropriately sized. For a 50A circuit, the minimum size is 8Ga (for most wiring methods). Larger wire is fine, and is desirable for a long run. The machine is rated at 40A input at 240V.

        The cord from the plug to the machine can be smaller, based on the rated duty cycle of the machine, but the manual recommends the same 8Ga to achieve rated performance.

        The ONLY reason to install a four-wire outlet is if it may be used later for an appliance that requires it. Even then, there is no need to change the cord on the machine... just don't use the un-needed stab (prong) on the plug (the groundED line).

        When wiring the plug, check the connections in the machine. Most likely, the cord has a black, a white, and a green. The green should go to the ground point in the machine and to the ground stab on the plug (the round prong that goes in the U shaped hole). The black and the white go to the two current carrying stabs. It is acceptable to use the white wire as a non-grounded current carrying line if it is part of a manufactured cable. For the supply, the white should be relabled with tape or paint to black or red at both ends to show that it isn't grounded. This is not required for the cord, but it is a good idea.


        • #5

          Got it; thanks. The part I wasn't getting was the purpose of the 4th neutral wire. So, I now have a 50a breaker, a 50a/240v 3-prong outlet and 6ga cable and will get started.


          • #6
            About the only place you find 4 wire besides feeding panels in residential is cloths dryers and some ranges.