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I've got 3 machines for my shop that are currently wired with a 12-3 4-wire NEMA 14-50 plug. Each machine uses only 15 amps at 240v, so how much power it uses is not a factor here with my question. My 3 wall outlets are NEMA 6-30 wired with 10-2 wire each on their own circuit (of course.) So my question is what do I do with the neutral wire when re-wiring these machines' plugs to 6-30? Just cap it?

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    Do the machines actually use the neutral for anything? Or is it simply vestigal? – ThreePhaseEel May 27 '18 at 0:59
  • I don't know for certain. I remember speaking with the builder of the machines over the phone and he said they "tend to work better" with neutral, and seem to remember him wanting me to tie it into ground, but I've read elsewhere that is a big no no, so that's why I decided to ask this question to the group. – Greg May 27 '18 at 17:47
  • Eh, what sort of machines are we talking about here? – ThreePhaseEel May 27 '18 at 18:02
  • They melt lead and pour the molten lead into moulds – Greg May 28 '18 at 18:04
  • That's strange....do the controls require 120V for some reason, or can they be set up to run the controls at 240V (perhaps using a 240->120 control transformer)? – ThreePhaseEel May 28 '18 at 18:12
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Cordage is the wire used in cords. In cordage, the numbering includes the ground wire (unlike in-wall building cables). So 12/3 cordage is black white ground. If you are running 240V then it is incapable of supporting a neutral wire, since the white must be used for the second hot. Feel free to mark the white wire with red tape. When connecting it to a NEMA 6, hook it up straight. When hooking it to NEMA 14, don't connect anything to the neutral lug.

If your equipment is using 12/3 in-wall building cable (e.g. Romex, NM, UF, etc.) then remove it and go to the local electrical supply and get proper cordage. You can also buy cordage with the plug you want already premolded onto the end. Or obtain an extension cord whose cable has correct ratings for cordage, and lop off the bits you don't need.

As far as I know, it's legal to put a 30A plug on a 15A device, and on this forum we've visited that question a few times and nobody's come up with a code cite saying it's not allowed. Just the same if building it from scratch, I would've gone NEMA 6-20 simply because the parts are significantly less expensive. Given where you are now, you could price it both ways and see:

  • 6-30 plug vs
  • 6-20 plug, 6-20 receptacle and 20A breaker.
  • Ok, so you're saying to cap off the neutral wire from the machine and just connect the rest to the 6-30 receptacle. I should not have included a lot of extra details in my question and just asked whether I should cap it or if there was some way to make use of it that was safe. I'm assuming the machine doesn't actually need neutral to run properly. – Greg May 27 '18 at 4:04

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