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It seems really easy to find the opposite (how to plug a 4 prong plug into a 3 hole outlet) but I actually need the reverse (I need to plug a 3 prong plug into a 4 hole outlet) and not sure if it even exists.

I have this welder: https://www.harborfreight.com/stick-225-inverter-welder-with-electrode-holder-64978.html

...which includes a 240, 3 prong plug. It's the kind that looks like your standard plug (two parallel prongs and a ground). It's not the kind dryers use where you have 3 angled prongs.

On the wall-side, I have a 40amp, 4-wire outlet (2 hot, neutral, ground).

Is there a way to use this welder with this outlet? If so, is there an off-the-shelf adapter for this? If not, could I wire up my own using a 4 prong plug wired into a box with a 3 prong outlet. If I can do the latter, what's the proper way to do that?

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  • Are you sure that your plug is a NEMA 6-50, or is it a NEMA 5-50? – ThreePhaseEel Dec 27 '20 at 0:09
  • @ThreePhaseEel It must be a 6-50 as it's for 240, right? – DA01 Dec 27 '20 at 0:54
  • According to the manual, it will use up to 42 A at 240V. If that is correct, a 40 A circuit is not enough, you need to bump up to 50 A. Which means not just a different breaker and receptacle (which you likely need anyway because of 3-prong vs. 4-prong) but also possibly larger wire between the receptacle and the breaker. – manassehkatz-Moving 2 Codidact Dec 27 '20 at 1:07
  • @DA01 -- can you get us the I1eff or I1max off the nameplate? – ThreePhaseEel Dec 27 '20 at 1:10
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    @ThreePhaseEel ahh! Those are the numbers I needed! For 240 it says I1max = 42A / I1eff = 19A – DA01 Dec 27 '20 at 1:26
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The simple solution (unless you drag it to other places with different receptacles) is to put the appropriate 4-prong plug on the cord, with the neutral prong of the plug not connected.

Another option is to add a box next to your box with the 4-prong receptacle and install the correct 3 prong receptacle in it. If you plug in and operate two things at once you may trip the breaker, but it spares you the "extra cord and plug" that your self-made adapter would have.

Edit to add: If you don't actually have something that uses the 4-prong receptacle, you could just replace the receptacle (capping the neutral wire in the box) and (if you are nice) write a note on the wall that there is a neutral available if someone wants to change the receptacle back to 4-prong later.

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    We installed it for the sole purpose of running the welder, so I think your second suggestion makes the most sense...just cap the neutral (and mark it). Thanks! – DA01 Dec 27 '20 at 4:40

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