I have some plugs in my warehouse that have L15-30 receptacles. Inside the boxes are four wires: Orange, Black, Red, and Green. I would like to swap one to an L21-30 receptacle. It looks like the main difference between the L15 and the L21 is that the L21 has an additional grounding wire. I don't have a neutral wire in the box and the green ground is ran outside of the box. All wiring throughout the warehouse is inside EMT conduit.

Do I need to run a neutral wire or is there another way to accomplish this? Thanks in advance!

  • 3
    What are you trying to plug into this? – ThreePhaseEel May 21 '19 at 22:52

You can't do that. It'll fry your equipment.

What you have is 240V "wild leg delta" (orange phase). This is 240V delta with a neutral point inserted halfway between red and black phases. It is 240V specifically, so it will be 120V from neutral to red and black hots, and 208V from neutral to orange hot. This gives it compatibility with 120/240V split phase (residential) service. The NEMA 15 doesn't use neutral, you are powering a 240V/3 phase machine which doesn't care about neutral.

The NEMA 21 plug you want to install is for 208V "wye" 3-phase, with neutral in the middle and 120V from each phase to neutral.

The difference in phase-phase voltage (240V vs 208V) and the high difference between neutral and wild leg (208V vs 120V) is going to break whatever equipment you try to plug into it.

You might be able to jumper the motor for 240, but you won't be able to jumper the auxiliary equipment for 208. You may be better off adding a 120/240V transformer inside the equipment to power its 120V auxiliaries.

Further, you can never bootleg neutral off another wire such as ground. If the above hadn't been a problem, you would need to fish an actual, white or gray neutral wire through the conduit to be neutral.

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The difference between the L15 and L21 series receptacles is the presence of a neutral wire, not a ground wire -- both types have a ground. The L21 series is intended for 3-phase Wye 120V/208V, whereas the L15 series is intended for three phase (either wye or delta) at up to 250V, where a neutral is not present or needed.

If your current L15 plugs are fed from a three-phase delta source, then without adding a new transformer, there's no way to get the netural wire you need for a L21 plug. However, if you have 120/208V three phase wye power, then you can simply run a new neutral wire back to your panel and switch the plug.

Three Phase Wye Vs Delta

If, at your breaker panel, you have a neutral bus that's 120V away from all of the hot phases (check all three in case you have high-leg delta), then you have three phase wye power, and you can run a neutral wire and install an L21. If not, you probably have three phase delta, and will need a transformer to manufacture a neutral locally, if you really need to do that. If that's the case and you're interested in going that route, then report back here and we can try and help -- we'll also need to know what you're planning on powering from it.

Image source: https://www.belden.com/blog/data-centers/3-phase-power-wye-it-matters

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  • 2
    the orange wire in the box usually indicates that it is a high leg delta system. – sww1235 May 22 '19 at 10:51
  • @sww1235 True, that's likely, but not guaranteed. If it's not high leg delta, then orange is a valid phase color for any leg. That's why I recommended measuring voltage on all three. – Nate S. May 22 '19 at 16:41
  • voltage measurement is always preferable. – sww1235 May 22 '19 at 17:02

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