Ok this is a weird one and maybe I just don't understand how mains work (though I feel I have a good enough knowledge to stay safe and do basic breaker work & wiring).

Buddy moved into a new shop and he called me to wire in a 240 plug into a closed off drop. He's heard the previous tenant was probably running a big air compressor or welder. So I wired in an outlet with the 3 wires that were in there, a black and 2 blue. Here's the resulting voltage:


After reading these values I figured I screwed up, so I removed the plug and put everything back to how I found it. However, there was another 240v plug in the shop that was already there so I decided to test it and it read the same values.

Violet and red make sense, but green seems impossible??

How is this possible? Is this normal? Was the previous tenant hooking up some non-standard machinery?

EDIT: Continuity between the 3 sockets is infinity, there's no short, the plug seems good. The black wire was attached to the vertical slit (where violet and green meet). Black wire had a very low resistance to the conduit so I would say it's "safe" to assume that's the common rail and tied to ground as it should be. The coloring would also suggest this.

Thanks in advance :D

  • 1
    Looks like I've discovered I have a high-leg or wild-leg delta system. I assume it's not safe to use dual-phase 240v equipment on this outlet?
    – Skinner927
    Mar 7, 2017 at 7:34
  • This is not a hobby to be doing work in a commercial space for other people! You could have burned up some expensive equipment because you didn't know what you were doing. Stick to installing fixtures and replacing outlets in your own home. Mar 12, 2017 at 13:27

1 Answer 1


Wrong socket! Wrong socket! Wrong socket!

That obsolete, dangerous NEMA 10 socket is bad news anywhere you see it. You wired this? Never use a NEMA 10 socket ever again. It should not have been sold to you, and make them take it back. It is illegal except for exact replacement of a broken one.

Do these look like your numbers?

enter image description here

That is wild-leg delta. A perfectly plausible thing to see in a commercial shop. You can see where, by grabbing the correct 2-3 wires, it can supply a variety of voltages including 120/240 split-phase (a-n-c). By law, the wild leg (208 from neutral) must be colored or taped orange.

NEMA 10 connectors are illegal** for new work and have been for some time, they are only legal and sold for replacing broken ones already installed. It has been fully replaced by NEMA 14, which is the same thing with a ground, or NEMA 6 if you don't need neutral. Any of them would grab phases A and C in this drawing.

It looks to me like they grabbed the wild leg by mistake. This would happen if they have a 3-phase service panel and they put the 2-pole breaker in the wrong breaker position (because they don't know what they were doing).

Panels and phases

As you probably know, on most split-phase panels (120/240), every other breaker row on the panel is the opposite leg, A B A B A B A B A B. And so if you use a 1-space breaker, you get 1 pole, and if you use a 2-space breaker, you get 2 poles and thus 240V. It doesn't really matter which 2 spaces you use.

A 3-phase panel works the same except it's A B C A B C A B C. With me so far? You can use 1-space, 2-space or 3-space breakers. (well, 1-space makes no sense on regular Delta.) Now if the 3-phase is wired Wye or regular Delta, power is symmetrical, so you can use any position without worrying about it.

However, a wild-leg delta system, watch out! Every third row in the breaker panel is the "wild leg". If you want 120/240 split-phase, you must plug your 2-pole breaker into specific places that avoid the wild-leg row. The problem is, the last guy didn't.

enter image description here

Mind you, the National Electrical Code specifies which particular phase must be wild-leg on the panel. If it's wrong, I'd fix it, but beware - there's a difference between swapping 2 phases and rotating all 3, and the difference is motors will run backwards.

I for one would paint the high-leg rows orange on the panel cover, so it's obvious.

Look out for other shoddy work

We are code mavens here, and when you really embrace it, code is your friend. Code protects you from wrecking stuff and killing people. The Oakland warehouse fire should be top-of-mind when working in converted spaces, so very preventable - and thousands of creative types are being evicted from similar spaces, a tragic loss to arts and culture. You want the fire marshal coming through and seeing really competent electrical.

Since you say you do have good grounds (presumably in the metal conduit), please eradicate those horrible NEMA 10 receptacles. Go NEMA 6 if you need 240V single phase (wild-leg is OK here). Or NEMA 14 if you need 120/240 split phase (cannot be wild-leg).

Generally in this type of wiring, you don't just install random receptacles. You wait until you know exactly what machine will go there, and then provision the power it requires, using the receptacle that's correct for that. (e.g. NEMA 15 if he wants 240 3-phase).

Here are the right colors when dealing with wild-leg. The lower phases can be any color except orange, white, gray, green or green/yellow.

enter image description here

If you see any existing neutrals, grounds or high-leg that aren't the right colors, please use some tape of that color to tag them, for practical clarity. Mind you, for neutrals and grounds 6 AWG or smaller, Code requires you use wire of the correct color, so do that in any new work.

** By context, we are in the native habitat of the NEMA 10 receptacle, i.e. US, Canada and the rest of North America. [we also claim Colombia because Shakira.] Eradicating NEMA 10 is a best practice everywhere it exists. Don't confuse "poorly enforced" with "legal".

  • Actually, the wild leg is the B-phase (except in certain metering environments where it is the C-phase. Funky, eh?) Mar 7, 2017 at 12:38
  • Rather than a mistake could it be that they were trying to balance the loads across phases and knew that the load previously at that location could tolerate connection to the wild leg? Mar 7, 2017 at 15:17
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    @PeterGreen quite possibly.. I was surprised that an industrial load would really want neutral. Perhaps the last guy wanted ground not neutral, and misused NEMA 10 when he wanted NEMA 6. Load-balancing a wild-leg system.is almost a lost cause. Mar 7, 2017 at 15:38
  • Would you probably state the country/region you're referring to when you talk about law?
    – SOFe
    Mar 7, 2017 at 15:51
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    Yes, conduit as grounding path is totally appropriate. But inspect the entire conduit path and make sure everything is well clamped and tight. When you're screwing down the receptacle yoke to the steel box, if it bottoms out HARD on the steel box, with no little paper squares in the way, that's a valid grounding path. Or most boxes have a hole tapped 10-32 for a ground screw - if not, tap your own, must be -32 pitch or finer for enough thread engagement. Using sheet metal screws is disallowed. Mar 7, 2017 at 19:14

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