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At work and our regular electrician is off. It could wait if necessary but I'd like to see if it's a simple fix.

When our building was constructed 50 years ago, apparently it had a 460V 3-phase pool pump. About 30 years back it was converted to 240V single phase; a 10 KVA step-down transformer was installed feeding a standard GE panelboard (separately derived service).

I'm in need of 120 volts (less than 2 amps total draw) for control power and I tapped in to this panelboard. When I tested it at close of business yesterday no bueno. Came back this morning to troubleshoot and found that the leg I tied into has only 76V to ground but the other leg has 166V.

I'm wanting to ask if I can balance these voltages by tying the X2-X3 connection on the transformer back to ground. If it's more complicated than that I'll wait for assistance.

Transformer nameplate:

Transformer nameplate

Panelboard:

Panelboard

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    It might be a simple fix, but you are not allowed to do it. You will held responsible if the building burns down. Either wait for your licence electrician comes back, or get another one
    – crip659
    Oct 7, 2022 at 13:17
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    I’m voting to close this question because it's a business concern and not a home improvement one.
    – JACK
    Oct 7, 2022 at 13:34

1 Answer 1

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As has already been pointed out in the comments, this is not something you yourself should be doing at a business.

However, in the interest of sharing knowledge, the short answer is you can't get 120V out of this panel.

Notice that the incoming feed has two hots going to the panel bus bars, a ground, and no neutral. If you want 120V, you need to run a new neutral wire from wherever the supply is.

Fun fact: if you search on this site for questions about the two hots being unbalanced, you'll find a lot of hits talking about a "lost neutral" - usually in the context of the neutral connection breaking somewhere between a homeowner's main panel and the pole transformer. You're seeing a similar effect here (likely due to the load you're trying to run, and probably damaging, on that pseudo-120V circuit). The only difference is, this panel was inteded for pure 240V with no neutral from day 1.

What you are proposing (and it looks like you've already done) is repurposing the ground as neutral which is very illegal and very unsafe. You just took your "safety shield" and turned it into a live, current-carrying part of your electrical system.

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  • Yes, that's the objective, to create a neutral in a separately derived service. [Edit To Add: This panelboard is the only load on that directly connected transformer.] But there's no rush; it can wait until our regular electrician returns.
    – ehbowen
    Oct 7, 2022 at 15:19
  • @ehbowen The only safe way to get 120 volts out of this panel is if the transformer has a center tap on the secondary to provide a neutral. Since you don’t need much current, the easiest way to get 120 volts is with a step-down transformer from the 240.
    – DoxyLover
    Oct 7, 2022 at 16:32
  • Yes, a 150VA control transformer is a workable option.
    – ehbowen
    Oct 7, 2022 at 17:14
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    Just a comment to close this out: Our electrician returned, and he was able to go into the transformer and connect a neutral leg to the center tap. Panel voltages are now in balance and the controller is working properly. Thanks for the inputs.
    – ehbowen
    Oct 15, 2022 at 5:33

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