I have a panel in a building I acquired and Im having a hard time figuring out how to pull 240.

Its a 225A 3 phase panel with only 2 hots. The center pole is a ground. The voltage from line to ground is 250v. A guy I know called it a 3 phase grounded panel. ??

I have some lights that run 208-277 single phase. Any ideas how to get that power off my panel?

Respectively, Jake

  • 2
    A picture would be very helpful
    – Ack
    Apr 3, 2020 at 22:58
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  • 1
    I suspect what you have here is what is called a "3 phase corner grounded" setup. It's a 3 wire connection to a delta transformer output with one corner of the delta grounded. So you can choose any two lines here and get the same output voltage with respect to ground. You said you want 240V and measured 250V but didn't say how you measured that. I suspect you have 240V here.
    – jwh20
    Apr 3, 2020 at 23:29
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    The center pole isn't a ground. The center pole might be at ground voltage either intentionally or due to a fault. You're kinda speed-running through the description here, either because you don't know or you're like "why bother with details, they'll know what I mean". We don't :) So is there a neutral? What are the voltages phase A-B, B-C, C-A, A-ground, B-ground, C-ground? Break 'em all down even if you think it's obvious. Apr 3, 2020 at 23:59
  • 1
    Can you post photos of the panel please? Apr 4, 2020 at 1:04

1 Answer 1


You have a corner grounded delta three-phase setup

While there's only one way to wire a split-phase service drop correctly, with the neutral point smack in the middle, with a three phase service, there's no clear place to put the bonding point that designates where neutral is, especially if the power company's transformers are wired in delta instead of the more modern/common wye configuration, considering that unless you are dealing with a continuous industrial process where uncontrolled breaker tripping could damage more than it saves, the NEC would really rather you not let your shop's electrical system float off to an obscene voltage relative to the earth, as that can cause insulation damage, shorts, sparks, smoke, and fires.

One system, called high leg delta, put the neutral point on a center tap of one of the windings. This had the advantage of providing 120V for lights and outlets; however, it came at a price, namely that the phase opposite the center tap shot up to 208V with regards to the neutral wire, rendering it useless for anything other than three-phase power. High leg systems also require a three-phase, four-wire panel, yet waste a significant number of spaces in that panel due to the B-phase being too much for most light-duty circuit breakers to handle.

Whoever had the utility run three-phase to your shop went the other way, though, and hooked the ground bond up to one of the incoming phases from the utility (typically the B-phase), yielding what you have, namely a corner grounded delta service. This provides a single voltage (240V in your case) with two hot phases, a grounded phase, and a ground (EGC). However, while you might think that single phase panels work with this, the matching breakers often are no good for you (your choices are QO...or heavier duty stuff rated for 277/480V systems), as corner-grounded delta is quite hard on breakers, requiring them to be specially rated for use in that system.

For 240V single phase loads, you can use this, but be careful!

If you have single phase loads that accept 240VAC (such as universal or 240V ballasts, or universal switching supplies), you can wire them line-to-grounded-leg (or perhaps even line-to-line) from your panel; however, there are a couple of caveats to this. First off, since you have a true three-phase panel on this system, you'll be using individual two-pole breakers for each circuit. Second, and more importantly, some loads are only rated for 150V-to-ground or less, and your system supplies 240V-to-ground, so you'll need to pay careful attention to the specifications of things; in general, anything rated for 277V maximum is safe, though.

For plug-in loads, furthermore, you'll need to use the correct receptacle for the task, too, most likely a NEMA 6 although one pole of the receptacle may wind up on the grounded phase. Finally, you won't be able to find GFCIs for this system, since GFCI receptacles are 120V-only contrivances, and GFCI breakers are not rated for 240V-to-ground service either.

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