High leg three phase distribution
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  1. #1
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    Default High leg three phase distribution

    Today someone asked me to look at a three phase outlet that I had hooked up. The service is 240 B phase grounded delta, the normal one in this area. The distribution panel has three wires coming in and two pole breakers as are common. I assumed that the dual breakers were connected to the two hot leads and didn't check the voltage. I pulled the breakers out to look at the strapping, which made no sense. Then I read the fine print in the label and found that the box was for a high leg service with the midpoint of one secondary grounded instead of corner grounding. Looking at the straps, it appears that you can get 120, 240 single phase and 240 three phase from the same box simultaneously by the placement of breakers. Is that the idea, to do it all in one box? I got everything working but it was bewildering for a while.

    Bill

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    Yes. That's a common distribution format. it allows for small manufacturing businesses to have a limited amount of 120V loads for lights, office equipment etc., but the MAIN power is intended to be 240V 3 phase.

    The thing that's odd is you saying that it's a B phase corner grounded? usually (and by code), B phase is the one that is the "high leg", meaning it is the point in the delta configuration that is NOT referenced directly to ground, so it measures out as 208V. I can't think of how a high leg delta system can work from a corner grounded configuration. Are you sure about that?


  3. #3
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    That isn't what I intended to say. The 240 three phase in this area is B phase corner grounded. That was the source of my confusion. Around here people frequently use single phase panels on three phase with the breakers in the two hot legs. With dual breakers, I assumed that was the case. When I read the fine print on the label, I realized that it was for high leg service, which I understand. With it connected to our service, you get some funny results, sometimes with a dual breaker, one is to a hot line and the other is putting a breaker in a ground lead. My question was

    "Is that panel designed to allow 120, 240 single phase and 240 three phase all from the same panel when connected to a high leg service?"

    I can make it work, but is it kosher? I am told that the former owner of the building was an electrician who wired it his own way and did not leave any documentation.

    Bill

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    Yes, you are supposed to be able to get all the above from it, although there can be a limit on the unbalance.... they may be done open delta with one transformer taking the single phase loads, although I have also seen 3 phase transformers set up for "lighting tap" 3 phase, aka high leg, "stinger", etc.

    If the panel is labeled for it, then yes it is good for it. But I see no reason or good way to use a 3 phase box for corner grounded.... far more reasonable to have a single phase box and breakers rated-up to use 240V to ground.

    Unless the box is marked as rated for it, it probably is not "kosher" to do. Breakers and possibly boxes for corner grounded should have a 1 & 3 phase dual rating marked on them..

  5. #5
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    Once I knew what the panel was designed for, I was able to pick a location for a dual breaker that would connect to the two hot lines so the owner can get his welding done. Fortunately it is for a Lincoln 350 series welder that isn't sensitive to phase rotation. Once they have the job at hand done, I will suggest that they get the service changed to a proper one. In the present state it shouldn't be dangerous, just weird.

    Bill

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    Quote Originally Posted by 9100 View Post
    ...... In the present state it shouldn't be dangerous, just weird.

    Bill
    There is a lot of stuff like that Things that are technically not "marked" for the usage, but are in no way actually dangerous. In some cases, the only difference between a rated-for-the-purpose part and the non-rated one is the marking.

    But, the inspectors are always looking for the label showing it is "listed for the purpose".. As they should, actually


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