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Thread: three phase transformer, single phase load

  1. #1
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    Jun 2005
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    three phase transformer, single phase load

    I recently did the electrical design for a small municipal wellhouse. The service entrance is 277/480vac three phase, feeding a motor control center. A branch circuit from the MCC feeds a transformer, which then feeds a panelboard for general loads like HVAC, lights, receptacles and so forth.

    The transformer is 480V:120/208V three phase. The problem is that I called out a 120/240V single phase panelboard (I don't know how I let that one slip by :mad: .) I wasn't lucky enough to have the contractor or supply house catch my mistake, so all the equipment was delivered and now the electrician is on site wondering what to do.

    So, my question is: can we safely go ahead and use the three phase transformer and single phase panelboard? We'd end up with 120/208V in the panel instead of 120/240V, and one leg of the transformer would not connected to anything.

    The panelboard is pretty lightly loaded, and the two pole devices connected to it (a couple small electric unit heaters, an a/c unit and a small electric water heater) are all rated for use at either 208V or 240V so they should be okay, but my main concern is the transformer - will there be circulating currents generated in the windings of that unused leg that will cause problems with heating, harmonics or ?

    I think the correct answer is to get the right transformer, but it would be nice if I didn't have to spend any more money.

    Thanks.

  2. #2
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    Re: three phase transformer, single phase load

    the real answer is to get the right panelboard. they are cheap.

    having said that, having a phase on the xfmr left unconnected is no different than if you just had no load on that phase. say you had 3 loads, one on each leg. and typically only one was ever on at a time (maybe a heater on one leg and an A/C on another. you would be no worse off with the a/c off and the heater on, than if the phase with the heater on it was disconencted.

    I am not so sure this is code compliant since the marking on the PB probably says single phase 3 wire 240-120, and thus this install would violate the listing.
    Bob

  3. #3
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    Re: three phase transformer, single phase load

    The marking on the panelboard would always list the maximum permissible voltage like 120/240V or 240V.
    Just because you can, doesn't mean you should.

  4. #4
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    Re: three phase transformer, single phase load

    This particular transformer is a Cutler-Hammer. I spoke to the C-H tech support guy; he told me the transformer wouldn't work in this case because they use a common core for the windings. Because of the common core, circulating currents will be induced in the unused windings, which will overheat the transformer.

    Replacing the panelboard is also an option, but since the electrician has already made most of the field connections from the panelboard to the lights, outlets, etc. I figured there would be less labor required to replace the transformer. I talked to the electrician today, and he's going to look at both options and let me know which one makes more sense from a cost standpoint.

  5. #5
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    Re: three phase transformer, single phase load

    Originally posted by stuart in mn:
    This particular transformer is a Cutler-Hammer. I spoke to the C-H tech support guy; he told me the transformer wouldn't work in this case because they use a common core for the windings. Because of the common core, circulating currents will be induced in the unused windings, which will overheat the transformer.
    Would you not have the same problem if you had no load on one leg of the transformer? So how would this transformer ever really have been suitable unless you could guarantee the current would be evenly split between the three legs?
    Bob

  6. #6
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    Re: three phase transformer, single phase load

    Good question, and I wish I knew the answer. He said all of their general purpose transformers are built this way, so it does makes me wonder - the load is never going to be perfectly distributed on a three phase transformer. My gut feeling is that it would work fine as is, but since he's the transformer expert and I'm not I'd rather get things right now instead of having to come back and fix it later if it doesn't.

  7. #7
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    Re: three phase transformer, single phase load

    I would call the C-H tech one more time and explain it again to him. Maybe ask for a different tech. I can't see why you can't do it the way you what. Explain to him what would happen if you put a 3 phase panel in it and only used 1 or 2 legs on it. Let us know what you find out.
    Jim

  8. #8
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    Re: three phase transformer, single phase load

    This is a problem I figured out some time ago. We live in a world that is so regulated that no one can make a decision without permission from some higher authority. The tech's at CH are not going to tell the customer what he wants to hear because that't not the purpose for which the transformer was designed and tested. If he makes an independent decision then his butt's on the line. If he says it will work, then he would never put it in writing. If you use a piece of equipment in a manner other that what it was designed and tested for, then the responsibility falls on you. Unless the instuctions on the transformer specifically say for single or three phase application don't use it. If it weren't for the liability issue the electrician would all-ready have hooked it up. In this case I agree with Bob, small three phase panels are not expensive. Good call Bob.
    90% of doctors don't graduate in the top 10% of the class.

  9. #9
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    Re: three phase transformer, single phase load

    May be the C-H tech misunderstood the question. His answer is valid if you were going to bring only single phase (2 wires) into the delta primary and then try to feed a 2 hot+N (3 wire) load off of the wye secondary.

    Although he is correct that a transformer in this arrangement will not be able to carry the "nameplate" load without over heating.

    His answer ignores the standard physics of transformers for a balanced three phase input with a "open" winding output. As long as the secondary load is not more than 57.% of the three phase rating, there should be no problem.
    Just because you can, doesn't mean you should.

  10. #10
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    Re: three phase transformer, single phase load

    What about NEC Article 110.3 (B).
    90% of doctors don't graduate in the top 10% of the class.

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