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Thread: Delta- Delta transformer

  1. #1
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    Delta- Delta transformer

    I have a 480v pri 240v sec Delta-Delta transformer. I'm questioning how to bond it. The transformer diagram has nothing on Grounding the B phase (x2). But I'm 90% sure that is what I need to do. Before I do this I want to be 100% sure. Any help would be grateful

  2. #2
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    Quote Originally Posted by EL Schulte View Post
    I have a 480v pri 240v sec Delta-Delta transformer. I'm questioning how to bond it. The transformer diagram has nothing on Grounding the B phase (x2). But I'm 90% sure that is what I need to do. Before I do this I want to be 100% sure. Any help would be grateful
    Will the transformer Name plate have a Grounded B phase secondary diagram on it if it is capable of being grounded?

  3. #3
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    Quote Originally Posted by EL Schulte View Post
    Will the transformer Name plate have a Grounded B phase secondary diagram on it if it is capable of being grounded?
    Probably not.

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  4. #4
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    I only have been around one corner grounded system, but as far as what the NEC says you pick one phase and ground it, B phase is customary but I don't think it's required. After that treat the grounded phase just like a neutral and follow 250.30 and 200.
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  5. #5
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    You don't actually HAVE TO ground one phase, you DO have to do that OR you must add ground fault monitoring and protection to the secondary side; one or the other. So the transformer nameplate diagram is not likely to show that, it's an installation decision on your part. It's a lot less expensive however to just use a corner grounded secondary and yes, B phase is the conventionally accepted one to ground, but it's not dictated.
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  6. #6
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    Thank you this helps. One more question. This transformer will be supplying a panel for 3 phase 240v equipment only. I can use 3 pole breakers right.

  7. #7
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    Quote Originally Posted by EL Schulte View Post
    Thank you this helps. One more question. This transformer will be supplying a panel for 3 phase 240v equipment only. I can use 3 pole breakers right.
    As long as the breakers are 240 volt and not 120/240 volt rated, but most 3 pole are not rated 120/240 AFAIK.

    The transformer will not care which leg you ground, as long as there is only one leg grounded, more then one leg grounded is short circuit.

    Make sure panel is rated for grounded phase system as well though. If so it may say "B" must be the grounded phase.

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