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Thread: how to read this 480V single phase transformer wiring diagram

  1. #1
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    how to read this 480V single phase transformer wiring diagram

    The utility provides the site with a 480V single phase service. The Owner has an existing 480V transformer that gets the voltage down to 120V. This xfmr is being relocated, specs on it are: 15kVA, 60Hz, single phase, Primary Volts 432/456/480, Secondary Volts 120/240.

    See attached photo which includes a wiring diagram - not sure what to make of it. If I read it correctly there are two wires coming in, they will connect to H1 and H4 - no neutral wire, only the two phase wires. I don't get what it is asking for on the secondary side - can I can get a 120/240 1p 3w service on this secondary?

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    Thanks!

  2. #2
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    Yes, you could get 120/240. For the 120V setup, X1 and X3 would be connected together, and X2 and X4 would be connected in parallel.

    To get 120/240, connect X2 and X2 together, this would be the neutral point.

  3. #3
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    Quote Originally Posted by malachi constant View Post
    See attached photo which includes a wiring diagram - not sure what to make of it. If I read it correctly there are two wires coming in, they will connect to H1 and H4 - no neutral wire, only the two phase wires. I don't get what it is asking for on the secondary side - can I can get a 120/240 1p 3w service on this secondary?


    Thanks!
    The primary needs only two wires. It's one winding.
    The secondary has two windings. The usual arrangement would be to connect X2 to X3. That would give you a 120-0-120 with three wires and 240V from X1 to X4.

  4. #4
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    Quote Originally Posted by david luchini View Post
    To get 120/240, connect X2 and X2 together, this would be the neutral point.
    I think that might work better with X2 connected to X3 rather than to itself................

  5. #5
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    Quote Originally Posted by Besoeker View Post
    I think that might work better with X2 connected to X3 rather than to itself................
    Personally, I always use the X2 to X2 connection Damn typos

  6. #6
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    Now that the others have you up to speed on the connections, I might remind you of the need to have a GES and propering bonding for the derived secondary system.

  7. #7
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    That was my next question. Had my head in the code book before I looked up to see these responses. (FYI I'm working with the 2011 NEC.)

    This transformer will be relocated to the side of an existing woodframe garage. We will provide a new 60A MCB panel within the garage. This panel will in turn feed a couple existing circuits in the garage and two new outbuildings - each of which is small enough to be fed with a single 120V 20A/1P breaker.

    Article 250 has changed significantly since I last took a class on service grounding, so let me take my best guess as to what needs to happen.

    Start with the outbuildings, they are easy. 250.32 Buildings Supplied by a Branch Circuit. (A) Grounding Electrode. "Exception: A grounding electrode shall not be required where only a single branch circuit supplies the building and the branch circuit includes an equipment grounding conductor." (That's not verbatim but close enough.) So no grounding electrodes required, correct? Just run an equipment grounding conductor to each device and we're good.

    Back to the garage: I assume 250.30 is where to start. Looks like I must follow (A)(1) through (A)(8):
    (1) System bonding jumper. Required, we will put it at the source.
    (2) Supply-side bonding jumper. Required.
    (3) Grounded Conductor: Not required as we have a system bonding jumper at the source [see (A)(1)].
    (4) Grounding Electrode. Required. (1) and (2) are not available, so exception allows connection to any electrode listed in 250.52(A). I assume a single ground rod (as described in 250.52(A)(5)(b)) at the 480V:240V transformer is acceptable.
    (5) Grounding Electrode Conductor. Required. Connect (1) to (4).
    (6) Grounding Electrode Conductor, Multiple Separately Derived Systems. Not required.
    (7) OK.
    (8) No metal, not required.

    I am foregoing the disconnect on the primary side of this transformer, as there is a main disconnect on the new panel on the secondary side. The two outbuildings each will have one single-pole main disconnect switch as the service enters the building.

    Man, this part of the code is confusing. Getting closer to understanding it. I think I have it right on this project. Please let me know if I am off on something. Thanks!

  8. #8
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    Quote Originally Posted by david luchini View Post
    Personally, I always use the X2 to X2 connection Damn typos
    Stuff happens....
    I had an old friend,Tommy. I took him on part time to do drawings. I pretty soon realised that his skills were far beyond that. He became contracts manager.
    He was full of pithy sayings.
    "I wanted to write a short letter but I didn't have time. So here's a long one."
    More subtle than it might seem, that.

    Another:
    "The man who never made a mistake never made anything."
    I still use it.

    Although he was a good many years my senior, about my father's age, Tommy and became good friends, particularly after I'd moved to the same street as him and close at that.
    Regrettably, Tommy passed away quite a few years ago. A car accident. It is thought that he had a heart attack and his car went off the road.
    I miss him.

  9. #9
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    Quote Originally Posted by david luchini View Post
    Yes, you could get 120/240. For the 120V setup, X1 and X3 would be connected together, and X2 and X4 would be connected in parallel.

    To get 120/240, connect X2 and X2 together, this would be the neutral point.
    To be more accruate X2 and X3 connected together and bonded to the ground, thus becoming the 'neutral' conductor.

  10. #10
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    Quote Originally Posted by weressl View Post
    To be more accruate X2 and X3 connected together and bonded to the ground, thus becoming the 'neutral' conductor.
    ....
    I'll never get there. No matter where I go, I'm always here.

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