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Thread: Transformer bonding question

  1. #11
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    Quote Originally Posted by electrofelon View Post
    I certainly agree it is not needed, but as an academic question hat happens if one were to connect the center tap to the supply neutral? I am well familiar with the case of the Delta-Wye transformer being reverse fed and supplied with a neutral, but this is different.
    Well, to start with, to do it at all, you would have to make certain that the source neutral is derived from the center tap of that transformer, and not from a wye, and then that you connect the source high leg to the new transformer's high leg.

    Even given that has been done, I would look at it this way: forget the high leg, and suppose you're feeding a 240v center-tapped (or a 120/240 wired as 240v) transformer from a 240v center-tapped transformer. Would you interconnect the taps?

    I wouldn't. I'd be concerned about less-than-perfectly-matching impedances and load currents creating currents within the transformers. In the same way that I wouldn't suggest using multiple transformers in parallel to supply a large load.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  2. #12
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    Hevi-duty Emerson. Catalog# E5H75S. I'd post a picture but I'm not sure how to do it from my phone

  3. #13
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    Name:  IMG_20181025_121022_671.jpg
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  4. #14
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    Quote Originally Posted by 73hemicuda View Post
    Hevi-duty Emerson. Catalog# E5H75S. I'd post a picture but I'm not sure how to do it from my phone
    Okay. Here is the specs.

    https://www.emerson.com/documents/au...-us-175176.pdf
    "Electricity is really just organized lightning." George Carlin


    Derek

  5. #15
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    Quote Originally Posted by 73hemicuda View Post
    It is a step up. Sorry I forgot to include that
    Actually it's a step down, wired in reverse. According to the catalog number

    X6 need not be grounded The high side output coil should be bonded though. or left ungrounded, needs ground fault monitoring.

    Not shown in the catalog data is, if it's approved for reverse connection.

    E85H75S would have been the better model for step up use with its 480/277 Wye output.
    Would have allowed for center point bonding of the output coil.
    Last edited by MTW; 10-26-18 at 02:26 PM.

  6. #16
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    Is there a reason you're using red instead of orange for your high leg?

  7. #17
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    Quote Originally Posted by Cow View Post
    Is there a reason you're using red instead of orange for your high leg?
    If you are referring to a "high leg" of a center-tapped Delta, there isn't one, is there?

    But it comes from one? I see the question now.

  8. #18
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    Quote Originally Posted by electrofelon View Post
    I certainly agree it is not needed, but as an academic question hat happens if one were to connect the center tap to the supply neutral? I am well familiar with the case of the Delta-Wye transformer being reverse fed and supplied with a neutral, but this is different.
    When you connect a center tapped transformer primary to a center tapped source secondary, you are in effect connecting a 1:1 autotransformer between L1-N and N-L2.
    If for any reason (mismatched taps, load related voltage drop....) the two source voltages do not match, you will cause a high current in your primary.

    Sent from my XT1585 using Tapatalk

  9. #19
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    Quote Originally Posted by Cow View Post
    Is there a reason you're using red instead of orange for your high leg?
    I changed it after the picture was taken. The wires were already marked black, red, blue from previous cutoffs from a different wire pull earlier in the day.

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