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Thread: Single Phase Transformer Secondary Grounding

  1. #1
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    Single Phase Transformer Secondary Grounding

    Hi,

    I have a singles phase 1.5Kva transformer, 480VAC 1ph primary to 120VAC 1ph secondary. H1 and H4 are my primary lines, H2 to H3 connected together. X1 to X3, X2 to X4 and secondary lines to X1, X4 for 1ph 120VAC. My question is, does any of the secondary need to be connected to ground? I know have seen some secondary 120VAC, 1ph be 180 - 190VAC.

    Thanks,
    Kilowatt

  2. #2
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    Yes you need to install a system bonding jumper and a GEC.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  3. #3
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    Quote Originally Posted by infinity View Post
    Yes you need to install a system bonding jumper and a GEC.
    So, I need to connect a ground wire to X2 and X4?

  4. #4
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    Quote Originally Posted by Kilowatt76 View Post
    So, I need to connect a ground wire to X2 and X4?
    You need to bond X2/X4 to the EGC and run a GEC to the nearest qualifying GE connected to X2/X4. often there is a stud on the transformer case for this.
    Bob

  5. #5
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    It's a separately derived system that falls under 250.21. You can elect either secondary conductor to be the grounded conductor and, as infinity states, bond that conductor and connect it to a grounding electrode as specified in 250.30
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  6. #6
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    Got it, thank you guys!

  7. #7
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    Quote Originally Posted by petersonra View Post
    You need to bond X2/X4 to the EGC and run a GEC to the nearest qualifying GE connected to X2/X4. often there is a stud on the transformer case for this.
    Unless there are specific instructions that state to do so on x2/x4, NEC is fine with grounding either secondary conductor.

    If one were using 120/240 volts on this secondary then NEC requires grounding the "neutral" which is going to be X2/X3 connection.
    I live for today, I'm just a day behind.

  8. #8
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    Quote Originally Posted by kwired View Post
    Unless there are specific instructions that state to do so on x2/x4, NEC is fine with grounding either secondary conductor.

    If one were using 120/240 volts on this secondary then NEC requires grounding the "neutral" which is going to be X2/X3 connection.
    The OP indicated X2 and X4 were connected together. that would make sense for a 120V secondary, as opposed to a 240/120 V secondary.
    Bob

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    Quote Originally Posted by petersonra View Post
    The OP indicated X2 and X4 were connected together. that would make sense for a 120V secondary, as opposed to a 240/120 V secondary.
    Yes, NEC doesn't care if you ground X1/X3, or X2/X4. Neither one when used in that configuration is "neutral".
    I live for today, I'm just a day behind.

  10. #10
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    It's funny how often it seems "scary" to ground a conductor that doesn't enter the building already so grounded.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

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