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Thread: Transformer grounding/bonding

  1. #1
    Join Date
    Jun 2012
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    Transformer grounding/bonding

    Hey All:

    I've got a few questions relating to transformers. I had a nice reference tool with pictures that
    described transformer grounding that I can't seem to find, but a few coworkers and I had some
    disagreement over a mock installation so here it goes. Situation: 480v primary 3 phase no
    neutral to a disconnect, then to a transformer with 120/240v secondary.

    The connection of the ground conductors in the disconnect became one talking point.
    Another was connection to ONLY building steel, not any other electrode.
    Finally, whether that GEC was routed through the secondary panel, and the bonding jumper
    locations, versus being run in a seperate conduit to the electrode.

    Any info would be appreciated, and if anyone knows the reference with pictures to which I'm
    referring, I love to know. Thanks in advance!

  2. #2
    Join Date
    Jul 2005
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    Tennessee
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    Name:  transformer.jpg
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Size:  141.4 KB

    Heres a pic that might help.
    Your equipment grounding conductor accompanying your 480 v supply is the grounding means on the primary side.
    Most often here, E/Cs bond their grounded conductor at the transformer and install a grounding electrode to that point (upper diagram). 250.30(A)(7) will prescribe your electrodes, but building steel is a common one.
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  3. #3
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    If it is pictures you want there are some helpful ones in the Handbook to be found around 250.30 and 250.52.
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  4. #4
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    Quote Originally Posted by augie47 View Post
    Name:  transformer.jpg
Views: 5054
Size:  141.4 KB
    ...
    The picture indicates the system bonding jumper (SBJ) can be made at either the source or first disconnecting means. It should be noted the GEC must connect at the same location where the SBJ is installed, or vice-versa.
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  5. #5
    Join Date
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    Location
    Ohio
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    Quote Originally Posted by Brownie326 View Post
    ... Situation: 480v primary 3 phase no
    neutral to a disconnect, then to a transformer with 120/240v secondary.
    For 3Ø secondary, the system should be referred to as 240/120V, or more completely 240/120V 3Ø 4W.

    The connection of the ground conductors in the disconnect became one talking point.
    For the primary, you should only have EGC "in" and EGC "out".

    For the secondary disconnect, you could have GEC, SBJ, SSBJ, and EGC "out"... or you could have SSBJ "in" and EGC "out". In either case, they all get bonded together. In the case where there is an SBJ and GEC, only these are permitted to be directly connected to a grounded [neutral] conductor or busbar, though many disregard this in panelboards.

    Another was connection to ONLY building steel, not any other electrode.
    If present, only the closest of water pipe or building steel electrodes are required for a separately derived system (SDS). You would only connect to other electrodes if either of the preceding was not present. Bonding to either of the mentioned electrodes connects it to the building grounding electrode system (GES) if there is already one established for the primary or another system.

    Finally, whether that GEC was routed through the secondary panel, and the bonding jumper
    locations, versus being run in a seperate conduit to the electrode.
    There is no requirement for routing of the GEC, but it cannot terminate at both source (transformer) and the system disconnect(s) locations.
    I'll never get there. No matter where I go, I'm always here.

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