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Thread: Please help me understand Transformer and Panel grounding/bonding.

  1. #1
    Join Date
    Jun 2018
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    NC, TN, SC
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    Please help me understand Transformer and Panel grounding/bonding.

    Hi all. I'm so unsure of myself right now. Can't wrap my head around grounding and bonding transformers. I've done panels, GFCI's, AFCI's, hundreds of lights, and helped on two PV installs. Transformers are so confusing to me. I'm not installing one but i am trying to understand the proper way before i actually have to one day. I'm sure ill get to watch someone first but it would be nice not to be clueless when i do see it done. Lets say i'm working with a 3-Phase, 4-Wire 480Y/277 V system and want to install a 75KVA transformer and 200amp panel.


    Primary Side grounding/bonding


    Using a 75KVA 480 primary 120/208 secondary transformer.
    Come off 125A MCB at switch gear. 90amps*1.25= 112.5 OCP device. Is This Correct?
    75000/(480*1.73)=90 90*1.25=112.5 USING 75 degree column i use 2awg. Is this Correct?


    I pull 3 2awg THHN wires, phase them brown, orange, and yellow.
    Since i'm using EMT i do not have to run a wire for the equipment grounding conductor. Under 250.122 it is considered an EGC. Is this correct? Is that all on the primary side i have to do?


    200A 208Y/120 three phase panel with a 200A main. Main in panel will serve as OCP and taps from transformer are under 10 feet in lenght.


    I run 350 kcmil X1-A Phase, X2-B phase, X3-C Phase X0-Neutral Bar of panel
    75000/(208*1.73)=208A 208*1.25=260A size secondary conductors need to be?
    Using 90 degree column and derating for 4 conductors (neutral is considered current carrying in this)
    350kcmil 350A*0.8=280A Is this correct?


    Secondary grounding/bonding (system Bonding Jumper At transformer only)
    Grounding electrode-Connect to X0 to ibeam (250.66) 2awg
    Supply Side Bonding Jumper- using EMT this is my SSBJ. Is this correct? Is using an LB ok or does it all have to be EMT?
    System Bonding Jumper- X0 to case of transformer and to ground bar of panel. (no neutral to ground connection in panel) Is this correct?


    I feel like i'm missing something. Any help would be greatly appreciated.

  2. #2
    Join Date
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    For someone who claims to not know what to do you have an incredible amount of detail in your question.
    If you go and decide to dance with a gorilla the dance ain't over till the gorilla decides it's over.

  3. #3
    Join Date
    Jun 2004
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    Cherry Valley NY, Seattle, WA
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    I didn't have time to thoroughly read through everything, but a few quick comments:

    For the supply side, yes just think of a transformer as any other device. All you need is an egc to the transformer grnd bar.

    For the secondary, you can make your N-G bond at the transformer or the first disco, your choice. GEC connection must be made at same point.

    Typically with three phases and neutral, you would only have 3 CCC so no derating required.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  4. #4
    Join Date
    Jan 2014
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    chicago, il, USA
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    One thing I'd like to add, you mentioned using the EMT for your EGC. I always use flex for the last 3 to 5 feet for my connection to the generator to minimize the 60 hertz hum. If that's the case, you would need an EGC wire for the whip.
    The world is round, you will get there no matter what path you take.

  5. #5
    Join Date
    Jul 2005
    Location
    Tennessee
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    Few comments:
    If I read correctly, you are feeding a 200 amp panel with your secondary. If so, 3/0s will normally suffice. In most cases the major portion of the load is linear, the neutral will not count as a current carrying conductor so no derating is necessary.
    408.36 requires OCP on your panel and if it's a 200 amp panel that OCP needs to be no greater than 200 amp.
    Keep the 240.21(C) secondary rules in mind as to conductor length.

    I think you have an excellent grasp of the subject.;
    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
    Join Date
    Jun 2018
    Location
    NC, TN, SC
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    I truly appreciate everyone who has replied. I am going over everyone's feedback and making notes. This forum is a great site with lots of nice and brilliant people.

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