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Thread: Bonding a Fence

  1. #1

    Post Bonding a Fence

    I have a 3000 KVA Transformer feeding a free stand 3000 amp distribution center. Outdoors......15kv / 380-220 3phase Y

    They have installed a 6 ft chain-link fence, with the rails and post welded together.

    Question ? what size ground wire and how many post needs to be clamped.

  2. #2
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    The NEC is vauge on this IMO. Probably why your asking. Lots of folks would get by with out bonding it.
    Is it likely to become energized?

  3. #3
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    Smile

    I don't see a need for the fence at all. Since it is there, it is fine and I would ignore it unless someone wants you to set up a grounding grid and/or some type of bonding (for a price of course). If this were fed with 35.5 kV or higher, you would be concerned about the step-touch potential that could be present. Under those conditions, I would turn to the NESC and a PE that designs substations for a living and probably works for the local electric utility.
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  4. #4
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    I had the same question this week!

    NEC 450.10 states that "Where grounded, exposed non-current carrying metal parts of transformer installations, including fences guards, and so forth, shall be grounded and bonded under the conditions and in the manner specified for electrical equipment and other exposed metal parts in Parts V,VI, and VII of Article 250".

    So you have to ground it.

    A review of Article 250 is not very clear to me when trying to figure out how to bond the fence. It would seem to be based on the size of the energized conductors in table 250.66 but I'm not positive on this since there seem to be a number of exceptions and it never calls out fencing anywhere in article 250.

    I ended up just running some 4/0 bare copper at the posts and using braided copper to the gates. They make some nice clamps for the fence posts and rails and use split bolts for the mesh. Probably overkill but safe. We were already installing some 4/0 grounding on the structural metal back to the ground grid, so it was easy to keep using 4/0.

    By the way, OSHA calls out for a 7' fence to limit access. I used 6' fence plus barbed wire to meet this.

    Hope this helps.

    TxEngr

  5. #5
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    Smile

    Very interesting. I have seen a multitude of pad mounted transformers with no fencing installed at all. If a chain link fence is placed around a pad mounted transformer in your back yard and it is grounded where it is close to the swimming pool, does that mean that the whole fence now has to be grounded and bonded (that may include the whole neighborhood)?
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  6. #6
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    Have you taken a peek at 110.31, and the table?
    Instructor, Industry Advocate

  7. #7
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    Quote Originally Posted by Pierre C Belarge View Post
    Have you taken a peek at 110.31, and the table?
    Dang it, where is that code book?

    The way I read 110.31 is....A fence is not required if the voltage is 600 or less.....correct?

    steve

  8. #8
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    Quote Originally Posted by jbolen View Post
    I have a 3000 KVA Transformer feeding a free stand 3000 amp distribution center. Outdoors......15kv / 380-220 3phase Y

    They have installed a 6 ft chain-link fence, with the rails and post welded together.

    Question ? what size ground wire and how many post needs to be clamped.
    Down on the farm when I was a kid, we had 15-20kV fences that were designed to sound an alarm when they became bonded.
    Oh forget it; the amperage on those fences was much lower than what you're talking about. Looks like Charlie and Pierre got this one covered, anyway.
    Karl J This is only the beginning of the real story. These adventures here are only the cover and the title page... C.S. Lewis in The Last Battle

  9. #9
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    Quote Originally Posted by jbolen View Post
    I have a 3000 KVA Transformer feeding a free stand 3000 amp distribution center.
    Just curious, why the overcapacity of the transformer, expansion?
    Lou (wannabe economist)

    If you relentlessly pursue perfection, you will eventually catch excellence.

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