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Thread: SDS on the 7th floor of a concrete-only building.

  1. #1
    Join Date
    Aug 2016
    Cincinnati, OH

    SDS on the 7th floor of a concrete-only building.

    We've recently had a client seek assistance with installing a large UPS on the 7th floor of a building that is built entirely of concrete. It's an older building and no one can seem to find any building steel what so ever. Also worth noting that this facility is in a large metropolis, 208/120V service, with a relatively high available fault current at the source. Fault current at the UPS location is much lower though.

    The UPS is a separately derived system (SDS), and as such needs a GEC connection. Without building steel nearby that would already be bonded to the grounding electrode system, we're forced to get a bit more creative than we would usually need to be. There is however an existing 4/0 CU from the water pipe in the basement to the 6th floor with a ground bar that is being shared by some other equipment. Bonding to this existing down conductor seems to be our way out.

    But... is there a calculation / formula for ensuring this existing 4/0 CU between the metal water pipe and the 6th floor will serve as an adequate GEC for this UPS? My concern is the length of the conductor, but without building steel there's only so much you can do. All signs seem to suggest we just use this. But it's not exactly in the vicinity of the SDS as is recommend and I'm not sure if this officially qualifies as an effective ground for a SDS. Should we run our own ground to the water pipe 7 floors below? Thoughts?

  2. #2
    Join Date
    Dec 2010
    (a) Common Grounding Electrode Conductor. The common
    grounding electrode conductor shall be permitted to be
    one of the following:
    (1) A conductor of the wire type not smaller than 3/0 AWG
    copper or 250 kcmil aluminum
    (2) The metal frame of the building or structure that complies
    with 250.52(A)(2) or is connected to the grounding
    electrode system by a conductor that shall not be
    smaller than 3/0 AWG copper or 250 kcmil aluminum

    Go to the 6th.
    The more they overthink the plumbing, the easier it is to stop up the drain

  3. #3
    Join Date
    Jul 2003
    New Jersey
    The GEC is not required to be larger than #3/0 so there is no calculation needed if you're connecting to a #4/0 common grounding electrode conductor.


    All responses based on the 2014 NEC unless otherwise noted

  4. #4
    Join Date
    Jul 2006
    If that ground bar is being used for "technical equipment", be prepared for some shouting. I'm not saying that existing users are right, but that doesn't stop anyone.

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