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    Exterior Switchboard Grounding

    Situation:

    Have a project on existing building throwing a curveball. The building service consists of an exterior combination service entrance switchboard/automatic transfer switch located in the middle of the chiller unit yard on a separate concrete pad about 6 feet from the utility transformer and 15 feet from the building. The combination switchboard/transfer switch is 1600A, 277/480V, 3 phase served by the utility company transformer. The incoming utility service feeders land at the OCPD to the automatic transfer switch and the automatic transfer switch feeds a switchboard with 7 separate OCPD's and feeders to the building landing on different panelboards in two separate electrical rooms. The existing "as-builts" show the incoming utility service feeder as 4 sets of (4) 600 MCM and (1) #4/0 AWG ground in 4" PVC conduit. The actual installed conditions appear to be 4 sets of (4) 600 MCM conductors in 4" PVC conduit and (2) bare copper #4/0 AWG grounds in separate 2" conduits. The building has a lightning protection system without a counterpoise. Cannot locate any grounding electrodes at the service entrance switchboard. The building is a tilt-crete construction with structural steel roof and steel interior columns. The 7 separate feeders to different panels inside the building each have a #6 AWG ground as no feeder exceeds 225 amps. Interior step down transformers each have their own ground rod. The building is not more than 10 years old.

    Issues:

    1) Cannot locate separate grounding electrode at the service entrance. It appears to be derived from inside the utility transformer.
    2) Cannot identify a common electrode grounding system with structural steel, water piping, gas piping, or concrete encased electrode.
    3) Cannot identify a bond to the lightning protection system.

    The building owner is wanting a main ground bus installed for a new telecommunications system to meet TIA standards but when trying to find a common ground point inside the building ran into all of the above.

    Questions:

    1) Does NEC Article 250.64 (D) apply to this situation? If yes, would the #6 AWG grounds need to be removed and replaced or is there a better solution?

    2) For the new main ground bus, would a better solution be to derive a new main ground bar (MGB) inside the building to the exterior switchboard/ATS with a separate ground conductor and bond the existing water piping, structural steel, gas piping, lightning protection system, and step down transformers to this new MGB?





    #2
    "The actual installed conditions appear to be 4 sets of (4) 600 MCM conductors in 4" PVC conduit and (2) bare copper #4/0 AWG grounds in separate 2" conduits."
    1. What is the function of the 4/0 in the separate conduit
    2. When you state ground, that is not a code term - what is it?
    3. 250.64, need to know which section 1, 2, or 3?
    Moderator-Washington State
    Ancora Imparo

    Comment


      #3
      1. The function of the 4/0 AWG in separate conduit appears to be grounding electrode conductors from the switchboard ground bus to ground rods but cannot confirm without digging up the yard area.
      2. The #6 grounds of the 7 separate feeders appear to be equipment grounding conductors as they are not sized per Table 250.66.
      3. Article 250.64 (D)(2).

      Comment

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