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Thread: Replacing Service Equipment on Ungrounded System

  1. #1
    Join Date
    Oct 2015
    Location
    York, PA
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    65

    Replacing Service Equipment on Ungrounded System

    I'm looking at a customer's industrial facility that has an extremely old electrical system. They have their own outdoor substation and receive a primary service from the utility. One of the outdoor transformers that feeds the building is a 13.2kV delta to 480V delta, with the secondary side being ungrounded. The secondary wires run into the building and land on an exposed bus arrangement in the electric room that has a number of exposed fused knife switches fed directly off of it.

    We would like to replace the secondary conductors and install a new switchboard/panelboard to replace the exposed bus arrangement and knife switches. The question is, given what we are replacing, do you believe the NEC requires us to turn the 480V side into a grounded system (assuming the system doesn't meet the requirements of 250.21(A))?
    Last edited by Sunny_92; 05-04-18 at 01:42 PM.

  2. #2
    Join Date
    Feb 2003
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    Seattle, WA
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    Not enough information. Your answer lies in 250.20(B). Its sub article (1) clearly does not apply. But you will have to tell us whether either of the other two sub articles applies. My guess is that they don't, based on the existing configuration not being grounded. So I suspect the NEC will not force you to ground the new configuration.

    That said, is there a good reason not to? Can the facility operate just as well if the service is grounded?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  3. #3
    Join Date
    Oct 2015
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    York, PA
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    Quote Originally Posted by charlie b View Post
    Not enough information. Your answer lies in 250.20(B). Its sub article (1) clearly does not apply. But you will have to tell us whether either of the other two sub articles applies. My guess is that they don't, based on the existing configuration not being grounded. So I suspect the NEC will not force you to ground the new configuration.

    That said, is there a good reason not to? Can the facility operate just as well if the service is grounded?
    Ah, good point! I breezed right over 250.20(B)(1). Your guess is correct; neither sub article (2) or (3) applies, so that really simplifies this situation.

    My guess is that the facility could operate just fine on a corner grounded system, barring any existing ground faults. My only concern about grounding one of the phases is that none of the downstream conductors would comply with the requirements for identifying grounded conductors in 200.6.

    Thanks!

  4. #4
    Join Date
    Oct 2015
    Location
    York, PA
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    Follow up question about the same portion of this system:

    Given that I'm dealing with a delta-delta transformer, the secondary conductors can be considered protected by a properly sized primary fuse (240.21(C)(1)). Assuming they are sized so that this is the case, is a disconnect at the nearest point of entrance still required by 240.21(C)(4) since the transformer is outside?

    I would not consider the current location of the service equipment to be "nearest the point of entrance of the conductors", so I'm trying to determine if code requires a disconnecting means outside the building (it is not feasible to locate a disconnecting means nearest the point of entrance of the conductors).

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