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Thread: Parallel feeders

  1. #1
    Join Date
    May 2003
    Location
    Leesburg, VA
    Posts
    6,933

    Parallel feeders

    Just when you think you’ve seen it all. Yesterday I responded to an emergency call for a tripped 4000 amp service bolted pressure switch. By the time I arrived at the job the on site engineers had reset the switch (not my recommendation) this is a service disconnect 208/120 4000 amp with blown fuse protection and ground fault protection. The engineer stated that the ground fault indictor was flagged when he first looked at the switch.

    There are two services in this building the one that tripped and a 4000 amp 480/277 services. It is unusually for PEPCO (the local utility) to provide 208/120 and 480/277.
    As I looked around I discovered that on the 208/120 service there is a 4000 amp 480/277 utility termination cabinet with a utility CT cabinet and a 4000 amp tap box. Connected to the tap box is the primary of a 1250 kva dry type transformer (customer owned). The secondary of this transformer feeds a 4000 250 VAC tap box which feeds the line side of the bolted pressure switch that is equipped with 4000 amp fuses. There are two inspection stickers TPF (temporary pending final) and final sticker. No line side over current protection for the transformer.

    Should a customer request an electrical outage in our area PEPCO will remove the CT’s.
    They lift the CT’s from an energized CT cabinet. Obviously; not in this case, and we will inform PEPCO and recommend signage to note this issue.

    Anyway, we looked for the normal causes for Ground fault trips and could find none, the ground fault relay was set at 700 amps .15 seconds. The switch had not tripped again so, I explained to the customer their options, they wanted to wait for a better time and I left the site. This is a hotel and shutting down at that time was not an option they liked. Big convention in town ECT…………….

    Today we got a second call from this customer; a tech went to the site and told me the following.

    The main line switched was tripped, (the customer had tried to reset it and it tripped a second time, wisely they stopped trying to reset the switch). Our tech looked for obviously problems and found, that a 400 amp fused safety switch that fed 400 amp MDP
    with 4 500 kcmils cu had the following A and B phase in one 2” conduit (EMT) and C and the grounded conductor/neutral in a separate 2” conduit (EMT). The conduits were hot (temperature) and the A and B phase conductors were in some places (were visible) melted into one mass of insulation and A and B phase conductors were shorted.
    Brian John
    Leesburg, VA

  2. #2
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    17,245

    Re: Parallel feeders

    I suppose that’s the reason for 300.5(I): Conductors of the Same Circuit. All conductors of the same circuit and, where used, the grounded conductor and all equipment grounding conductors shall be installed in the same raceway or cable or shall be installed in close proximity in the same trench. Exception No. 1: Conductors in parallel in raceways or cables shall be permitted, but each raceway or cable shall contain all conductors of the same circuit including grounding conductors.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
    Join Date
    Mar 2003
    Location
    Texas
    Posts
    11

    Re: Parallel feeders

    Brian,
    Not an expert on the subject but many years ago when I first started at a municiple owned electric company we had a problem similar to this. We had a 4160/2400 generation station with a sub station bus inside. The lines feeding out became bad from sheer old age so they were replaced by a contractor with 1000kcml copper. Each were placed in its own 3" rmc where they entered the building. It wasn't a few weeks and they started tripping the reclosures and melting. We shut down and replaced the rmc with a larger conduit and pulled new wire all in the same conduit. It lasted for another 15 years until we upgraded the system and done away with the generation.

    Barry

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