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Thread: 400a service/feeder questions

  1. #1
    Join Date
    May 2005
    Location
    Richmond, Virginia
    Posts
    22,989

    400a service/feeder questions

    Hey, all. I'm back.

    I'm looking at a 400a 3ph existing service to feed a new panel in the ground floor of a gutted building. There are four 750mcm conductors in a 4" EMT that run from an outside disconnect, through the wall, and feed a 3ph panel. The customer wants the panel relocated several feet farther away, and to have the new ceiling hug the existing ceiling closer than the 4" EMT would allow.

    I'm suggesting using MC cable above the new ceiling to feed the panel, transitioning from the EMT at the wall penetration by stripping the MC and pulling the conductors through the outside existing EMT. My understanding is there are two possible objections: the conductors must be individually marked, and an approved transition device must be used where the MC sheath meets the EMT.

    The other question is how small can I go with the conductors and still comply with the requirements that they be protected from over-current.
    '
    1. I'm looking at a Bridgeport 290-XS to join the MC to the EMT. Am I okay here as long as the conductors are labeled?

    2. Can I use 600mcm aluminum at its 385-amp rating in the 90-degree column (presuming the right insulation type) in Table 310-16, on 400-amp fuses, or must I use 750mcm aluminum at the same 385 amps as in the 75-degree column? Or must the conductors be sized at the full 400-amp rating?
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  2. #2
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
    Posts
    4,401
    Quote Originally Posted by LarryFine View Post
    Hey, all. I'm back.

    I'm looking at a 400a 3ph existing service to feed a new panel in the ground floor of a gutted building. There are four 750mcm conductors in a 4" EMT that run from an outside disconnect, through the wall, and feed a 3ph panel. The customer wants the panel relocated several feet farther away, and to have the new ceiling hug the existing ceiling closer than the 4" EMT would allow.

    I'm suggesting using MC cable above the new ceiling to feed the panel, transitioning from the EMT at the wall penetration by stripping the MC and pulling the conductors through the outside existing EMT. My understanding is there are two possible objections: the conductors must be individually marked, and an approved transition device must be used where the MC sheath meets the EMT.

    The other question is how small can I go with the conductors and still comply with the requirements that they be protected from over-current.
    '
    1. I'm looking at a Bridgeport 290-XS to join the MC to the EMT. Am I okay here as long as the conductors are labeled?

    2. Can I use 600mcm aluminum at its 385-amp rating in the 90-degree column (presuming the right insulation type) in Table 310-16, on 400-amp fuses, or must I use 750mcm aluminum at the same 385 amps as in the 75-degree column? Or must the conductors be sized at the full 400-amp rating?
    Larry,

    All of the large "feeder type" MC cable I have used has had "normal" marked inner conductors, sometimes THHN sometimes XHHW, so I dont think that is an issue. IT looks like that fitting is technically for greenfield. It would use the Arlington 84XX series of connectors mated to RGS coupling then EMT connector.

    http://www.aimedia.co/media/spec-sheets/8412-8417.pdf

    I think 700 AL is the smallest you can go and still next size up to 400 (375 is not a standard size). You could use 600 only if you spliced with a 90 degree rated connector to a piece rated 351 or higher at 75 degrees.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  3. #3
    Join Date
    Feb 2003
    Location
    Pennsylvania
    Posts
    2,398
    if the ceiling space is that tight why not switch to a copper feed and reduce the size,

    or would that not give enough advantage

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