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Thread: Same phases in conduit

  1. #1
    Join Date
    Aug 2006
    Posts
    1,122

    Same phases in conduit

    Hello All

    I am involved with a new construction project where we are running feeders from a main substation transformer to a main bus in one of the electrical rooms. The feeder run calls for 9 runs of 1000 kcmil per phase, for a total of 27 1000 kcmil feeders run through 9 conduits with 3 feeders in each conduit.

    I was told that you could not have three feeders of the same phase run in a conduit (example: having all nine A phases run through three conduits, with three A phase feeders in each conduit) and that you had to have one of each of the phases running in each of the conduits. This would mean that each of the nine conduits would one feeder from each of the phases in them. Is there a reason why each of the conduits has to have three different phases in them and cannot be all the same phase run in a conduit.

    Thanks for the help

    Mull982

  2. #2
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    19,048
    The impedace of the circuit goes up if you use an isolated phase arrangement. Also if any the raceways or enclosures are of ferrous material, then you will have a huge inductive heating problem that can destory the installation.
    On another note why are they calling for 1000kcmil? I would take a look at bus duct for this installation.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #3
    Join Date
    Jul 2006
    Location
    Ohio
    Posts
    153
    Paragraph 300.20. Running equal sizes of all three phases through a conduit will balance the magnetic fields, and minimize any induced currents generated in the conduit. If you run just one phase in a conduit, the magnetic field will not be cancelled but amplified, and induced currents in ferrous materials will heat the conduit.

    I can say that I witnessed the end result of what happens when you run the phase conductors in an unbalanced way. Our instructions were VERY clear to our customer, and they were ignored. The conduit got so hot it started MELTING!

  4. #4
    Join Date
    May 2004
    Location
    MA
    Posts
    1,472
    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. 2: Isolated phase, polarity, grounded conductor, and equipment grounding and bonding conductor installations shall be permitted in nonmetallic raceways or cables with a nonmetallic covering or nonmagnetic sheath in close proximity where conductors are paralleled as permitted in 310.4, and where the conditions of 300.20(B) are met.
    Charlie

  5. #5
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    19,048
    Charlie,
    Note that the section that you have cited only applies to underground installations.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  6. #6
    Join Date
    May 2004
    Location
    MA
    Posts
    1,472
    Quote Originally Posted by don_resqcapt19
    Charlie,
    Note that the section that you have cited only applies to underground installations.
    Don
    DON, I agree, and I precieved, the op as an underground installation although on second read maybe it's not??
    Charlie

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