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Thread: Derating Parallel conductors in a raceway

  1. #1
    Join Date
    Apr 2012
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    Gainesville,GA,USA
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    Derating Parallel conductors in a raceway

    Raceway:
    I have a 3” raceway (part pvc and part aluminum) that has (8) 3/0 thhn copper conductors. (2 sets of a,b,c and 2 neutrals).

    I’m looking at table 310.15(B)(2)(a) and wondering if I should adjust by 80 (6 conductors (the 2 sets of a,b,c)) or if I should adjust by 70 (all 8 wires(the 2 sets of a,b,c and the 2 neutrals)).

    Assuming that’s the correct table, I guess it boils down to whether or not the neutrals are considered ‘current-carrying conductors’ – are they?

    Setup:
    GA Power Transformer is feeding a 480/277 3/0, 4-wire, 1200amp main breaker via (3) of the raceways described above.

    What would be the adjusted rating of the main breaker in this scenario?

  2. #2
    Join Date
    Jan 2009
    Location
    Easton, Maryland NEC: 2011
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    7,851
    This section defines when it it is a CCC or not.


    310.15(B)(4) Neutral Conductor.
    (a) A neutral conductor that carries only the unbalanced
    current from other conductors of the same circuit
    shall not be required to be counted when applying the provisions
    of 310.15(B)(2)(a).
    (b) In a 3-wire circuit consisting of two phase conductors
    and the neutral conductor of a 4-wire, 3-phase, wyeconnected
    system, a common conductor carries approximately
    the same current as the line-to-neutral load currents
    of the other conductors and shall be counted when applying
    the provisions of 310.15(B)(2)(a).
    (c) On a 4-wire, 3-phase wye circuit where the major
    portion of the load consists of nonlinear loads, harmonic currents
    are present in the neutral conductor; the neutral conductor
    shall therefore be considered a current-carrying conductor.
    "Electricity is really just organized lightning." George Carlin


    Derék

  3. #3
    Join Date
    Jul 2005
    Location
    Tennessee
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    If I did my math correctly, you are in trouble either way.
    If you are supplying a 1200 main breaker, your total conductor ampacity must be at least 1200 amps.
    Parallel 3/0 (225 x 2 x .8) in 3 conduits comes up to 1080 amps

    In direct answer to your question, the neutral being a ccc depends on 310.15(B)(4) and if a majority of your load is linear or non-linear. We would need to know a little more about the load to determine. (Normally the neutral does not count on services)

    (jumper answered that part as i was typing, sorry)
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  4. #4
    Join Date
    Jun 2006
    Location
    Ohio
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    13,069
    Quote Originally Posted by StanRob View Post
    ...

    What would be the adjusted rating of the main breaker in this scenario?
    Well hopefully the main breaker is adjustable.

    3 sets of 4x3/0 THHN Cu with a 75°C terminal temperature limitation [110.14(C)] would put you at 400A max per conduit x 3 = 1200A. That's before ampacity adjustments. Using full ampacity rating, you have 225A x 6 = 1350A. At the very least you have to derate 80% for ccc's. That puts you at 1080A...
    I'll never get there. No matter where I go, I'm always here.

  5. #5
    Join Date
    Apr 2012
    Location
    Gainesville,GA,USA
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    Thank you all very much for the quick and thorough replies!

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