Page 1 of 2 12 LastLast
Results 1 to 10 of 12

Thread: Parallel conductors count for derating factor?

  1. #1

    Parallel conductors count for derating factor?

    I helped an electrician at work the other day, do some calculations for a power run for our new press.

    He was figuring it off of 125% of listed FLA.
    I showed him section of 670

    670.4 Supply Conductors and Over current Protection.
    (A) Size. The size of the supply conductor shall be such as to have an ampacity not less than 125 percent of the full-load current rating of all resistance heating loads plus 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all other connected motors and apparatus based on their duty cycle that may be in operation at the same time.

    (B) Over current Protection. A machine shall be considered as an individual unit and therefore shall be provided with a disconnecting means. The disconnecting means shall be permitted to be supplied by branch circuits protected by either fuses or circuit breakers. The disconnecting means shall not be required to incorporate over current protection. Where furnished as part of the machine, over current protection shall consist of a single circuit breaker or set of fuses, the machine shall bear the marking required in 670.3, and the supply conductors shall be considered either as feeders or taps as covered by 240.21.
    The rating or setting of the over current protective device for the circuit supplying the machine shall not be greater than the sum of the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device provided with the machine, plus 125 percent of the full-load current rating of all resistance heating loads, plus the sum of the full-load currents of all other motors and apparatus that could be in operation at the same time.

    Well the largest motor was 120A, second and third motor both 93.1 A each. The heaters are 17,500w; 21,600w; 22,100w, and 1,200w. So I came up with calculations as fallows.

    Largest motor 102A*125%= 150.0A
    Second motor + 93.1 A
    Third motor + 93.1 A
    Total motor load = 336.2 A


    Heaters 17,500W
    + 21,600W
    + 22,100W
    + 1,200W
    Total heater W = 62,400 W
    Heater Voltage is 480V 3phase so = 480V*sqarroot of 3
    So total heater Amps = 75A
    Then 125% 0f 75A = 93.82A
    Total = motors +heaters = 336.2A + 93.8A = 430A
    The name plate says FLA of 442A so all good
    Now we would rather not run over 500kcm so we parallel them. This gives us 4/0 wire.
    4/0 THHN = 260A 260A+260A = 520A


    Number of Current-Carrying Conductors Percent of Values in Tables 310.16 through
    310.19 As Adjusted for Ambient
    Temperature if Necessary
    4–6 = 80%


    Now the question is do we count the two sets of wires as current carrying conductors when they are paroled? I can not find this answer any where.
    520A * 80% = 416A Not enough.

    Also what about This?

    310.4 Conductors in Parallel.
    Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and larger, comprising each phase, neutral, or grounded circuit conductor, shall be permitted to be connected in parallel (electrically joined at both ends to form a single conductor).
    Exception No. 1: As permitted in 620.12(A)(1) .
    Exception No. 2: Conductors in sizes smaller than 1/0 AWG shall be permitted to be run in parallel to supply control power to indicating instruments, contactors, relays, solenoids, and similar control devices provided
    (a) They are contained within the same raceway or cable,
    (b) The ampacity of each individual conductor is sufficient to carry the entire load current shared by the parallel conductors, and
    (c) The over current protection is such that the ampacity of each individual conductor will not be exceeded if one or more of the parallel conductors become inadvertently disconnected

    Does this mean that the over current protection would have to be 260A on each wire?
    We have never done it this way. Have we been wrong? :o

    Any help would be appreciated
    Sorry about the length
    Dan
    Dan Thomas
    Industrial Electrician, Indiana

  2. #2
    Join Date
    Feb 2003
    Location
    Seattle, WA
    Posts
    17,135

    Re: Parallel conductors count for derating factor?

    Originally posted by electricaldoc: Now the question is do we count the two sets of wires as current carrying conductors when they are paralleled? I can not find this answer anywhere. 520A * 80% = 416A Not enough.
    I didn't check any of your math or your code references. I'll just respond to this question. If you have two sets of parallel conductors in the same raceway (e.g., 2 phase A, 2 phase B, 2 phase C, and let's not count the neutrals), then you must count all six conductors as being "current carrying." So if your math is right, then you cannot use the conductors you had selected.

    Is it possible to run parallel conduits? If you do, then you don't have to derate for more than three current-carrying conductors in the same raceway.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
    Join Date
    Feb 2003
    Location
    Cleveland, Ohio
    Posts
    1,321

    Re: Parallel conductors count for derating factor?

    Dan,

    Looks like you will need to run 6 250kcmil if your conductors are in one conduit. If you run two conduits, 4/0 will do.
    Dave Nix

  4. #4
    Join Date
    Aug 2004
    Posts
    11,542

    Re: Parallel conductors count for derating factor?

    by the way, there is nothing "wrong" with sizing the thing at 125% of FLA. You will just get larger wires than the code actually requires.

    You might want to look at the way the equipment is arranged and see if some creative thinking might reduce your wire sizes some. It might be possible to put the thing on two or more seperate circuits.
    Bob

  5. #5
    Join Date
    Mar 2005
    Location
    North Texas, DFW area
    Posts
    381

    Re: Parallel conductors count for derating factor?

    Originally posted by electricaldoc:
    Does this mean that the over current protection would have to be 260A on each wire?
    We have never done it this way. Have we been wrong?
    It's a good thing you've never done it that way, cause it would be wrong. Paralleled conductors would need to be fed from the parallel lugs of one single circuit breaker (or from one set of fuses).

    Just run your parallel sets of conductors in separate conduits and you'll be fine. Also make sure that you have one set of each phase (A,B & C) and the neutral (if any) in each conduit.

    And your equipment grounding conductor (if you install one -- metal conduit would meet code if done correctly) must be run full size per 310.4. For instance if the MOCP called for you to feed this machine with a 450A trip CB (and that's what you actually used), then you would have to install a full size #2 Cu. EGC (sized from 250.122) in each parallel conduit.

    The EGC can't be reduced in size when run parallel.

    Edited to add -- but I agree with everyone else, if you put them in one conduit, then you'll have to derate them.

    [ August 08, 2005, 12:26 AM: Message edited by: tx2step ]

  6. #6

    Re: Parallel conductors count for derating factor?

    Ok now to add to this mess the people incharge have desided they want a 480V @ 225A panle on same circuit. Now I know this will not actualy pull 225A & I know that press will probably never pull all of 430A. So i should be able to use same circuit, but can not find this in code. That way I can know hwo I can derate they two together.

    Anyone know where I can find this?
    Dan Thomas
    Industrial Electrician, Indiana

  7. #7
    Join Date
    Apr 2003
    Location
    Westchester County, New York
    Posts
    3,552

    Re: Parallel conductors count for derating factor?

    The code reference for parallel conductors and ampacity adjustment is 310.4 (the very last sentence)
    "Conductors installed in parallel shall comply with the provisions of 310.15(B)(2)(a)."

  8. #8
    Join Date
    Feb 2003
    Location
    lancaster
    Posts
    164

    Re: Parallel conductors count for derating factor?

    Saw an installation today that i need help. 1200 amp single phase(120/240) in two conduits. (1) had 2-500s+1-350 (2)4-500s+2-350s.Is this ok meaning only two conduits? not looking at derating yet.they go from service head to splice box.THANKS GEO

  9. #9
    Join Date
    Mar 2003
    Location
    Utah
    Posts
    780

    Re: Parallel conductors count for derating factor?

    Geo,

    I'm assuming the the instalation you describe has one conductor for each phase in one conduit, and two for each phase in the other?. I would say the instalation would be a violation. 310.4 says in part that "Where run in separate raceways or cables, the raceways or cables shall have the same physical characteristics." I think the reason for this requirement is so that the conductors in the separate raceways will perform the same. With more conductors in one raceway than the other, they will not perform the same, at least not in terms of heating. Are the two conduits of which you speak the same size? Does anyone think that it would be too much of a stretch to call this a violation based on the code section I have cited?

  10. #10
    Join Date
    Aug 2004
    Posts
    11,542

    Re: Parallel conductors count for derating factor?

    Unless they used the same size raceway the physical characteristics of the raceways are different. Violation.

    while maybe not a good idea to do so, the code seems to allow parallel runs of different numbers of conductors in seperate conduits as long as the conduits are the same type and size.

    [ August 11, 2005, 03:46 PM: Message edited by: petersonra ]
    Bob

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •