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Thread: Condit fill calculations

  1. #1
    Join Date
    Jan 2013
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    Condit fill calculations

    I'm confused on the tables and annex c part of the nec.

    Say I have the following....

    All THHN in a schedule 80 pvc
    #12 x10
    #10 x5
    #8 x3

    Since I have over 2 wires in one conduit do I calculate the cross sectional area of the conduit at 40% and then add up cross sectional area of each wire and add them up and see what size the Will fit into?

  2. #2
    Join Date
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    Somewhere over the rainbow
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    First start on Table 5 ch9
    find THHN row and use Approximate area square inches.
    then add the total of all of the conductors of your example.

    Then go to raceway properties Table 4 of ch9
    Find rigid PVC conduit
    scroll down the 40% column until you find a number that is higher than the total area that you calculated.
    then look to the left of that row you will see what conduit is needed.

    Added: Based on my calculation you need a 1.25" conduit
    Edward
    Education is a progressive discovery of our ignorance.


  3. #3
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    That's pretty much what I was thinking. Thank you for the clarification. Does the annex c take the 40% into consideration when it states the maximum number of conductors in any given raceway?

  4. #4
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    Quote Originally Posted by crtemp View Post
    That's pretty much what I was thinking. Thank you for the clarification. Does the annex c take the 40% into consideration when it states the maximum number of conductors in any given raceway?
    A

    Annex C is only used when all the conductors are the same size with the same insulation. It is based on the table for 1 wire (53%), 2 wires(31%) or over 2 wires (60%)

  5. #5
    Join Date
    May 2013
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    Massachusetts
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    Quote Originally Posted by Dennis Alwon View Post
    A

    Annex C is only used when all the conductors are the same size with the same insulation. It is based on the table for 1 wire (53%), 2 wires(31%) or over 2 wires (40%)
    Corrected.

    Essentially, it represents about 70% of the conduit diameter filled with cable, if you were to pack the cables as tightly as possible. Since two circles have a lousy packing factor in a circle, the area fill percentage is significantly less for 2 wires. 1 circle obviously has a 100% packing factor in a circle, so its area fill percentage is high. Then when you look at 3+ circles in a circle, the packing factor takes off to a steady cruising value between 80% & 90%.

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