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Thread: Jam Ratio

  1. #1
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    Jam Ratio

    Does jam ratio for conduit sizing apply only to three or more equal sized conductors/cables. What if the conductors/cables are not of the same size?

  2. #2
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    ok may i ask what you mean by "jam ratio"?
    are you referring to conduit fill?
    how many you can "jam" into one conduit?

  3. #3
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    FPN No. 2: When pulling three conductors or cables into a raceway, if the ratio of the raceway (inside diameter) to the conductor or cable (outside diameter) is between 2.8 and 3.2, jamming can occur. While jamming can occur when pulling four or more conductors or cables into a raceway, the probability is very low.
    It appears to apply to 3 conductors of the same size.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  4. #4
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    Quote Originally Posted by Gac66610 View Post
    ok may i ask what you mean by "jam ratio"?
    are you referring to conduit fill?
    how many you can "jam" into one conduit?
    I believe he is talking about note 2 in ch 9 tables. I don't have my code book with me but he is correct about asking.
    The thought is that when 3 cables are pulled into a conduit they will flaten out and jab in the conduit
    (Not to scale.. 1inch id with 3 cables with .35 od when laying flat they are wider then in id of the pipe)


  5. #5
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    Did not know that, i'm a resi

    Back to skwell


  6. #6
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    The fact that you have 1 cable that is larger can still cause the jamming. The cables tends to flatten out as they are pulled through the 90 degree turn. There are means
    to ensure the cable remain in the triangular configuration while making this pull.
    The sites below have some good information. The tables assume the 3 cables are the same.

    http://www.polywater.com/jamming.html]
    http://ecmweb.com/mag/electric_simple_calculations_cable/

    Timm have you been through this process before?
    Very good question.
    Last edited by bob; 03-16-12 at 09:56 PM.

  7. #7
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    I have not had this problem in the field, but still want to be sure. There is lot of literature available on the internet about jam-ratio. But there are two questions unanswered.

    1. Does jam-ratio apply only to 3 conductors, or more than 3 conductrors as well?
    2. Does jam-ratio apply to conductors/cables of the same size (in same conduit), or to different sized conductors/cables (in same conduit) as well?

  8. #8
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    The three percentages for conduit fill in Chapter 9 should take care of all jam ratio scenarios. That why for one conductor you're allowed 53% fill, for two conductors 31% fill and for 3 or more 40% fill.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

  9. #9
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    Quote Originally Posted by timm333 View Post
    I have not had this problem in the field, but still want to be sure. There is lot of literature available on the internet about jam-ratio. But there are two questions unanswered.

    1. Does jam-ratio apply only to 3 conductors, or more than 3 conductrors as well?
    2. Does jam-ratio apply to conductors/cables of the same size (in same conduit), or to different sized conductors/cables (in same conduit) as well?
    I think jamming issues can exist in those other situations but with 3 conductors same size and the correct to raceway size ratio is probably when it happens the easiest and has most effect on pulling force needed.

  10. #10
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    Quote Originally Posted by infinity View Post
    The three percentages for conduit fill in Chapter 9 should take care of all jam ratio scenarios. That why for one conductor you're allowed 53% fill, for two conductors 31% fill and for 3 or more 40% fill.
    Rob,
    I don't understand what you are telling us here. Can you expand on it?
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

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