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Thread: Minimum Bending Raduis for MV Shielded Cable

  1. #1
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    Minimum Bending Raduis for MV Shielded Cable

    Ok, 300.34 reads,

    300.34 Conductor Bending Radius.
    The conductor shall not be bent to a radius less than 8 times the overall diameter for nonshielded conductors or 12 times the overall diameter for shielded or lead-covered conductors during or after installation. For multiconductor or multiplexed single-conductor cables having individually shielded conductors, the minimum bending radius is 12 times the diameter of the individually shielded conductors or 7 times the overall diameter, whichever is greater.

    The top potion is discussing single conductors and the potion discussing the multiconductors or multiplexed shielded is what we are dealing with.
    As strange as it my seem the contractor, believes that the calculation refers to one cable, but I contend that it is refering to all 3 cables in the multipex (triplexed is what the cables are configured). The difference is great when you speak of all three cable diameter, instead of one of course. In this case the single conductor 500kcmil EPR Shielded Cable diameter is 1.38 by the manufacturer detail, that would mean that the bend radius would be somewhere near 49" inches. I just wanted to know the correct interpretation of the code. Is it the one single conductor or all three in the multiplex configuration? My calculation would look like this:

    MV cable diameter = 1.38" X 3 (number of MV cables in the multiplex) = 4.14"

    12X4.14" = 49.68" bending radius

    The code is definitely concern with maintaining the protection of the MV cable, by stating of the two calculation take the one that has the greater bend radius. So it is clear they are not trying to give any allowances on the bending of the cable and I am talking about a 4160V cranking motor.

    The bend radius may not come into play today or even a year from know, but maybe a few years down the line, my company would be absorbing the cost of replacing the cable and a shut down of the Units, if the MV cable is damaged because of a bad install.

  2. #2
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    Quote Originally Posted by rob569 View Post
    Ok, 300.34 reads,

    300.34 Conductor Bending Radius.
    The conductor shall not be bent to a radius less than 8 times the overall diameter for nonshielded conductors or 12 times the overall diameter for shielded or lead-covered conductors during or after installation. For multiconductor or multiplexed single-conductor cables having individually shielded conductors, the minimum bending radius is 12 times the diameter of the individually shielded conductors or 7 times the overall diameter, whichever is greater.

    The top potion is discussing single conductors and the potion discussing the multiconductors or multiplexed shielded is what we are dealing with.
    As strange as it my seem the contractor, believes that the calculation refers to one cable, but I contend that it is refering to all 3 cables in the multipex (triplexed is what the cables are configured). The difference is great when you speak of all three cable diameter, instead of one of course. In this case the single conductor 500kcmil EPR Shielded Cable diameter is 1.38 by the manufacturer detail, that would mean that the bend radius would be somewhere near 49" inches. I just wanted to know the correct interpretation of the code. Is it the one single conductor or all three in the multiplex configuration? My calculation would look like this:

    MV cable diameter = 1.38" X 3 (number of MV cables in the multiplex) = 4.14"

    12X4.14" = 49.68" bending radius

    The code is definitely concern with maintaining the protection of the MV cable, by stating of the two calculation take the one that has the greater bend radius. So it is clear they are not trying to give any allowances on the bending of the cable and I am talking about a 4160V cranking motor.

    The bend radius may not come into play today or even a year from know, but maybe a few years down the line, my company would be absorbing the cost of replacing the cable and a shut down of the Units, if the MV cable is damaged because of a bad install.
    The number closer to 4 feet is the correct one. If you bend a 500 MCM triplexed cable at a 4 inch radius (if that is even possible) you will destroy it.

    It does say overall diameter. What part of that is your contractor having trouble with?
    Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and anyone else who can't handle the truth.

  3. #3
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    Is there a shield around all three conductors??

    If not, it looks to me like you would use the larger of:

    12 x 1.38", or
    7 x the entire cable diameter.

    Anyway, your 4.14" number doesn't seem right: (3) 1.38" cables won't have a total diameter of 3 x 1.38. (They actually fit within a 3" diamter circle - but that doesn't allow much for an outter covering on the bundle.) You need a cut sheet that has the total cable diameter, or measure it.

    steve
    Last edited by steve66; 05-28-09 at 05:40 PM.

  4. #4
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    If in doubt, contact the cable manufacturer and ask them what the recomended bending radius is for that type cable. They should have bending radius, side wall pressure and pulling tension calcs for every cable they make.

  5. #5
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    Quote Originally Posted by steve66 View Post
    Is there a shield around all three conductors??

    If not, it looks to me like you would use the larger of:

    12 x 1.38", or
    7 x the entire cable diameter.

    Anyway, your 4.14" number doesn't seem right: (3) 1.38" cables won't have a total diameter of 3 x 1.38. (They actually fit within a 3" diamter circle - but that doesn't allow much for an outter covering on the bundle.) You need a cut sheet that has the total cable diameter, or measure it.



    steve
    This is EPR shielded cable and that number was the manufacturer's O.D. for the cable. I didn't make that up. They are ran in a 4" rigid conduit, but it comes out of the pipe and they put a pretty tight bend on it, my Siemens rep. really brought it to my attention, I just followed up on it.

  6. #6
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    Quote Originally Posted by drbond24 View Post
    The number closer to 4 feet is the correct one. If you bend a 500 MCM triplexed cable at a 4 inch radius (if that is even possible) you will destroy it.

    It does say overall diameter. What part of that is your contractor having trouble with?

    No they did not have a 4" bend radius, they orginally had like a 10" or 12" and they mess with it and it went to almost 15", but that is stil not enough, even if you take the smaller of the 2 calculations. I think this is one of those cases where no one actually question it and now all I'm hearing is I've seen worst. If they modify it and put it in tray coming out of the conduit, they would have to meet the requirement, becuase of the tray they would have to use.

  7. #7
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    Anyway my question was if you have the cables in a triplex, three cables wrapped in a triangle configuration (this is what they would call a multiplexed single conductor cables) how would you calculate it. Like I did or using only one cable out of the three, I think given how the code reads you would use the three totaled diameters together and then multiply that.

  8. #8
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    Quote Originally Posted by steve66 View Post
    Is there a shield around all three conductors??

    If not, it looks to me like you would use the larger of:

    12 x 1.38", or
    7 x the entire cable diameter.

    Anyway, your 4.14" number doesn't seem right: (3) 1.38" cables won't have a total diameter of 3 x 1.38. (They actually fit within a 3" diamter circle - but that doesn't allow much for an outter covering on the bundle.) You need a cut sheet that has the total cable diameter, or measure it.

    steve
    The ratio of diameters of one circle circumscribing three of equal diameter to and abutting each other is (1+1/cos30):1... approximately 2.1547.

    The overall diameter would be 2.97.

    It's kind of hard to physically measure the overall diameter of triplex :cool:

    7 × 2.97 = 20.8
    12 × 1.38 = 16.56

    Minimum bend radius is 20.8
    Last edited by Smart $; 05-28-09 at 08:14 PM.

  9. #9
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    Quote Originally Posted by rob569 View Post
    Anyway my question was if you have the cables in a triplex, three cables wrapped in a triangle configuration (this is what they would call a multiplexed single conductor cables) how would you calculate it. Like I did or using only one cable out of the three, I think given how the code reads you would use the three totaled diameters together and then multiply that.
    Given those two choices, the answer is neither. It is certainly not the diameter of just one of the conductors. However, the overall diameter of the cable is also not the sum of the individual diameters. The number you want is somewhere in between.

    Quote Originally Posted by Smart $
    The ratio of diameters of one circle circumscribing three of equal diameter to and abutting each other is (1+1/cos30):1... approximately 2.1547.

    The overall diameter would be 2.97.

    It's kind of hard to physically measure the overall diameter of triplex

    7 × 2.97 = 20.8
    12 × 1.38 = 16.56

    Minimum bend radius is 20.8
    It is still too early in the morning for me to test this. I've never actually calculated it. I've always drawn the cable in AutoCAD and then just put a circle around it and measured the diameter of that circle. I don't have AutoCAD handy at the moment to do that now.

    Your best bet is to just ask the manufacturer what the overall diameter of the cable you have is.
    Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and anyone else who can't handle the truth.

  10. #10
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    Quote Originally Posted by Smart $ View Post
    The ratio of diameters of one circle circumscribing three of equal diameter to and abutting each other is (1+1/cos30):1... approximately 2.1547.

    The overall diameter would be 2.97.

    It's kind of hard to physically measure the overall diameter of triplex :cool:

    7 × 2.97 = 20.8
    12 × 1.38 = 16.56

    Minimum bend radius is 20.8
    So you think the way the code reads it is suggesting that you take just one cable diameter out of the triplex and then calculate that. Interesting. I was waiting for that intereptation, I guess I will have to see what the manufacturer says about this now.

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