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Thread: Duct bank calculations

  1. #1
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    Feb 2009
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    Duct bank calculations

    Is there a formula out there for calculateing duct banks? I need one that takes into account the area of the conduit and the rebar for steel reinforcement.

    Thanks
    Hank

  2. #2
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    Feb 2003
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    Calculating what aspect of ductbanks? Width and height? Minimum burial depth? Area? Space between conduits? Length? Weight? Amount of concrete? Ampacity of conductors within? Cost per linear foot? Time to install?
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  3. #3
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    I need to know the amount of concrete. It needs to have 5" of concrete above and 5" of concrete below the and 5" on either side I have 9 4" conduits, stacked 3 wide and 3 high. Sch 40 pvc the trench is 1600'

    Thanks
    Hank
    Last edited by HankCar; 02-19-09 at 11:29 AM.

  4. #4
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    Don't bother trying to deduct the pipe and rebar area unless you are installing trench forms. When using dirt forms the real world is far from perfect and you will use that small amount of extra concrete both in a not perfectly cut excavation and the men not pouring exactly 5" on top.
    BTW: You didn't give the amount of spacing between the conduits and around the conduits to the rebar.
    Last edited by Nick; 02-19-09 at 11:53 AM.

  5. #5
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    Just useing factory-fabricated Duct seperators Industry standard rigid pvc interlocking spacers sized for 4" pvc

  6. #6
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    NOTE: The following discussion is a mathematical one. I have never done this work, so I do not know how this is calculated in the "real world." But it should give you a general notion of the amount needed.

    The formula is as simple as “area times length equals volume.” Depending on the degree of precision you need, you can be as detailed as you like in determining the area. Here is one example. I will assume you want the conduits to be spaced 7.5 inches center to center, as shown in Detail 4 of Figure B.310.2.

    Draw a square, and draw your pattern of conduits inside that square. The distance from the center of the left circle to the center of the right circle is 15 inches. Add 2 inches from the center of the left circle to the left edge of that circle, and 2 more from the center of the right circle to the right edge of that circle. Finally, add 5 inches each to the left and right sides. You now have a square of 29 x 29 inches. That has an area of 841 square inches.

    From Chapter 9, Table 4, Articles 352 and 353, the total area of a 4” sch 40 PVC is 12.554 square inches. You have nine of these. They will take up 113 square inches of the available area of the square.

    That leaves 728 square inches of concrete. Divide by 144 to get 5.056 square feet of concrete. Multiply by1600 to get a total of 8089 cubic feet of concrete. Finally, divide by 27 to get 300 cubic yards of concrete.

    This will not be precisely the amount you need. For starters, the Chapter 9 table gives us the internal area of the conduit, not the external area. So the calculated amount of concrete will be slightly high. Also, you will need to put spacers in the trench, to hold the conduit in place, and they will take up some room. So here again the calculated amount of concrete will be slightly high. Finally, nobody is going to be able to dig a trench that is sized to three decimal places of precision.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

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