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Thread: Manhole Size Calculation

  1. #1
    Join Date
    Jun 2010
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    Manhole Size Calculation

    Question: If i have 12 Each 4" Conduit entering from the North and 8 Each 4" Conduit Leaving from the South and 6 Each Leaving from the West.
    What Size is my Manhole?

    I think it is 8X4"+7X4"+1 1/2" x 8 +(24" working space) = Length
    6X4"+5x4" + 1 1/2" x 6 +(24" working Space)=Width

    6 1/2 Feet Head room

    Is this Close to Correct per NEC
    Thanks
    Rob

  2. #2
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    Well what are you installing meaning low voltage high voltage ? Whats going into manhole sometimes it matters do you need a ladder what kind of cover or entrance to manhole give just a little bit more info .

    Do the conductors cables get racked ?

  3. #3
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    manholes

    there are feeders 400volt 3 ph running from switchgear to switchgear about 90m. 3000amp main, the 24" NEC min is the manhole cover all the way down as clear working area,yes they will be racked 18" L-bracket design.

  4. #4
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    Well read 314.52 what do you think?

    We always size our manholes bigger than the code do to just being able to pull wire better and when someone has to work inside it but you can go 8 x for straight back and 6 times for a u pull plus add others in row as the nec says if you want .

    If your cover is as big as the box meaning wide iam a little confused ? 24 inches in a manhole is kinda tight for a electrician to climb down into dont understand what your asking but 6 1/2 head room in a 24 wide manhole do you see what iam asking ?

  5. #5
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    manholes

    the total of 8 Feet by 6.5 feet by 6 1/2' feet deep with a 24" round whole at the top is what i was asking (as Minium allowed per NEC) in real terms the Cover will be 3 feet round almost 8 foot deep and 9 by 8

    i just wanted to know if there is anything in the calculation missing

  6. #6
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    What are the 1-1/2" allowances for?

    Are you looking at 110.72? Minimum working space for this voltage is 30", 36" if the conductors are on both sides.

    Otherwise, the pullbox dimensions look right to me.

  7. #7
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    Quote Originally Posted by Roblawro65 View Post
    the total of 8 Feet by 6.5 feet by 6 1/2' feet deep with a 24" round whole at the top is what i was asking (as Minium allowed per NEC) in real terms the Cover will be 3 feet round almost 8 foot deep and 9 by 8

    i just wanted to know if there is anything in the calculation missing
    NOP nothing in the calculation 8 feet x 6.5 thats fine didnt understand what you wrote the last post go for it.

  8. #8
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    Working Space?

    Does the working space have to be added to the dimensions or just be there?
    A typical 5x7 precast vault has the knockouts centered at 5-1/2" and 12-1/2" from the inside wall. That gives you 3' of clearance.

    In my opinion this is to many cables for one vault. I would use two 5x7 vaults, one for each circuit. Fires are the most common failures in vaults.
    Cables can be in vaults a long time and nothing happens until someone gets in there and starts pulling around on them.

    The advantages to two vaults are:
    1.) You ever have to send a man into a vault with energized cable. IF work needs to be done use your LOTO proceedure and kill everthing in that vault.

    2.) If you need to replace cable in one circuit while the other is energized good luck. Pulling cable in vaults is not a delicate operation, things are banged around, dropped etc.

    3.)You don't know exactly in which ducts you'll be working ( they will usually be behind enegized cable?)

    IF you are going to pull out of the vault and back in a 24" lid is not big enough. You need three times the minimum cable bending radius to pull it and out again.

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