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Thread: Underground Conduit Questions

  1. #1
    Join Date
    May 2018
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    Underground Conduit Questions

    I have three questions regarding underground conduit and I am wondering if you would be able to provide a brief sentence on each. Thank you.

    1) is there a minimum required size for underground conduits? IE- If the cables I need to run only require a 2" conduit, is that acceptable or must it be no smaller than a certain size, say 3"?

    2) what are the rules for transition from underground PVC to above ground, does it need to transition from PVC to RGC above ground?

    3) what are the conditions when the underground conduit must be encased in concrete?

    Thanks

  2. #2
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    May 2005
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    Richmond, Virginia
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    1. No, unless you want to upsize for larger conductors for possible future additions/upgrades or for easier conductor pulling.

    2. You only need either Sched. 80 or RGS conduit for physical protection, or steel for not burning through a PVC elbow with the pulling rope.

    3. Concrete can be used for shallower burial depths and for conduit that needs to be inside a building but rated for not inside a building.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  3. #3
    Join Date
    Dec 2015
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    Texas
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    171
    Quote Originally Posted by LarryFine View Post
    1. No, unless you want to upsize for larger conductors for possible future additions/upgrades or for easier conductor pulling.

    2. You only need either Sched. 80 or RGS conduit for physical protection, or steel for not burning through a PVC elbow with the pulling rope.

    3. Concrete can be used for shallower burial depths and for conduit that needs to be inside a building but rated for not inside a building.
    In addition to Larry's notes, local codes, project requirements, utilities may all dictate additional requirements. For example, we are required to concrete encase with red-dye underground conduits crossing gas pipelines through the entirety of the easement per the utility requirements. Different jurisdictions may have more stringent requirements. Conduits from the meter to the utility transformer may require rigid metal only, or conduit only depending on the utility. Minimum conduit size from a utility transformer to the service entrance disconnect may be only 4" if dictated by them.

  4. #4
    Join Date
    Apr 2016
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    Georgia
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    Quote Originally Posted by Gshell82 View Post
    I have three questions regarding underground conduit and I am wondering if you would be able to provide a brief sentence on each. Thank you.

    1) is there a minimum required size for underground conduits? IE- If the cables I need to run only require a 2" conduit, is that acceptable or must it be no smaller than a certain size, say 3"?

    2) what are the rules for transition from underground PVC to above ground, does it need to transition from PVC to RGC above ground?

    3) what are the conditions when the underground conduit must be encased in concrete?

    Thanks

    In the Petro-chem industry most clients require minimum 1" - it can be PVC or Rigid Galv Steel but almost always its PVC.
    Typically we transition from PVC to RGS with an adapter, then stub up using RGS 90 degree elbows.
    Within operating units, typically we run PVC conduit in concrete duct banks - in remote areas such as Tank Farms it may not be as stringent - depends on the Client and specifications.

  5. #5
    Join Date
    Apr 2009
    Location
    Williamsburg, VA
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    5,438
    Many phone companies required two 4 inch conduits for a commercial building even if they were only running a single 25 pair cable... No idea what they're doing now for fiber or if that requirement still exists
    Electricians do it until it Hertz!

  6. #6
    Join Date
    Mar 2014
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    South Florida
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    we require minimum 1" for underground. easier pulling and the cost of burying a 1" compared to a 3/4" is negligible.

    our details transition to either RGS or aluminum with bitumastic at the elbow turning up. burning a pvc elbow cited as the main concern.

    if you anticipate large machines regularly digging nearby (industrial plant) i would say you want to encase it depending on how critical the conductors are. consider the risk of damage to the conduit and/or conductors within. how much would it cost the owner, in lost production and emergency repair costs, to get it fixed? segment of fiberoptic ring connecting automation of plant to eachother, 600A worth of medium voltage ring bus conductors? better encase that.... site lighting? nah, they'll fix it when they get to it.

  7. #7
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    Quote Originally Posted by drktmplr12 View Post
    we require minimum 1" for underground. easier pulling and the cost of burying a 1" compared to a 3/4" is negligible.

    our details transition to either RGS or aluminum with bitumastic at the elbow turning up. burning a pvc elbow cited as the main concern.

    if you anticipate large machines regularly digging nearby (industrial plant) i would say you want to encase it depending on how critical the conductors are. consider the risk of damage to the conduit and/or conductors within. how much would it cost the owner, in lost production and emergency repair costs, to get it fixed? segment of fiberoptic ring connecting automation of plant to eachother, 600A worth of medium voltage ring bus conductors? better encase that.... site lighting? nah, they'll fix it when they get to it.
    if you are going to put it in concrete then it seems like it would make sense to use PVC.
    Bob

  8. #8
    Join Date
    Mar 2014
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    South Florida
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    Quote Originally Posted by petersonra View Post
    if you are going to put it in concrete then it seems like it would make sense to use PVC.
    yes, concrete encasement is all pvc schedule 40. direct bury or concrete cap schedule 80

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