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Thread: Underground Wiring in PVC Conduit

  1. #1
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    Underground Wiring in PVC Conduit

    When running wires underground through PVC conduit to feed a garage, is THHN or THWN the proper type to use? Suppose a garage is to be wired with a subpanel fed from the house by a 40-amp 240V breaker. They would use each a red, white, black, and green #8 AWG copper conductor, but what type of insulation? Since the conduit is dry when placed in the ground, and glued with PVC cement so that water can never enter it, would that still be considered a wet location?

    Also, suppose a 3-way switch was to be added for the outside garage light to be operated from the house as well. A red, black, and blue #14 AWG would each need to be pulled through the conduit as well. What is the minimum size conduit that can hold four #8 conductors and three #14 conductors?

  2. #2
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    A PVC pipe under ground, even if under a house slab, is considered a wet location. Pvc conduit runs outside almost always fill with water, unless you clean the joints real well and use primer then they might stay dry but it really doesnt matter, unless the its downhill into a basement. So you need a wet location wire, however I doubt there is much thhn now that is not also rated as thwn - maybe in smaller sizes, double check. Check the NEC for the conduit fill, 3/4 should be fine, 1" would be nice.

  3. #3
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    Quote Originally Posted by electrofelon View Post
    A PVC pipe under ground, even if under a house slab, is considered a wet location. Pvc conduit runs outside almost always fill with water, unless you clean the joints real well and use primer then they might stay dry but it really doesnt matter, unless the its downhill into a basement. So you need a wet location wire, however I doubt there is much thhn now that is not also rated as thwn - maybe in smaller sizes, double check. Check the NEC for the conduit fill, 3/4 should be fine, 1" would be nice.
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  4. #4
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    Quote Originally Posted by jeff48356 View Post
    When running wires underground through PVC conduit to feed a garage, is THHN or THWN the proper type to use? Suppose a garage is to be wired with a subpanel fed from the house by a 40-amp 240V breaker. They would use each a red, white, black, and green #8 AWG copper conductor, but what type of insulation? Since the conduit is dry when placed in the ground, and glued with PVC cement so that water can never enter it, would that still be considered a wet location?

    Also, suppose a 3-way switch was to be added for the outside garage light to be operated from the house as well. A red, black, and blue #14 AWG would each need to be pulled through the conduit as well. What is the minimum size conduit that can hold four #8 conductors and three #14 conductors?
    You didn't say what schd. PVC you would use. As stated, 3/4" will work. if it is schd. 40. If you use schd. 80 you would need 1". Of course, bigger is better in either case. PVC can be a bear to pull through.
    If you aim at nothing, you will hit it every time!

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    Quote Originally Posted by electrofelon View Post
    A PVC pipe under ground, even if under a house slab, is considered a wet location. Pvc conduit runs outside almost always fill with water, unless you clean the joints real well and use primer then they might stay dry but it really doesnt matter, unless the its downhill into a basement. So you need a wet location wire, however I doubt there is much thhn now that is not also rated as thwn - maybe in smaller sizes, double check. Check the NEC for the conduit fill, 3/4 should be fine, 1" would be nice.
    Seal joints however you wish, it will not matter. Most of the water that ends up in the raceway is from condensation and not from leaking joints. Earlier this summer I ran an underground PVC on a Friday. On Monday when I pulled conductors there was enough water in the raceway that the pull rope was wet. It did not rain over the weekend either, humid air condensed in the cooler underground portion of the raceway.

  6. #6
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    XHHW

    My 2-bits, THWN is PVC based thermoplastic insulation. XHHW is PE based thermoset insulation. XHHW is more tolerant of wet locations. THWN is more flexible and easier to strip. POCOs generally use XLPE underground which is XHHW-2. I always spec XHHW-2 for wet locations.
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  7. #7
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    Quote Originally Posted by Little Bill View Post
    You didn't say what schd. PVC you would use. As stated, 3/4" will work. if it is schd. 40. If you use schd. 80 you would need 1". Of course, bigger is better in either case. PVC can be a bear to pull through.
    Do explain your thought on this one? The difference between schd 40 and schd 80 is in the wall thickness, but the inside diameter will remain the same, meaning that it is the outside overall diameter that will increase. All of your conduits are I.D.

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    Quote Originally Posted by racerdave3 View Post
    Do explain your thought on this one? The difference between schd 40 and schd 80 is in the wall thickness, but the inside diameter will remain the same, meaning that it is the outside overall diameter that will increase. All of your conduits are I.D.
    Better go out to the truck and look again. I use the same fittings for.sch 40 as I do sch 80. The id changes. Your code book will also tell you the same.
    Tom
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    Quote Originally Posted by racerdave3 View Post
    The difference between schd 40 and schd 80 is in the wall thickness, but the inside diameter will remain the same, meaning that it is the outside overall diameter that will increase. All of your conduits are I.D.
    You have it backwards: the outside diameter remains the same (so that schd 40 and schd 80 can be interchanged with the various fittings); it's the inside diameter that changes with the different wall thickness. And the sizes are nominal, not actual. You may wish to refer to the tables on this page:

    http://www.engineeringtoolbox.com/pv...ons-d_795.html

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    Quote Originally Posted by Jon456 View Post
    You have it backwards: the outside diameter remains the same (so that schd 40 and schd 80 can be interchanged with the various fittings); it's the inside diameter that changes with the different wall thickness. And the sizes are nominal, not actual. You may wish to refer to the tables on this page:

    http://www.engineeringtoolbox.com/pv...ons-d_795.html

    Chapter 9 table 4 is a good place to look also.

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