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Thread: Can THW wire be used underground in conduit?

  1. #1
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    Default Can THW wire be used underground in conduit?

    I want to add a sub-panel to my detached garage about 50' away. I have a bunch of #6 wire I would like to use if I can. The black is marked "THW 600v". The red and white I have is marked "MTW - Type THW 600v. From what I've gathered a W in the wire designation means it is sutible for wet locations ie. conduit. I know THWN is suitable for use in conduit but I've never heard specifically that THW can be used. Does anyone know for sure?

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    Electrical Contractor jbfan74's Avatar
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    Quote Originally Posted by pyro9862 View Post
    I want to add a sub-panel to my detached garage about 50' away. I have a bunch of #6 wire I would like to use if I can. The black is marked "THW 600v". The red and white I have is marked "MTW - Type THW 600v. From what I've gathered a W in the wire designation means it is sutible for wet locations ie. conduit. I know THWN is suitable for use in conduit but I've never heard specifically that THW can be used. Does anyone know for sure?
    See my reply on the other forum you asked this question on.
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  3. #3

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    Yes you can use it as long as you properly size it, protect it and terminate it.
    http://www.inspectpa.com/forum/forum.php
    My answers are based mostly on the ICC codes. Advice given is my personal opinion and every person performing work should acquire a permit from his/her jurisdiction and get the work inspected. My opinions are not directions to follow for DIYs or professionals

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    DIY Member jetlag's Avatar
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    Default THW ok

    Quote Originally Posted by pyro9862 View Post
    I want to add a sub-panel to my detached garage about 50' away. I have a bunch of #6 wire I would like to use if I can. The black is marked "THW 600v". The red and white I have is marked "MTW - Type THW 600v. From what I've gathered a W in the wire designation means it is sutible for wet locations ie. conduit. I know THWN is suitable for use in conduit but I've never heard specifically that THW can be used. Does anyone know for sure?
    You can run the thw in pvc underground, The #6 copper is good for 65 amps to a 60 amp sub panel . You will need 4 conductors to a sub panel , 2 hot , a neutral and a ground. The ground could be # 4 if you like. The code says they can be put in a 1" PVC but I think you will wish you had used 1 1/4 ".

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    Moderator & Master Plumber hj's Avatar
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    Default W

    HOW does conduit qualify as a "wet" location?

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    Code Enforcement codeone's Avatar
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    Quote Originally Posted by hj View Post
    HOW does conduit qualify as a "wet" location?
    The deffinition in article 100 of the NEC
    Location, Wet= Installations underground or in concrete slabs or masonry in direct contact with the earth; in locations subject to saturation with water or other liquids, such as vehicle washing areas; and in unprotected locations exposed to weather.

    If you have ever pulled much wire in underground piping you would see very soon underground piping is a wet location. Because of temperature changes condensation builds up in the conduit.

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    Code Enforcement codeone's Avatar
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    Also article 300.5 of the NEC
    Underground Installations at 300.5(B)
    Wet Locations = The interior of enclosures or raceways installed underground shall be considered to be a wet location. Insulated conductors and cables installed in these enclosures or raceways shall be listed for use in wet locations and shall comply with 310.8(C). Any connection or splices in an underground installation shall be approved for wet locations.

    310.8(C) gives the specs for the wire.

  8. #8

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    HOW does conduit qualify as a "wet" location?
    If it is "normaly subject to wetness" ie underground (or outdoors in some areas), it is considered wet.

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    Code Enforcement codeone's Avatar
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    Quote Originally Posted by pyro9862 View Post
    "MTW - Type THW 600v. From what I've gathered a W in the wire designation means it is sutible for wet locations ie. conduit. specifically that THW can be used. Does anyone know for sure?
    Yes the "W" stands for use in wet locations.
    and a "U" would stand for underground which is also approved for wet locations.

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    DIY Member jetlag's Avatar
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    THW can be submerged in water, the conduit is just for physical protection

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    Moderator & Master Plumber hj's Avatar
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    Default condensation

    Condensation ONLY occurs when humid air is subjected to a temperature below its "dew point". In the summer time, when it is humid, the temperature is ABOVE the dew point. In the winter, when the temperature could be below the dew point, the air is "dry", or "dryer", but in any case, HOW does the humid air get into a "sealed" circuit, and even if it does how does ENOUGH air enter to cause more moisture than evaporation would eliminate, and even if it gets in, HOW does it circulate enough to pass through the pipe in order to be subjected to the low temperature? Now THAT is "voodoo science".

  12. #12
    Code Enforcement codeone's Avatar
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    Quote Originally Posted by hj View Post
    Condensation ONLY occurs when humid air is subjected to a temperature below its "dew point". In the summer time, when it is humid, the temperature is ABOVE the dew point. In the winter, when the temperature could be below the dew point, the air is "dry", or "dryer", but in any case, HOW does the humid air get into a "sealed" circuit, and even if it does how does ENOUGH air enter to cause more moisture than evaporation would eliminate, and even if it gets in, HOW does it circulate enough to pass through the pipe in order to be subjected to the low temperature? Now THAT is "voodoo science".
    Go dig up any underground conduit and prove there is no water in it.
    Besides you miss the point. The code panel sees enough water in underground piping to make it a mandatory wet location. And this is whether your opinion agrees with it or not.
    Last edited by codeone; 12-16-2009 at 08:45 AM. Reason: spelling

  13. #13
    Jack of all trades frenchie's Avatar
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    Quote Originally Posted by hj View Post
    Condensation ONLY occurs when humid air is subjected to a temperature below its "dew point". In the summer time, when it is humid, the temperature is ABOVE the dew point.
    Yeah, the above-ground temperature. Below ground is usually a fair bit cooler than above-ground. Above-ground air subjected to below-ground temperatures = condensation.

    HOW does the humid air get into a "sealed" circuit
    Wet-location rated boxes & conduit aren't vapor-proof. That's a whole other level.

    and even if it does how does ENOUGH air enter to cause more moisture than evaporation would eliminate
    I'm sorry, but that question isn't making sense to me. The amount of condensation vs evaporation, is related to the total amount of air movement... how? You have it exactly backwards. If there was enough/more air movement, the conduit could dry out.

    Compare to wall cavities - back in the old days, before insulation, the inside of the walls would get damp, but because there was a lot of air movement in there, they'd also dry out easily. It's only nowadays, with our tightly-sealed construction & cavities stuffed with insulation, that we get condensation problems in walls if we aren't careful.

    and even if it gets in, HOW does it circulate enough to pass through the pipe in order to be subjected to the low temperature? Now THAT is "voodoo science".
    Again, compare to wall cavities. You only need a teeny-tiny hole for enough air movement to transport a whole lot of moisture.





    I got one for you - electrical boxes on the outdoor lighting, on Fire Island? Have to be checked out every year. Saltwater air gets in, vapor condenses inside, then water evaporates out I guess - they're usually dry inside, but also usually half-full of salt.
    Last edited by frenchie; 12-16-2009 at 10:15 AM.
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    Extreme DIY Homeowner Scuba_Dave's Avatar
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    Default

    Plenty of conduit has been dug up that is full of water
    It's a wet location, get used to the idea
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  15. #15

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    Quote Originally Posted by Scuba_Dave View Post
    Plenty of conduit has been dug up that is full of water
    It's a wet location, get used to the idea
    Yes, plan on it being wet 100% of the time it is in underground conduit. It is a fact. I don't care how tight your joints are or how careful you are. Underground conduit fills with water all of the time in most soil categories.

    Our local POCO even has stipulations for drainage holes to be drilled into the conduit and the NEC addresses this issue too.
    http://www.inspectpa.com/forum/forum.php
    My answers are based mostly on the ICC codes. Advice given is my personal opinion and every person performing work should acquire a permit from his/her jurisdiction and get the work inspected. My opinions are not directions to follow for DIYs or professionals

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