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Thread: 300.7 method of sealing conduit

  1. #1
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    300.7 method of sealing conduit

    Im retired, only service calls I make now are in my home and I find I am still learning stuff.

    Latest issue is I have condensation inside emt that is in my attic. I am familiar with 300.7 but that is something I never bothered to do.... until today. I plugged up entry into a box in my attic with duct seal. I did the same for the box on the warm side too. Lots of work for a senior citizen.

    Do you think this alone will solve my issue?

    Also in regards to your method of plugging raceways... would you plug both ends? Or just the warm side or cold side?

  2. #2
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    We have been using duct seal on service raceways for years and it works well.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  3. #3
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    Welcome to The Forum. I would use it on both ends otherwise you are likely to have a slug of water accumulate at the duct seal and drip anyway.

    EMT in an attic? What is this unicorn of which you write?
    Electricians do it until it Hertz!

  4. #4
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    thats what I was thinking but the flow of air is interrupted with only one side plugged too...wouldnt that alleviate the condensation as well?
    Last edited by ricry; 05-16-18 at 08:43 PM.

  5. #5
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    The NEC only requires it on one end which is sufficient. The concern is the movement of air which cannot happen if either end is closed off.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  6. #6
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    Quote Originally Posted by ricry View Post
    thats what I was thinking but the flow of air is interrupted with only one side plugged too...wouldnt that alleviate the condensation as well?
    Condensation is a process of warmer moist air reaching its dew point on or inside a colder object. Airflow has little to do with it.

    I suppose in theory that if one side of the conduit was perfectly sealed, and there were no air leaks in any fittings, no water would ever accumulate inside the conduit. In practice, conduit is rarely perfectly sealed, even EMT with set screw connectors. Any raceway run underground is considered a wet location, to a much lesser extent, EMT in a dry but variable humidity and temperature attic will eventually become a wet location. There are almost always very localized temperature differences that will cause air to enter the conduit.

    an attic though is normally a dry location and the sort of problem does not usually crop up unless there are roof leaks, bathroom fans vented to the attic, and so on.
    Electricians do it until it Hertz!

  7. #7
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    When you have a temperature differential on either end of a raceway or even between an end and the interior of the raceway - you have a good chance of condensation forming at times within that raceway.

    Sealing just one end won't necessarily stop all condensation and seals aren't always perfect seals, but this significantly inhibits air flowing through the raceway reducing how much humidity may enter the raceway and condense while in there.

    One humid summer day I buried a PVC raceway in the ground. It never rained overnight, but when I pulled conductors through it the next day - my pull rope was a little wet. That much condensation had formed inside just overnight from the combination of warm humid air being able to flow through the pipe but the underground temp was cool enough to allow it to condense in there.
    I live for today, I'm just a day behind.

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