300.7 method of sealing conduit

ricry

Member
Location
USA
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?
 

JFletcher

Senior Member
Location
Williamsburg, VA
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? :D
 

ricry

Member
Location
USA
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?
 
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infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
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.
 

JFletcher

Senior Member
Location
Williamsburg, VA
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.
 

kwired

Electron manager
Location
NE Nebraska
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.
 
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