Condensation in EMT Conduit

[Edge]

I have a two story house with an attic. The attic has a bare bulb utility light that is wired using EMT metal conduit. The conduit rises about three feet above the attic insulation. The conduit runs straight down through a wall to an outlet box on the second floor. I have noticed water in the outlet box in the winter (Midwest). Warm moist air is going up the conduit, condensing in the cold of the attic and running back down to the outlet box. The conduit and metal box are rusting along with the potential problem of shorts from the water. I have tried to stop the air infiltration by using foam cutouts behind the outlet cover. This has helped some but does not stop it completely. I want to know if it is safe to caulk the conduit openings either at the second floor end or the attic end. I worry that blocking the second floor end might be trapping moisture in the conduit. Will blocking the attic end cause heat buildup? Can another material be used instead of caulk, like fiberglass? What does the electric code say about this situation.

[gfretwell]

Use duct seal on both ends. (available from a supply house and maybe a good hardware store) It is designed not to damage the wire. The NEC actually requires this in situations like you have where you have different environmental conditions at each end of the raceway. In real life "rope caulk" will probably work fine but it doesn't have the U/L blessing. Don't use silicone tube caulk. The acetic acid is tough on the wire and if you ever want it out of there it is a pain to remove.

300.7 Raceways Exposed to Different Temperatures. (A) Sealing. Where portions of a cable raceway or sleeve are known to be subjected to different temperatures and where condensation is known to be a problem, as in cold storage areas of buildings or where passing from the interior to the exterior of a building, the raceway or sleeve shall be filled with an approved material to prevent the circulation of warm air to a colder section of the raceway or sleeve.

[Edge]

Thanks for the advice on Duct Seal. I decided to look at another utility light, in my garage attic. It is mounted about ten feet above the attic floor so I have never looked at the conduit that runs to it. The conduit has several 90 degree bends with a horizontal section that is exposed to the cold. In examining the bend at the horizontal section, it cracked, with rusty water leaking out. Water must have accumulated there for years rusting out and weakening the metal. I am going to replace the conduit, but with no horizontal runs, and plug both ends with duct seal.

[Just Joshin]

imho:

"Duct Seal" migth be your friend. ;) If you use it, use it on the end of the conduit inside the home. This way it doen't over heat, summer time, or shrink, winter time, too much.

Just a guess....

tom @

[Just Joshin]

I was referring to attic heat in the summer. So use it on the climate controled side.

Just guessing....

tom @