On Mar 19, 3:16 pm, wrote:
On 19 Mar 2007 11:02:22 -0700, "Edge" wrote:



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.

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.

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.