I know it’s a biggie code violation to run wires in a plenum, such as a cold air return, specifically romex is forbidden. However, I have this interesting question no one has answered.
The code says you can run a wire perpendicular to an air return, such as running wires across joists where one joist bay is panned, it’s permisible. But if it’s ok to run romex perpendicular and thru the air return, why is it not ok to run parallel inside an air return plenum? I mean, whatever happens to the wire (and I don’t know what happens) when run parallel inside an air return plenum will certain happen to the piece exposed inside the plenum that runs perpendicular. What’s the logic of this?
Replies
Probability
Amount of wire in the duct. more wire = more insulation and paper filler = more toxic smoke.
What CAP said. Perpendicular = less wire insulation to burn off, and less toxic smoke.
Also inline would have much more of a tendency to blow the flames down the line causing it to spread.Crossflow the there is no new material to light and it will die out in a short time.
In addition to what the others have said, there's the practical, "human factors" point that, if wires are allowed in plenums without restriction then, as the path of least resistance, plenums will come to be seen as raceways (which would be a bad thing).
The current allowance for crossing a plenum provides no real incentive to make plenums become raceways. And it acknowledges the practical issue that often the wire is there first, and then the tin man comes along and nails steel to the bottom of the joists and turns it into a plenum.
Thanks, good point(s). I never would've thunk the wires in plenum restriction was due to fire/smoke issues. Makes sense to me as well as what you said.
my guess .. if a wire crosses through a plenum and you need to open it up, you can just pull the cleats and take the section in front or behind out ... if a wire runs the length of the duct then all the pieces are like beads on a chain, and you can't add or remove any pieces of duct .. just a guess ..
You become more aware of these restrictions in commercial situations when the "plenum" is the entire area above a suspended ceiling. Even low-voltage wiring in such areas must meet rigid restrictions, either enclosed in conduit or with a fireproof, non-toxic-gas-emitting Teflon jacket.