According to Clark W. Griswold, Jr. 73115 dot 1041 at compuserve dot com:
(Chris Lewis) wrote:

| If the wire overheated or in case of a fire, you'd get toxic fumes in
| the plenum from the PVC and shortly thereafter, from the romex too.
| That's why (we) ban bare romex in air plenums in the first place - toxic
| fumes from plastic sheathing...

So what non-metallic can be used?

Essentially none. Consumer-available non-metallic conduit is pretty well
all PVC. Almost all plastics have a noxious fumes issue anyway.

A little bit of reality goes a long ways in this situation. While I wouldn't run
romex for any length inside a plenum, crossing 18" of floor joist is not a major
threat to the occupants of the building. If there is enough flame to burn
through the plenum and vaporize 18 inches of plastic, the occupants have far
worse problems to deal with.

Of course, the incremental risk of 18" worth of PVC isn't very high.

It's not hard to see that hot air entering the return vent could melt the PVC
and produce fumes that are 100% captured and ejected into other rooms, adding
quite a bit of toxic fume load to prevent occupants from waking up - toxic
fumes directly generated from the fire tend to rise away from the flame point,
and not go into a return duct (which is on or near the floor).

I'm sure the rule is primarily focused at using the duct for a raceway, but they
decided to simplify it as much as possible - it generally doesn't cause that
much of a hardship anyway.

It's more of a hardship in buildings which use dropped ceilings as a air plenum.
Here, _any_ wire in a dropped ceiling used as a plenum _must_ be in conduit,
tho, I believe they now accept teflon (despite the fact that at higher
temperatures teflon produces _really_ nasty fumes).
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.