According to :
In sci.engr.electrical.compliance Chris Lewis wrote:
| According to :
| | There's an exception for teflon coated wire (certain types of CAT-5 for
| | example). And conduit of course. If you use EMT, you could probably
| | get away with strapping it to the ducting for ground.

| I wouldn't trust ducting for ground. How about a PVC conduit throught
| the duct?

| 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.

| Better still, use a PVC conduit sleeve to route the wire _under_ the plenum.
| Just don't go _through_ the plenum. The PVC conduit is sufficient physical
| protection for bridging the joists, even without the sheet metal on the
| bottom of the plenum.

Then you're committing space between any future ceiling finish and the
joists,

For the most part you'd be at least strapping the bottoms of the joists for
a ceiling finish, no matter how you routed the wire, so, you're losing that
3/4" of headroom anyway.

in which case the crossmember could do the job, too.

True enough.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.