Is the suspended ceiling being used for returning the room air to the air handler? If so, the entire space above the suspended ceiling would be considered to be a "Return Air Plenum".
No wiring, or anything else for that matter, can be installed in an air plenum (or air duct) unless it has been rated by the manufacturer to be used in an air plenum. That includes old newspapers, and that includes PVC conduit that has not been rated to be used in the plenum.
This is a life safety issue. If something like PVC conduit smolders or catches fire, it sends out smoke and toxic fumes. The smoke and fumes can injure or kill someone who breathes the fumes. If return air spaces, like areas above suspended ceilings, get smoky or full of toxic fumes, the air handlers will spread the smoke and fumes to all parts of the building.
Spreading smoke and toxic fumes so everyone in the building can breathe it is a bad thing.
Unless the area above the suspended ceiling is verified by the architect, in writing, that it is not going to ever be used as a return air plenum, PVC conduit that is not rated by manufacturer for plenum use cannot be used above the suspended ceiling.
Douglas Krantz
