Question : To Plenum or not to plenum?

I am looking to install in a warehouse that has a typical false drop down ceiling.  That ceilng is about 8 feet high.  Since this is a warehouse, the actual roof of the building is aproximately 30 feet.  So when you lift a tile of the false ceiling you see the ceiling of the builiding aprox 20 feet further.  I would like to know if this falls under Plenum(HVAC) or non-plenum cabling.  If you could help me with any of your personal experience or direct me to a website, it would be much appreciated.

Answer : To Plenum or not to plenum?

winkingtiger,

Sean is correct on that what typically dictates the use of plenum rated cable is how the HVAC return duct is installed. The easiest way to tell is to find  a return grille (hold a dollar bill to it, if it stays its a return, if it blow back to you its a supply) and see if ductwork is directly attached to it. If you can see thru the grille, then the cavity above the ceiling is the actual return plenum, requiring the use of plenum rated cable. If the ductwork is attached to the return grille (and all others) then its safe to use non-plenum rated cable, but as the previous posts comment on, the local building code supersedes all. In some areas they might require for the cables to run thru metal conduit regardless how the duct work is installed.

Also my gut feeling is that if you have a 20 foot high cavity above the ceiling, the return plenum is directly attached to the grilles. Remeber that the air above the ceiling is consider conditioned air, so adding an addtional 20 feet into the room height would add directly to the heating and cooling load. Also to note the reason that plenum rated cable is required when the ceiling cavity is used as the return plenum; is that if PVC cable was used the toxic gases would be transported by the HVAC system to other parts of the building.

Also if there are cables now installed you can tell by looking at the outerjacket of the cable. Your are looking for the letters CMR (PVC) or CMP (Plenum rated).

Here is a link for cabling in general:

http://cablingdb.com/Standards/568B/TIA-EIA-568TOC.asp

Kindest regards,

Joel_Sisko