Q: We have decorative wood panels in our cafeteria ceiling which do not provide a seal to limit the passage of smoke. In fact, there are wide gaps between the wood panels. HVAC supply ductwork is mounted above the wood panels and return air is drawn through the open space above the wood panels as well. Are we required to use plenum rated cabling above the wood panels?
A: The Life Safety Code, 2000 edition, section 3.3.150 defines a plenum as a compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.Given that the above-ceiling space is used as a chamber for the return air of the HVAC system, the space appears to be a plenum and I would conclude that plenum-rated cable should be used. NFPA 70 (1999 edition), section 300-22(b) specifies that Type MI or Type MC cable must be installed in ventilating air plenums. This standard does allow limited lengths of flexible metal conduit and liquid-tight flexible conduit to devices permitted to be in the plenum.