Section 300.22/Wiring in Ducts and Plenum Spaces
NEC 300.22 restricts wiring methods in plenums to prevent smoke and toxic fume propagation through air-handling spaces.
Wiring in ducts and plenums used for environmental air is highly restricted. Within ducts specifically fabricated for air handling, no wiring is permitted except as necessary for the duct system itself. In other spaces used for environmental air (above-ceiling plenums), only wiring methods with inherently low smoke and toxicity are permitted: MI cable, MC cable with impervious outer jacket, and plenum-rated cables (CMP, CATVP, etc.). Standard NM cable (Romex), AC cable, and non-plenum-rated cables are not permitted in plenum spaces.
Why this section exists
The space above a suspended ceiling is often used as a return air plenum. Air from the occupied space passes through ceiling tiles into this space and returns to the air handler. If a fire starts in the plenum or if cables burn, the smoke and toxic fumes from cable insulation are distributed throughout the building by the air handling system. Plenum-rated cables use insulation that produces minimal smoke and toxic fumes when burned, reducing the hazard.
What plan reviewers look for
Plan reviewers check the architectural reflected ceiling plan and the mechanical drawings to identify which ceiling spaces are used as return air plenums. They then verify that the electrical specifications call for plenum-rated cables in these spaces. They check that conduit or MC cable is used for power wiring and that low-voltage cabling (data, fire alarm, communications) is plenum-rated.
Common violations
Related NEC requirements
NFPA 90A Section 4.3 covers fire protection requirements for duct construction and materials in plenum spaces. Section 725.154 covers plenum-rated Class 2 and Class 3 cables. Section 760.53 covers plenum-rated fire alarm cables.