Q. What wiring methods does the NEC allow above a suspended ceiling or other spaces used for environmental air?

A. Electrical metallic tubing (EMT), rigid metal conduit (RMC), intermediate metal conduit (IMC), armored cable, metal-clad (MC) cable without a nonmetallic cover, and flexible metal conduit (FMC) can be installed in a cavity plenum space. If accessible, surface metal raceways or metal wireways with metal covers can be installed in a cavity plenum space [300.22(C)(1)].

PVC conduit [Art. 352], electrical nonmetallic tubing [Art. 362], liquidtight flexible conduit, and nonmetallic cables aren't permitted to be installed in spaces used for environmental air because they give off deadly toxic fumes when burned or superheated. Plenum-rated control, signaling, and communications cables and raceways are permitted in a cavity plenum space. See these specific sections in the NEC:

• CATV [820.179(A)]

• Communications [800.21]

• Control and signaling [725.154(A)]

• Fire alarm [760.7]

• Optical fiber cables and raceways [770.113(C)]

• Sound systems [640.9(C) and 725.154(A)]

Any wiring method suitable for the condition can be used in a space not used for environmental air-handling purposes.

Metal cable tray systems can be installed to support the wiring methods and equipment permitted by this section [300.22(C)(2)].

Electrical equipment with metal enclosures is permitted to be installed in a cavity plenum space [300.22(C)(3)]. Examples of electrical equipment permitted in a cavity plenum space would be air-handlers, junction boxes, dry-type transformers; however transformers must not be rated over 50kVA when located in hollow spaces [450.13(B)].