Code Q&A: Approved Wiring Methods in Environmental Air Spaces

March 7, 2013

Your most pressing National Electrical Code (NEC) questions answered

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)].

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.