Code Q&A: Installing Wiring in Plenum Spaces for Environmental Air

Jan. 12, 2023

Test your knowledge of the NEC.

Q. What are the NEC requirements related to the installation of wiring within plenum spaces for environmental air?

See answer below.

A. The requirements of Sec. 300.22 apply to the installation and uses of electrical wiring and equipment in plenum spaces for environmental air.

(C) Plenum Spaces for Environmental Air. This section applies only to the space above a suspended ceiling or below a raised floor used for environmental air. It does not apply to habitable rooms or areas of buildings, the prime purpose of which is not air handling.

Note 1: The spaces or cavities above a suspended ceiling and below a raised floor used for environmental air are examples of the type of plenum spaces to which this section applies. (Figure)

(1) Wiring Methods. Metal raceways, Type AC cable, and Type MC cable without a nonmetallic cover, electrical metallic tubing, intermediate metal conduit, rigid metal conduit, flexible metal conduit, or (where accessible) surface metal raceways or metal wireways with metal covers are permitted to be installed in the plenum space.

Cable ties for securing and supporting cables must be listed for use in a plenum space.

Plenum-rated Chapter 7 and Chapter 8 wiring methods are permitted in plenum spaces according to the following:

Class 2 Power-Limited Cables [Sec. 725.3(B)]
Coaxial Cables [Sec. 800.3(C)]
Fire Alarm Cables [Sec. 760.3(B), Exception No. 2]

(2) Cable Tray Systems.

(a) Metal Cable Tray Systems. Metal cable tray systems can be installed to support the wiring methods permitted to be installed in a plenum space.

(3) Equipment. Electrical equipment with a metal enclosure or a nonmetallic enclosure listed for use in an air-handling space can be installed in a plenum space.

Examples of electrical equipment permitted in plenum spaces are air handlers, junction boxes, and dry-type transformers although transformers are not permitted to be rated over 50kVA when in hollow spaces [Sec. 450.13(B)].

These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.

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.