Extensive revisions have been made to the language of this section, and a new subsection was added addressing cable trays.

300.22 Wiring in Ducts Not for Air Handling, Fabricated Ducts for Environmental Air, and Other Spaces For Environmental Air (Plenums). The provisions of this section apply to the installation and uses of electrical wiring and equipment in ducts used for dust, loose stock, or vapor removal; ducts specifically fabricated for environmental air, and spaces used for environmental air (plenums).

(A) Ducts Used for Dust, Loose Stock, or Vapor. Ducts that transport dust, loose stock, or vapors must not have any wiring method installed within them.

(B) Ducts Specifically Fabricated for Environmental Air. If necessary for direct action upon, or sensing of, the contained air, Type MC cable that has a smooth or corrugated impervious metal sheath without an overall nonmetallic covering, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, or rigid metal conduit without an overall nonmetallic covering can be installed in ducts specifically fabricated to transport environmental air. Flexible metal conduit in lengths not exceeding 4 ft can be used to connect physically adjustable equipment and devices within the fabricated duct.

Equipment is only permitted within the duct specifically fabricated to transport environmental air if necessary for the direct action upon, or sensing of, the contained air. Equipment, devices, and/or illumination are only permitted to be installed in the duct if necessary to facilitate maintenance and repair.

(C) Other Spaces Used for Environmental Air (Plenums). This section applies to wiring and equipment in spaces not specifically fabricated for environmental air-handling purposes (plenums) but used for air-handling purposes as a plenum. This requirement doesn’t apply to habitable rooms or areas of buildings, the prime purpose of which isn’t air handling.

Note 1: The spaces above a suspended ceiling or below a raised floor used for environmental air are examples of the type of space to which this section applies. (click here to see Fig. 12)

Note 2: The phrase “other space used for environmental air (plenum)” correlates with the term “plenum” in NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, and other mechanical codes where the plenum is used for return air purposes, as well as some other air-handling spaces.

(1) Wiring Methods. Electrical metallic tubing, rigid metal conduit, intermediate metal conduit, armored cable, metal-clad cable without a nonmetallic cover, and flexible metal conduit can be installed in environmental air spaces. If accessible, surface metal raceways or metal wireways with metal covers can be installed in environmental air spaces.

(2) Cable Tray Systems.

(a) Metal Cable Tray Systems. Metal cable tray systems can be installed to support the wiring methods and equipment permitted by this section. (click here to see Fig. 13)

(3) Equipment. Electrical equipment with a metal enclosure or nonmetallic enclosures listed for use within an air-handling space (plenum) and having adequate fire-resistant and low-smoke-producing characteristics can be installed.

Analysis: There’s long been confusion about the terms “plenum” and “other space(s) used for environmental air.” While nearly all mechanical codes (NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, International Mechanical Code (IMC), and Uniform Mechanical Code (UMC)) use the term “plenum” for anything that moves environmental air, the NEC only referred to a physically constructed duct as a “plenum.” The space beneath a raised floor and the space above a suspended ceiling (when used for air handling) were referred to as “other spaces used for environmental air.” Obviously, the Code shouldn’t be the document that defines air system components, yet manufacturers were forced to have their product literature include provision for these “other spaces,” which created massive confusion for people installing, designing, and inspecting mechanical systems. The confusion was rampant enough to require an Informational Note in the NEC trying to explain what these spaces really were. With the change, a component of an air-handling system that’s created for the sole purpose of moving air (such as a duct made of tin or drywall) is now called a “duct specifically fabricated for environmental air.” The space above a ceiling or below a floor that’s used for air moving isn’t fabricated for that sole purpose; therefore, it isn’t a “duct,” according to mechanical codes. These spaces are now referred to as “other spaces used for environmental air (plenums),” which correlates with other codes and also has the desired effect of using commonly accepted trade language — most people refer to the space above a suspended ceiling as a “plenum ceiling,” not an “other space used for environmental air ceiling.”

Additional changes to this section include a new provision dealing with cable trays in other spaces used for environmental air (plenums), which requires these cable trays to be metallic.

Solid metal cable trays with metal covers can be used to support and enclose wiring methods that traditionally weren’t allowed in these locations.