Q. What wiring methods are allowed in ducts and other spaces used for environmental air handling?

A. Section 300.22 addresses this question. 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.

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.

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.