For the past few National Electrical Code (NEC) cycles, there has been a concentrated movement by parts of the construction industry to permit fire alarm cables, burglar alarm cables, communications cables and signaling system cables to be installed in a fabricated duct or plenum used for environmental air. Is this an acceptable use of this duct or plenum? What wiring methods are currently permitted to be installed in these ducts or plenums in the NEC? These questions and their answers may help keep the installer from erroneously installing a system that must be removed based on a violation notice from an inspector or a call back notice from a customer due to “unknown noise” whenever the air conditioning system operates.

The answers to these and related questions actually start in Article 100 of the NEC with the definition of “plenum,” since there isn’t a definition of a “duct.” A plenum is defined as “compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.” Not very clear is it? This is the same definition located in 3.3.22 of NFPA 90A document, the Standard for the Installation of Air--Conditioning and Ventilating Systems, the origin of the definition.

NFPA 90A further defines plenum as “air-handling unit room plenums,” “apparatus casing plenums,” “ceiling cavity plenums,” and “raised floor plenums.” Then there is an annex entry that further explains plenum in A.3.3.22 of NFPA 90A, covering supply, return, exhaust, outside, and mixed air plenum. NFPA 90A defines “duct” or “air duct” as “a conduit or passageway for conveying air to or from heating, cooling, air conditioning, or ventilating equipment but not including a plenum.” Easy? Not exactly.

Determining the wiring methods permitted in these areas would be difficult at best, if not for the NEC. The NEC adequately covers all of the aformentioned areas in Section 300.22. Section 300.22(A) covers ducts specifically fabricated for transporting dust, loose stock or flammable vapors, and no wiring of any type is permitted in these ducts. In addition, no wiring shall be used in a duct used for vapor removal or ventilation from commercial cooking hoods, other than luminaires permitted by 410.10(C) within the hood itself.

Section 300.22(B) covers ducts that are specifically fabricated to transport environmental air. Within these specifically fabricated ducts, electrical equipment and devices are only permitted to be installed if necessary for the direct action on, or sensing of, the contained air within the duct.

Only the following wiring methods are permitted to be used to make the connection to the electrical equipment and devices within the fabricated duct: Type MI cable, Type MC cable with a smooth or corrugated metal sheath (not the interlocking type MC cable) without a nonmetallic covering on the cable, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, or rigid metal conduit without nonmetallic jacket on the conduit. Flexible metal conduit can be installed only for lengths not exceeding 4 feet and then only to connect to physically adjustable equipment and devices. Section 300.22(C) covers all “other spaces used for environmental air” and provides the installation requirements for wiring methods and electrical equipment permitted in these spaces. To make it simple, if it is specifically fabricated as a duct for use in air movement for environmental air, refer to 300.22(B). For any other space used for environmental air, refer to 300.22(C).

Section 725.154(A), covering plenum rated Class 2, Class 3, and power limited tray cable (PLTC) in ducts, and Sections 760.3(B), 760.53(B)(1) and 760.154(A), covering plenum-rated non-power-limited and plenum-rated power-limited fire alarm cable, only permit these plenum-rated cables to be installed in accordance with the requirements in Section 300.22. Sections 800.154(A), covering plenum-rated communications cables; 820.154(A), covering plenum-rated CATV (coaxial) cables; and 830.151(A), covering plenum-rated medium--powered broadband communications cables, and 830.154(A), covering plenum-rated low-powered broadband communications cables, also require compliance with 300.22. Effectively, plenum--rated cables may be installed only in a specifically fabricated duct using the wiring methods provided in 300.22(B).

To answer the question raised in the first paragraph of this article, fabricated ducts never should have open plenum cables, or unlimited lengths of any wiring methods, installed within the duct, since air movement can cause noise, and over a period of time, the constant movement may damage the outer cable jacket, thereby affecting the operation of the system. A fabricated duct used for environmental air should never be used as a conduit for cabling systems.

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 and at mark.c.ode@us.ul.com.