Navigating the requirements in the National Electrical Code (NEC) for ducts, plenums, and other air-handling spaces can be difficult. Air-handling systems have the potential to convey smoke, hot gases, and flames from one area of a building into another, as well as providing air supply for a potential fire.

Electrical wiring methods used in these areas should neither add to the fuel load of a fire nor add smoke to the air-handling system.

NEC Section 300-21 provides a very basic statement. It requires that electrical installations in hollow spaces, vertical shafts, and ventilation or air-handling ducts be made so the possible spread of fire or products of combustion, such as smoke, will not be substantially increased. Before determining the wiring methods that can be used in ducts, plenums, or other air-handling spaces, it is necessary to understand the terminology used when dealing with air-handing systems.

Section 1-6 of National Fire Protection Association 90A, the “Standard for the Installation of Air Conditioning and Ventilating” (1996 Edition) provides definitions for air-handling systems––a good starting point in understanding terminology.

An air distribution system is defined as a continuous passageway for the transmission of air. This distribution system can consist of air ducts, air connectors, duct fittings, dampers, plenums, fans, and accessory air-handling equipment.

An air duct is defined as a conduit for conveying air. Environmental air is air that is supplied, returned, re-circulated, or exhausted from spaces for the purpose of modifying the existing atmosphere within the building.

Finally, a plenum is defined in NFPA 90A as a compartment or chamber to which one or more air ducts are connected that form part of the air distribution system. A plenum cannot be used as an occupied space or for storage of any materials. Article 100 of the NEC provides the same definition for plenum as that found in NFPA 90A.

The word “plenum” in the NEC is used to describe both an air space and also various types of wiring methods that are specifically designed and tested for installation within these areas.

Section 300-22(a) of the NEC provides information on the installation and uses of electrical wiring and equipment in ducts that are used to transport dust, loose stock, or flammable vapors. These ducts can be used for vapor removal or for ventilation of commercial-type cooking equipment.

These ducts can be connected into a larger vertical or horizontal shaft or can be installed as a single duct without a shaft connection. No wiring or electrical equipment can be installed in these ducts; however, lighting fixtures (luminaires in the 2002 NEC) are permitted in the commercial cooking hood where they meet all of the conditions stated in Section 410-4(c).

Section 300-22(b) provides requirements for ducts or plenums used for environmental air. These ducts or plenums, such as metal or fiberglass ducts, are those specifically fabricated to transport environmental air. These ducts can be either fabricated at the site or brought in pre-fabricated and assembled at the site.

Rigid Metal Conduit (RMC), Intermediate Metal Conduit (IMC), Electrical Metallic Tubing (EMT), Mineral Insulated cable (MI cable), and Metal-Clad Cable (Type MC cable) employing a smooth or corrugated impervious metal sheath without a nonmetallic covering can be used for wiring within these fabricated ducts or plenums.

Electrical equipment and devices are permitted within these ducts or chambers only if they are needed for the direct action on or sensing of the air contained within the duct. Flexible metal conduit and liquidtight flexible metal conduit can be used within fabricated ducts to connect adjustable equipment and similar devices but are limited to four feet. If illumination is required inside the fabricated duct to assist in maintenance and repair of equipment located within the duct, an enclosed and gasketed fixture must be used.

An “other space used for environmental air” is one that is not specifically fabricated for environmental air but is used for transportation of either supply or return air. The space over a drop ceiling is a typical “other space.” The same wiring methods can be used in these “other spaces” as used in fabricated ducts, with the addition of Armored Cable (Type AC cable), and totally enclosed, nonventilated, insulated busway without provisions for plug-in connections.

Factory-assembled multiconductor control or power cable specifically listed for use in these other spaces can be used. Surface metal raceways or metal wireways can also be used. In an “other space for environmental air,” liquidtight flexible metal conduit is permitted in single lengths of 6 feet or less.

Electrical equipment installed in these areas must be metal enclosed. However, electrical equipment with a nonmetallic enclosure and associated wiring materials can be installed if they are listed for use in these areas. They must have adequate fire-resistance and be low-smoke producing. Any wiring method used in these areas must be suitable for the ambient temperature in the other space.

Following these special rules will help provide the safety to life and property to which the NEC is dedicated and will help restrict the spread of fire and products of combustion within buildings.

ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or via e-mail at mark.c.ode@us.ul.com.