NFPA 731 Section 4.6.3.1 requires the installation of all wiring, cable, and equipment to be performed in accordance with NFPA 70 (NEC), Article 725 or Article 800, where applicable. Many circuits in a security system are “signal circuits” and NEC^® Article 725 must be followed.

One requirement in NEC^® 725.3(C) is to ensure any Class 1, 2 or 3 remote-control, signaling, and power-limited circuits are installed in accordance with NEC^® 300.22. This section has specific rules when circuit wiring or equipment is installed in ducts used for dust, loose stock, or vapor removal; ducts specifically fabricated for environmental air; and other spaces used for environmental air (plenums).

The NEC^® defines a plenum in Article 100 as follows:

Plenum. A compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.

The International Mechanical Code defines a plenum in Chapter 2 as follows:

Plenum. An enclosed portion of the building structure, other than an occupiable space being conditioned, that is designed to allow air movement, and thereby serve as part of an air distribution system.

The area above some suspended grid ceilings are constructed as a plenum but not all.

Only certain wiring methods are permitted in a plenum. Plenums can exist in the space above a suspended ceiling where an air conditioner may be present. Here are two rules to determine if the space above a grid ceiling is a plenum:

• If the above grid AC unit pulls return air from the office space below, through a dummy grill, instead of return air ductwork, then the return air circulates above ceiling where wiring methods also exist. This is a plenum (see image).

• If the above grid AC unit pulls return air from the office space below, through a ducted return air grill, then the return air does not circulate among the wiring methods above ceiling. This is NOT a plenum.

Wiring methods in a plenum are generally limited to metal cables, metal boxes, metal raceways and certain nonmetallic jacketed plenum rated cables with low smoke and heat release properties. Testing and listing of plenum rated wiring methods and equipment has nothing to do with the toxicity of the fumes released when something melts but has everything to do with visible smoke release and optical density levels. The idea is to make sure that if a wiring method melts in a plenum, the smoke released back into the return air, and recirculated back into the building supply air system isn't so dense that it blocks emergency lights and exit signs from being seen by building occupants during egress.

Below is a sample of NFPA 731. For the complete section, see the actual NFPA 731 text at NFPA.ORG. Once there, click on the free access link to the latest edition of  NFPA 731.

4.6.3 Wiring.

4.6.3.1 The installation of all wiring, cable, and equipment shall be performed in accordance with NFPA 70, Article 725 or Article 800, where applicable.

Below is a sample of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the latest NEC^® edition of NFPA 70.

300.22 Wiring in Ducts Not Used for Air Handling, Fabricated Ducts for Environmental Air, and Other Spaces for Environmental Air (Plenums). The provisions of this section shall 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 other spaces used for environmental air (plenums)…

(C) Other Spaces Used for Environmental Air (Plenums). This section shall apply to spaces not specifically fabricated for environmental air-handling purposes but used for air-handling purposes as a plenum. This section shall not apply to habitable rooms or areas of buildings, the prime purpose of which is not air handling.

Exception: This section shall not apply to the joist or stud spaces of dwelling units where the wiring passes through such spaces perpendicular to the long dimension of such spaces.

(1) Wiring Methods. The wiring methods for such other space shall be limited to totally enclosed, nonventilated, insulated busway having no provisions for plug-in connections, Type MI cable without an overall nonmetallic covering, Type MC cable without an overall nonmetallic covering, Type AC cable, or other factory-assembled multiconductor control or power cable that is specifically listed for use within an air-handling space, or listed prefabricated cable assemblies of metallic manufactured wiring systems without nonmetallic sheath. Other types of cables, conductors, and raceways shall be permitted to be installed in electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, rigid metal conduit without an overall nonmetallic covering, flexible metal conduit, or, where accessible, surface metal raceway or metal wireway with metal covers.

Nonmetallic cable ties and other nonmetallic cable accessories used to secure and support cables shall be listed as having low smoke and heat release properties.