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 which has specific rules when circuit wiring is installed in an air plenum.

NEC Section 300.22(C)(1) specifically allows factory-assembled multiconductor control or power cable without being installed in a metal raceway or box if the cable is specifically listed for use within an air-handling space.

NEC 725.179 provides additional requirements for the proper listing and marking of Class 2 and 3 cables often used in security system wiring. Part of the testing of a plenum rated cable includes ensuring that the cable exhibits a maximum peak optical density of .50 or less, an average optical density of .15 or less, and a maximum flame spread distance of 5 feet or less when tested in accordance with NFPA 262 which is the Standard Method of Test for Flame Travel and Smoke of Wires and Cables for Use in Air-Handling Spaces.

Testing and listing of plenum rated wiring methods 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 cable 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.

725.179 Listing and Marking of Class 2, Class 3, and Type PLTC Cables. Class 2, Class 3, and Type PLTC cables, installed as wiring methods within buildings, shall be listed as resistant to the spread of fire and other criteria in accordance with 725.179(A) through (I) and shall be marked in accordance with 725.179(J).

(A) Types CL2P and CL3P. Types CL2P and CL3P plenum cable shall be listed as suitable for use in ducts, plenums, and other space for environmental air and shall be listed as having adequate fire-resistant and low-smoke producing characteristics.

Informational Note: One method of defining a cable that is low smoke producing and fire resistant is that the cable exhibits a maximum peak optical density of 0.50 or less, an average optical density of 0.15 or less, and a maximum flame spread distance of 1.52 m (5 ft) or less when tested in accordance with NFPA 262 -2019, Standard Method of Test for Flame Travel and Smoke of Wires and Cables for Use in Air-Handling Spaces.