Class 2 Cable in a Return-Air Plenum
A general contractor is renovating a 4,000 sq ft office suite in Portland. The cavity above the suspended ceiling is used as a return-air plenum. The low-voltage subcontractor installed a listed 120V/24V Class 2 power supply rated 100 VA above the ceiling and fed it from a receptacle located above the ceiling. From the supply, the installer ran 16 AWG CL2 cable to 12 LED downlights and a wall dimmer. In one area the cable is tie-wrapped to a sprinkler pipe. In another, it passes through a 2-hour rated corridor wall in a 1-inch sleeve with no sealant. The electrical inspector red-tags the job. The subcontractor argues that because the circuit is only 24V, Article 725 does not apply and that plenum rules are only for line-voltage wiring. The general contractor wants to know whether the cable can stay if the splices are moved into a metal box.
1. Which NEC articles govern the 24V circuit, and is the subcontractor correct that Article 725 does not apply? Explain how the plenum classification changes the answer.
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Article 725 applies to Class 2 circuits regardless of the 24V level; low voltage is not exempt. Article 300.22(C) governs wiring in a return-air plenum. In a plenum, cables must be listed for plenum use, such as CL2P or CL3P, or installed in metal raceway, and equipment must be listed for the environment. The installed CL2 cable is not plenum-rated, so it fails 300.22(C) even though it is Class 2. The power supply must also be listed and installed per its listing; a cord-and-plug connection above a suspended ceiling may be inaccessible and may violate the requirement that the receptacle be accessible. Article 725 still requires separation, support, and listing. The subcontractor is wrong on both points.
2. List each defect in the installation and the correction. Address support, the rated wall, and the plenum.
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Defects include: (1) CL2 cable exposed in the plenum: replace with CL2P cable or install it in EMT, IMC, or RMC with metal boxes; (2) cable tie-wrapped to a sprinkler pipe: sprinkler pipe cannot support cable, so provide independent support per 300.11; (3) penetration of a 2-hour rated wall with an unsealed sleeve: firestop the annular space with a listed system per 300.21 and local code; (4) power supply and receptacle above the ceiling: verify the supply is listed for the location and the receptacle remains accessible; (5) splices: use listed boxes and connectors, keep them accessible, and do not bury them. Moving splices into a metal box does not cure the plenum-rating defect if the cable remains exposed in the plenum.
3. If the contractor wants to keep the existing CL2 cable, what code-compliant alternatives exist for the plenum and the rated-wall crossing?
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You cannot keep CL2 cable exposed in the plenum. Alternatives are to install the CL2 cable inside EMT, IMC, or RMC with listed metal boxes; the raceway provides the required protection. For the rated wall, use a fire-rated sleeve or firestop system and keep the raceway continuous through the wall, sealing the annular space with a listed firestop material. Alternatively, replace the cable with CL2P and still firestop the wall penetration. Any box must remain accessible, and splices must be made with listed connectors. Verify local amendments and the manufacturer's listing before choosing the method.