Plenum-Rated Class 2 Cabling in a Return-Air Ceiling
An Albany-area integrator is installing a 24 Vdc, Class 2 access-control system in a renovated 1970s office building in Rochester. The panel is a listed power-limited supply rated 2 A. From the panel, 16 CL2R cables leave a closet and run 35 ft through the return-air plenum above a suspended ceiling, then another 40 ft in 3/4 in. EMT to door controllers. In the plenum, the installers bundled all 16 cables with zip ties every 24 in. for a 12 ft straight section, then crossed two 120 V branch circuits at 90 degrees without a barrier. The ceiling is used as an air-return plenum. The local AHJ red-tagged the job, citing plenum rating, bundling, separation, and support. The contractor argues that the cable is Class 2 and therefore "low voltage" and exempt. The electrical inspector will return in five days.
1. Does the CL2R cable satisfy the plenum requirement? Explain what cable markings would be acceptable and whether CL3R or CL3P can be substituted.
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No. A return-air ceiling is an environmental air space, and NEC 300.22(C) requires cables installed there to be listed for plenum use. In Article 725, that means a CL2P or CL3P marking, not CL2R. The "R" suffix is riser-rated and is intended for vertical shafts, not air-handling spaces. A CL3P cable can generally be used where a CL2P cable is required because Class 3 is a higher circuit classification, but CL3R does not become a plenum cable just because it is Class 3. The same logic applies to communications cable: CMP is the plenum marking, while CMR is riser-rated. The contractor should not rely on the phrase "low voltage" as an exemption. The correction is to replace the 35 ft plenum section with CL2P/CL3P cable, or install the existing cable in a raceway or enclosure listed for plenum use. The replacement cable must be marked and supported per the applicable Article 725 installation requirements, and the AHJ should be shown the cable reel markings or submittals before re-inspection.
2. Evaluate the bundling and separation issues. What correction is needed before the re-inspection?
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The 16-cable bundle and the unbarriered crossing are both legitimate inspection issues. NEC 725.55 requires Class 2 circuit conductors to be separated from electric light and power conductors by at least 2 in. or by a listed barrier. A 90-degree crossing without a barrier is not a listed barrier; wrapping the crossing with tape would not change that. The 120 V branch circuits must be moved, the Class 2 cables must be moved, or a listed barrier/raceway must be installed. Bundling also matters: when many cables are tied together for a sustained length, the AHJ can require an ampacity adjustment and a check of the cable's temperature rating and support method. Even though the access-control load is small, the inspector is entitled to see the calculation or a manufacturer's installation instruction that permits the bundle. The practical correction is to replace the plenum section with plenum-rated cable, spread or support the cables with listed supports instead of over-tightened zip ties, maintain the required separation from the 120 V circuits, and document the method for the inspector. Simply calling the circuits "power-limited" does not eliminate the separation or plenum rules.
3. The contractor proposes to wrap the crossing points with electrical tape and proceed. Is that acceptable? What should be done before re-inspection?
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No. Electrical tape is not a listed barrier for separating Class 2 circuits from power conductors, and it does not correct the plenum-rating violation. Before re-inspection, the contractor should stop work in the affected ceiling area, replace or sleeve the non-plenum cable with a listed plenum method, restore the required 2 in. separation or install a listed barrier, correct the bundling and support, and photograph or otherwise document the corrected installation. The contractor should also verify that the access-control power supply is listed as a Class 2 power source and that the cable is marked for the application. If any of the 120 V crossing work requires opening a junction box or moving branch-circuit conductors, that work should be done by a qualified person under the proper permit. The AHJ should be contacted before the re-inspection so the inspector knows the proposed correction; proceeding with tape would likely result in a second rejection and possible stop-work order.