Low-voltage cable types and where they can go
Power-limited cables, such as Class 2 doorbell, thermostat and control cable, must be listed and the right type for the space: plenum-rated in air-handling plenums and generally riser- or plenum-rated in vertical runs between floors. Table 722.135(E) lists allowed substitutions.
What the code requires
Section 722.135 is part of Article 722, new in the 2023 NEC, which gathers general cable rules for power-limited circuits that used to sit in Articles 725 and 760. That includes Class 2 and Class 3 circuits, such as doorbell, thermostat and control wiring, power-limited fire alarm circuits, and the new Class 4 fault-managed power circuits. Cables installed in buildings must be listed.
Table 722.135(B) sets which cable type may go where. In plenum spaces used for air handling, only plenum-rated cable is allowed unless it is in a metal raceway that meets 300.22(C) or in a solid-bottom metal cable tray with solid metal covers. Vertical runs through floors and in shafts generally need riser- or plenum-rated cable, with exceptions such as metal raceways, fireproof shafts and one- and two-family dwellings. Limited-use cable is allowed only in one-, two- and multifamily dwellings and only if it is under 0.25 in. in diameter, and its exposed length in nonconcealed spaces is limited to 10 ft.
Table 722.135(E) lists which cable types may substitute for others. For example, CL2 can be replaced by CL2R, CL2P, CL3, CM or CMR, among others, while CL2P can be replaced only by CMP or CL3P. In industrial establishments serviced only by qualified persons, exposed PLTC cable must be secured at intervals of no more than 6 ft, and in hoistways cables must run in specific raceway types, such as rigid metal conduit or EMT.
Four-pair cables that carry both power and data, such as Power over Ethernet, must also follow 725.144. Circuits that run beyond one building and are exposed to lightning or to accidental contact with power conductors over 300 V to ground follow listed Chapter 8 protection rules and Part I of Article 300.
On the job
- Ordinary non-plenum doorbell or thermostat cable run loose through a plenum space covered by 300.22(C) does not comply; that space needs plenum-rated cable unless a permitted raceway is used.
- Unlisted cable does not comply. General-purpose cable in a vertical run between floors generally does not either, unless the table allows it, such as in a metal raceway, a fireproof shaft or a one- or two-family dwelling.
- When the specified cable is not on hand, a substitute is acceptable only if Table 722.135(E) allows it.
What changed in 2023
The 2023 NEC created Article 722 for cable rules shared by these circuits, moved Class 1 circuits to Article 724, and left Article 725 covering only Class 2 and Class 3 circuits.
Related sections
- NEC 722.24
- NEC 725.144
- NEC 300.22
- NEC 300.21
- NEC 722.179
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