Low Voltage and Power Limited Circuits
Article map
Article 725 covers Class 1, 2, and 3 remote-control and signaling circuits. Article 760 covers fire alarm, 770 optical fiber, 800 communications, 820 community antenna and cable TV, and 830 network-powered broadband. Class 2 and 3 circuits are defined by their power source and are limited by 725.121 and its tables; the source must be a listed Class 2 or 3 power supply, a listed transformer, or another permitted source.
Separation and installation
- 725.136(A): Class 2 and 3 conductors cannot share a raceway, box, or enclosure with power, Class 1, or non-power-limited fire alarm conductors unless separated by a permanent barrier.
- 300.22(C): cables in environmental air spaces must be plenum rated.
- 725.144 addresses ampacity of bundled Class 2 cables, which matters for long PoE bundles.
- Cable markings such as CL2, CL3, CL2P, and CL3P tell you the permitted location.
Worked example
A 24 V Class 2 cable for a door strike crosses a ceiling used as a return-air plenum and shares a gutter with a 120 V circuit. Reasoning: the plenum requires a CL2P or CL3P cable under 300.22(C); inside the gutter, 725.136(A) requires a permanent barrier or a separate compartment. A standard CL2 cable fails both tests even though the circuit is only 24 V.
Traps
- Assuming low voltage means no rules, no permit, or no separation.
- Using riser-rated cable in a plenum.
- Exceeding the Class 2 source limits in 725.121.
- Mixing Article 725 and Article 800 substitution rules.