Low Voltage and Limited Energy Circuits, Cable, and Raceways
The article map
Low-voltage work is not one NEC article. Article 725 covers Class 2 and Class 3 circuits; Article 800 covers communications circuits; Article 820 covers coaxial cable; Article 830 covers network-powered broadband. Cable markings tell you where a cable may be installed: CM/CMR/CMP for communications, CL2/CL2R/CL2P and CL3/CL3R/CL3P for Class 2 and Class 3, and FPL/FPLR/FPLP for fire alarm. The suffix R means riser-rated and P means plenum-rated.
Core rules and concepts
- A plenum ceiling generally requires a plenum-rated cable (CMP, CL2P, CL3P, or FPLP); a riser-rated cable is not a substitute.
- Cable substitution is permitted only where the article allows a higher-rated cable to replace a lower-rated one, such as plenum-rated cable in a riser.
- Class 2 and Class 3 cables must be separated from power conductors unless the installation method or a barrier is permitted.
- Supports, bend radii, and firestopping must follow the cable article and the building code.
- Limited-energy contractors must stay within Oregon's limited-energy scope and permit rules.
Worked example
A communications cable runs through a return-air plenum and then up a riser. The plenum portion requires CMP. A CMR cable is not acceptable in the plenum even though it is acceptable in the riser. If the contractor installs CMP throughout, that is permitted because the plenum rating exceeds the riser requirement.
Traps that cost marks
- Using CL2 cable for a fire alarm circuit that requires FPL.
- Running Class 2 cable in the same raceway as line voltage without an allowed separation method.
- Treating "accessible" as equivalent to "plenum-rated."
- Forgetting firestopping where cable penetrates a rated assembly.