Low Voltage Cabling and Wiring Methods
Cable markings you must read
Low-voltage cable is listed for a use, not just a voltage. Communications cable uses CM, CMR, and CMP; the P suffix means plenum and the R means riser. Power-limited control and signal cable uses CL2, CL3, and their P/R versions. Fire alarm cable uses FPL, FPLR, and FPLC. CATV/coax and network-powered broadband have their own Article 800/820/830 listings.
Substitution and location
- A higher fire/location rating can replace a lower one in the same family: CMP may be used where CMR or CM is required, and CMR where CM is required. The reverse is not permitted.
- In a plenum, NEC 300.22(C) requires a plenum-listed wiring method; typically CMP, CL2P/CL3P, or FPLC as applicable. “Low smoke” is not the same as plenum-listed.
- Use 300.15 boxes or fittings wherever a cable terminates or is spliced; do not leave an open-ended jacket in a ceiling.
- Support horizontal cable at the intervals required by the article and manufacturer, typically 4.5 ft (1.4 m) for UTP, and keep bend radius within the cable datasheet-often four times the outside diameter for UTP.
Separation and fill
Class 2/3 and communications cables generally may not be installed in the same raceway with power conductors unless the article permits it and separations or barriers are maintained. Raceway fill is limited by Chapter 9, Table 1; for more than two conductors, the usual limit is 40 percent of internal area.
Worked example
A 1 in. EMT has an internal area of 0.864 in²; 40 percent is 0.346 in². Four Cat 6 cables at about 0.049 in² each use 0.196 in². Six 18/2 cables at about 0.020 in² each use 0.120 in². Total is 0.316 in², just under 0.346 in², so the bundle fits. Adding one more Cat 6 would raise it to 0.365 in² and exceed the limit; use a larger raceway or a second pathway.
Traps
- Assuming any cable with a “P” is acceptable in a plenum; check the circuit type and article.
- Using CL2P for fire alarm initiation or notification; fire alarm cable must be FPL-rated.
- Overfilling conduit because “data cable is small.”
- Bundling many cables and ignoring ampacity/heat rise, especially with PoE.