Cable Fire Ratings Explained: CM, CMR, CMP, CL2 and CL3

CM, CMR, CMP, and CL2 or CL3 are fire ratings, not performance ratings. They describe how a cable behaves in a fire and how far flame travels along its jacket. Building code decides which one you are allowed to install, and getting it wrong can fail an inspection or, worse, not fail one.

The communications cable ratings

These apply to network cable and are defined in NFPA 70 (the National Electrical Code), Article 800.

• CM (Communications, general purpose): the baseline. Suitable for general use but not for risers or plenums. Typically what patch cables and non-permanent runs use.
• CMX: limited use, generally residential and restricted to dwellings. Also the rating you will see on outdoor and direct-burial cable.
• CMR (Riser): rated for vertical runs between floors. Riser cable resists carrying flame from floor to floor. Required in shafts and vertical pathways.
• CMP (Plenum): the highest rating. Required in plenum spaces, meaning any space used for air circulation, most commonly the void above a drop ceiling that returns air to the HVAC system. Plenum jackets produce less smoke and are more flame-resistant, which is why they cost noticeably more.

The hierarchy runs CMP > CMR > CM > CMX. A higher rating can substitute for a lower one. CMP cable can go anywhere CMR can. The reverse is not true.

CL2 and CL3

These come from a different article of the code (Article 725, Class 2 and Class 3 circuits) and you will see them on HDMI, speaker, and control cable rather than on network cable. They indicate the cable is rated for in-wall installation and specify voltage and power limits.

CL3 permits higher voltage than CL2. For in-wall HDMI runs, CL2 or CL3 is what building code typically requires, and an uncertified HDMI cable run inside a wall is a code violation regardless of how well it works.

How to decide

1. Is it going in a plenum space? Above a drop ceiling that handles return air, or in a raised floor used for air handling. Use CMP.
2. Is it running vertically between floors? Use CMR at minimum.
3. Is it going inside a wall, but not a riser or plenum? CM for network, CL2 or CL3 for HDMI and low-voltage AV.
4. Is it outdoors or buried? Use CMX with a UV-resistant jacket, or direct-burial rated cable with a PE jacket.
5. Is it a patch cable in a rack? CM is fine.

Why it is worth getting right

Plenum cable costs more than riser, and riser costs more than general purpose, so there is a real temptation to run the cheaper one. Two reasons not to. An inspector who catches it will make you pull and replace, which costs far more than the cable did. And in a fire, a non-plenum jacket in a return-air plenum puts smoke into every room the HVAC serves.

Nippon Labs stocks bulk Cat5e, Cat6, and Cat6a in CM, CMR, and CMP, plus outdoor CMX and direct-burial. If you are unsure which the job calls for, the answer usually comes from the pathway rather than the network.