CM Cable Jacket
CM stands for “Communications Multipurpose.” It is a cable jacket fire resistance rating defined in Article 800 of the NEC. In the case of CM, the standard outlines general use, or use in cases where the fire code does not place any restrictions on cable type.
The CM cables are generally the minimum jacket ratings suitable for in-wall installation of Ethernet cables and are appropriate for installation inside a residence or a single-story commercial building. As with any in-wall rated cable, the goal is to prevent fire from traveling along a cable from one part of a building to another. The Ethernet cables labeled “Type CM” must pass a standardized flammability test before the cables are put into use.
CMR Cable Jacket
CMR cable stands for “Communications Multipurpose Cable, Riser”. It is a cable jacket fire resistance rating defined in Article 800 of the NEC. CMR cable is also referred to as “riser-rated cable” and designed to prevent fires from spreading between floors through risers or vertical shafts.
The goal of CMR cable is to be flame-retardant enough to prevent the spread of fire from one floor to another. The fire resistance requirements for CMR are much stricter than that of CM. And CMR cable is more flame-retardant and consequently more expensive than CM cable.
CMP Cable Jacket
CMP cable stands for “Communications Multipurpose Cable, Plenum”. It is a cable jacket fire resistance rating defined in Article 800 of the NEC. CMP cable is also referred to as “plenum-rated cable”. It is constructed to restrict flame propagation to no more than five feet and limit the amount of smoke emitted during a fire.
CMP or plenum-rated cables adopt materials that burn more cleanly and self-extinguish more easily. Air travels throughout a building via plenum spaces, so it is critical that cables installed in such spaces use this kind of material in the construction — won’t give off toxic fumes if burned.
Thus, a CMP cable is suitable for installation into air plenum spaces. An example of the most common types of plenum spaces would be a dropped ceiling or raised floor.