What is CMP cable?
CMP stands for Communications Multipurpose cable, Plenum. It's a fire-safety rating from the U.S. National Electrical Code (NEC), printed on the jacket of communications cable, such as Cat5e or Cat6, that is listed for use in plenum spaces: the air-handling areas of a building, such as the space above a drop ceiling or below a raised floor that is used for return air.
Air in a plenum moves through the building's ventilation system, so smoke from burning cable there would spread quickly. CMP cable uses jacket and insulation materials that resist flame spread and give off less smoke in a fire. It has to pass the strictest of the communications cable fire tests (NFPA 262).
The cable rating hierarchy
| Marking | Meaning | Where it can go |
|---|---|---|
| CMP | Plenum | Plenum spaces, plus anywhere the lower ratings are allowed |
| CMR | Riser | Vertical runs between floors and general areas; not plenums |
| CM / CMG | General purpose | Horizontal runs in ordinary spaces; not risers or plenums |
| CMX | Limited use | Limited uses, such as short runs in houses |
A higher rating can always replace a lower one, so CMP can be used anywhere CMR, CM or CMX is allowed. The reverse is never true. Fiber has matching ratings: OFNP (plenum) and OFNR (riser) for nonconductive optical cable.
Why it costs more
The fire-resistant compounds in plenum cable cost more than the ordinary PVC used in general-purpose cable, so CMP is the most expensive of the communications ratings. Running cheaper general-purpose cable above a return-air ceiling to save money breaks code, can fail an inspection and creates a real safety hazard. Local building codes and the inspector (the authority having jurisdiction) have the final say on what a space requires.
Identifying it
Read the printed legend on the jacket. It usually shows the category (for example Cat6), the rating (CMP), the listing mark and footage markers. Plenum cable often feels stiffer than general-purpose cable, but the marking, not the color or feel, is what counts.
For the CompTIA A+ exam
Core 1 (220-1201) objective 3.2 lists plenum-rated cable among the network cable types. The objectives don't spell out CMP, but it's the marking you'll find on plenum cable. If a question describes running cable through a drop ceiling used for air return, the answer is plenum-rated cable.