Why the jacket matters
Communications cable is fire-rated based on where it is installed. The rating printed on the jacket tells an inspector whether the cable is allowed in that space.
The common ratings
| Rating | Name | Where it is typically used |
|---|---|---|
| CMP | Plenum | Air-handling spaces such as drop ceilings and raised floors used for return air |
| CMR | Riser | Vertical runs between floors in shafts and risers |
| CM | General purpose | Horizontal runs that are not in plenum or riser spaces |
| CMX | Limited use | Limited-use applications such as some residential runs |
Substitution rules of thumb
- A higher rating can generally replace a lower one: CMP can be used where CMR, CM or CMX is called for, and CMR can replace CM or CMX.
- A lower rating cannot replace a higher one — riser cable is not allowed in a plenum space.
- Plenum cable costs more, so use it where it is required rather than everywhere by default.
- Always confirm with the project specs and your local authority having jurisdiction (AHJ).
General guidance only — always follow project specifications, manufacturer instructions and local code.