What is CMR cable?
CMR stands for Communications Multipurpose cable, Riser. It's a U.S. National Electrical Code (NEC) fire rating printed on the jacket of communications cable, such as Cat5e or Cat6, that is listed for riser runs: vertical paths between floors, such as riser shafts, vertical conduits and wall cavities that connect one floor to the next.
A vertical shaft can act like a chimney in a fire. Riser cable is built so that flame won't climb from floor to floor along the cable bundle, and it has to pass a vertical shaft flame test (UL 1666) to earn the rating.
Where CMR can and can't go
- Allowed: risers and shafts between floors, and anywhere general-purpose (CM/CMG) or limited-use (CMX) cable is allowed.
- Not allowed: plenum spaces, such as a drop ceiling or raised floor used for return air. Those need plenum-rated (CMP) cable.
- Substitution: CMP can replace CMR, but CMR can never replace CMP.
CMR vs. other ratings
| Rating | Fire performance | Typical location |
|---|---|---|
| CMP (plenum) | Strictest: low flame spread and low smoke | Air-handling spaces |
| CMR (riser) | Resists flame spreading up a vertical run | Floor-to-floor runs |
| CM / CMG (general purpose) | Basic flame resistance | Horizontal runs in ordinary spaces |
| CMX (limited use) | Lowest | Limited residential runs |
Riser cable isn't held to plenum cable's low-smoke requirement, which is why it costs less and why it can't go in air-handling spaces. The matching rating for nonconductive fiber cable is OFNR.
On hard drive spec sheets, CMR means something unrelated: conventional magnetic recording, where data tracks don't overlap, as opposed to SMR (shingled magnetic recording).
For the CompTIA A+ exam
Core 1 (220-1201) objective 3.2 lists plenum-rated cable; riser cable isn't named separately. Know the order anyway: plenum (CMP) is the highest rating, riser (CMR) is next, and a higher rating can always stand in for a lower one. A question that contrasts a run between floors with a run above a return-air ceiling is testing exactly that difference.