One of the most common NEC questions is: “Is CMR cable plenum-rated?” The answer is no. For plenum spaces, only CMP-rated cable is permitted.
CMP Cable
What Is a Plenum Space?
A plenum is any area in a building where air circulates for heating, cooling, or ventilation—such as drop ceilings, raised floors, or HVAC ducts. These spaces spread smoke and toxic gases quickly during a fire.
“Not sure how CMP compares with CM and CMR? Start with our ultimate cable fire rating guide.”
What Is CMP Cable?
CMP (Communication Plenum): Highest fire rating for communication cables.
Fire Resistance: Low smoke, low toxicity, and minimal flame spread.
Applications: Required in plenum spaces by NEC.
Why CMR Cannot Be Used in Plenum Areas
CMR is riser-rated, not designed for air-handling areas.
It produces more smoke and toxic gases, which is unsafe in plenum spaces.
Conclusion
If your installation involves plenum spaces, CMP is the only safe and legal choice. While more expensive than CMR, it guarantees fire safety and code compliance.
“Looking for Ethernet solutions? Here’s our breakdown of Cat6 CMR vs CMP cables.”
