What do CMR and CMP stand for?
These two acronyms are confusing as the “C” and the “M” do not indicate separate words, but are an abbreviation for “CoMmunication”:
- CMR = Communications Riser
- CMP = Communications Plenum
What is "Riser"?
A riser is usually a space that runs between floors in an office or a commercial building. Think of it as an elevator shaft without the elevator… If you have a large building with multiple floors, each floor will house one or more “rooms” the are dedicated to distributing network to that floor. Each one of those rooms are usually placed on top of each other, and are connected to each other via a “riser” shaft.
The riser is then used to run all the network cables (copper, fiber, coax, etc.) to interconnect all the floors.
What is Plenum, or a Plenum Space?
A Plenum Space is the part of a building that is used as part of the air conditioning and heating system. This usually is the area above a suspended ceiling that is used as a forced air return. Unlike a riser, the flow of air is greater and distributed around the entire floor that the area services.
So why does this matter?
As you can tell by the above descriptions the differences between the two are the amount of airflow and the distribution of that flow around a given area. This means that any combustible material (such as a network cable) would cause smoke and fumes that would be distributed very quickly in the event of a fire.
In the case of a Plenum space, the speed and distribution would be much greater than in a riser.
How are CMR and CMP cables different?
The difference between the two is the materials that the jacket and wires are made from. A plenum “rated” cable is made from less hazardous and lower smoke materials than other cables. This is so in the event of a fire an area does not fill with choking, dense smoke.
Why isn’t every cable rated for plenum spaces?
Plenum cable is expensive, much more expensive than riser-rated cable. Additionally, the practice of running cables in plenum spaces is becoming less popular, and therefore demand for plenum rated cable is much less than riser-rated cable.
How do I know which one I should buy?
Unfortunately this is something that we cannot help or advise you with as each individual building and installation has to be evaluated. Usually, building or air conditioner plans and installation documentation will indicate which areas are considered plenum spaces. You may need to involve an inspector to evaluate what cable you may need.