Planning for your next cable run project, installing Ethernet or a home theater for example? You have researched cable types and measured the length you will need. Chose Cat5e cable or HDMI cable and are ready to buy them and get to it. But then you learn that not all cables are made equal (they never are). There are different cable jacket ratings. CMP, CMR, CL2, what????
Before allowing the confusion take over, let’s make sense of the different cable jacket ratings, what they mean, who specifies them and why.
The National Electric Code
Cable jacket ratings are specified by The National Electric Code (NEC), which is published by the National Fire Protection Association. The NEC contains minimum standards for electrical wiring – from the cables themselves to signs, machinery and alarm systems.
While the NEC guidelines aren’t a federal law, they are checked by the building inspector before a CO (certificate of occupancy) is granted at the end of construction. And for a good reason, since they are designed for fire prevention.
The different cable jacket ratings refer to the outer jacket of a cable. This outer jacket protects the inner wires from damage caused by moisture, UV rays and the physical elements around it. Most cable jackets for low voltage applications are made of PVC. While PVC is a flexible and inexpensive material, it may release smoke as well as hydrogen chloride and other toxic gases in case of a fire.
For that reason, PVC jackets have different resistance to fire depending on their use. Here are different uses that call for specific cable jacket rating:
Riser / vertical shaft is the space between floors of a building. This is ducts, conduits, cables and pipes are installed to run power, low-voltage wiring etc.
Plenum is where a building’s air ducts are connected for HVAC, for example a drop ceiling or raised floor.
Cable Jacket Ratings
Being that the NEC guidelines are around 1000 pages long, here are the most common ratings:
CL/CM/CMG Jacket Rating for main stream installations.
CL cables are used for audio/visual applications. They do not reduce smoke or toxic gas emission, but minimize the possibility of shock.
CM/CMG cables are for communications applications. These ratings indicate that the jacket has passed a vertical tray flame test. In simpler words, they do not spread flames more than 8’. These cables can be installed in walls or ceilings.
CMR rated cables are used in riser spaces thanks to their limited flames spread. They can be used anywhere but plenum spaces.
CMP rated cable can be used in plenum spaces as well as anywhere else (but it’s a bit of an overkill for ‘anywhere else’). These jackets’ material retards flame spread and does not emit toxic gas when burnt.