Difference Between Plenum (CMP) Rated Cable & Riser (CMR) Rated Cable

Both Plenum Rated Cable (CMP) and Rise Rated Cable (CMR) are cables often used in industrial and residential settings, especially when required legally by the National Electrical Code (NEC) and fire codes.

These wires are chosen based on where the electrical cables will be installed and how a fire could, in theory, spread in that space.  While industrial and residential locations can utilize a variety of cables, specific environments require plenum or riser cable in order to minimize or prevent smoke inhalation. 

Plenum Cable

Plenum rated cables are used in plenum spaces as part of commercial and residential spaces.  A plenum space is any area used for air circulation related to heating and cooling. Plenum spaces are often found above a drop ceiling that returns air to HVAC, under raised floors, or any space that is used for "air handling". Because of this system, it's vital that if there is a fire, plenum spaces don't spread the smoke through the ventilation. In sttuctures with these plenum spaces,  plenum cables are used as they have a higher fire rating and are designed with fire-retardant plastic jacket to avoid high levels of smoke from being created and spread.

Check out a few of our Plenum Rated cables below:

• CMP Plenum, Stranded, Unshielded Security Alarm Cable
• Interlock Armor Plenum Fiber Optic Cable
• Plenum Thermostat Wire

Riser Cable

Riser Cable, also known as CMR or Communications Multipurpose Cable, Riser, is a classification defined by the NEC for low-voltage communication cabling. Riser cables are used when a cable runs vertically connecting one floor to another, and is not installed in a plenum space (air handling area). CMR cables are engineered to resist flame spread vertically, self-extinugish when possible, and prevent fire from "wicking" up the cable jacket. Based on these attributes, riser cables are often utilized for network cabling between stacked IT closets, vertical runs inside walls, within elevator shafts, and floor-to-floor cabling.

While riser cables are engineered to resist flame spread, they are not low smoke, which is why they can only be used in non-airflow vertical spaces. 

Check out a few of our Riser Rated cables below:

• CMR Riser, Stranded, Shielded Security Alarm Cable
• CMR Riser, Stranded, Unshielded Security Alarm Cable
• Belden CMR Riser Rated Cable, Unshielded
• Indoor/Outdoor Dry Loose Tube Fiber Optic Cable
• Indoor Prysmian Riser-Rated Fiber Optic Cable
• Indoor/Outdoor Riser-Rated Fiber Optic Cable

TL;DR (To Summarize)

Electricians are bound by local building laws, NEC regulations and guidelines, and Federal Marshal enforcement, as well as other inspections and best practices. Failing to use the correct cables when wiring can result in massive needs to retrofit, increase costs, and voided insurance coverage. 

While Plenum Cable (CMP) can replace Riser Cable (CMR), Riser Cable (CMR) cannot replace Plenum Cable (CMP)

General Rules of Thumb:

• If air flows through it, go with Plenum Cable. 

• If it connects floors, go with Riser Cable.

• If air does not flow through it and it doesn't connect floors, general purpose wire may be allowed. 

When in doubt, always double-check with an inspector and default to the highest required rating when choosing cabling and wiring. 

Looking for plenum and rise options? Shop our Fire Alarm Cable and Security Alarm Cable categories.