How to Choose Between Plenum Vs Riser Rated Cables

September 24, 2019 | Blue Wave Communications

It can be all too easy to underestimate the complexities of low-voltage cabling requirements for an office or data center. Many people aren’t fully aware of the National Fire Protection Association (NFPA) and assume that just any cable will work for their needs—neglecting to verify that their cables actually meet National Electrical Code (NEC or NFPA 70) requirements. 

There are several distinct cable ratings referenced in the NEC that businesses need to be aware of when planning and budgeting for a new cable installation or when hiring a low voltage cabling contractor. Some of these ratings include CL/CM cables, CMP cables, and CMR cables—commonly referred to as general purpose/residential, communications plenum, and communications riser, respectively. There are a few other ratings for communications cable that apply to outdoor environments, but that is a topic for a future blog. 

It should be noted that the CL/CM, CMP, and CMR designations only refer to the rating of the cable “jacket” and its fire resistance properties—they have no bearing on the data transmission capabilities of the cable. Most commercial spaces will have to use either plenum or riser rated cables to meet fire safety codes. As such, many people ask about the difference between plenum and riser rated cables.

What is a Plenum?

To understand plenum cabling, it’s important to know what a plenum is. As noted in a Medium article, “a plenum is a separate space used for air circulation for heating, ventilation, and air conditioning.” These spaces may be used as a pathway to return air to the building’s heating, ventilation, and air conditioning (HVAC) system..

Any materials, including cables installed within a plenum space need to be rated for that environment—largely because any smoke from burning cables could easily be sucked into the HVAC system and spread throughout the building. 

What is a Riser?

A riser is a term given to a vertical shaft or series of rooms within a building that allow primary utilities such as electrical conduits, water supply lines, and communications cabling to be distributed vertically.  For communications cabling, the riser could be a separate room on each floor or a room shared with the electrical gear. In either case, the room is self-contained and not used for recirculating air to the HVAC system. Thus the fire resistivity requirements for materials installed within these rooms are less stringent than that required for plenum spaces.

What is Plenum Cabling? 

Plenum cabling, designated as CMP by the NFPA’s NEC guidelines, is cabling that uses materials such as polyvinyl chloride (PVC) or fluorinated ethylene polymer (FEP) for their jacket’s construction. These materials have a higher degree of fire resistance and emit fewer toxic fumes compared to other plastic polymers.

When running cables within plenum spaces, plenum cables are a must. Because plenum cables are built to a higher fire resistance standard than riser cables, plenum cabling is more expensive than riser cabling. While you can substitute plenum cabling for riser cabling in a “riser” space, you cannot substitute riser rated cables for plenum rated cables in a plenum space.

What is Riser Cabling?

Riser cables, designated as CMR by the NEC, is cabling that meets the fire resistivity standards for installation within a vertical riser or within an area designated specifically as a non-plenum environment. 

One example of this would be an HVAC system that uses a ducted-return configuration because separate ducts are used to recirculate air to the system instead of using the entire space above the drop ceiling. This helps to prevent the spread of smoke and toxic fumes throughout the HVAC system from any burning cables (or drop ceiling tiles) in the event of a fire.

To summarize the difference between plenum and riser rated cables, cost is a major factor but it’s really a matter of how fire-resistant their jackets are and how much smoke is emitted if they burn. The higher fire resistance of plenum cables make them a lower risk in a building fire than riser cables (which are, in turn, safer than general-purpose cables). 

Terms to Know when Choosing Plenum Vs Riser Cabling

Here’s a short list of some acronyms and other terms to know when looking for plenum or riser rated cables:

  • CL/CM/CMG Cables. The term for residential-grade or general-purpose cabling. NOT SUITABLE FOR COMMERCIAL USE IN RISER OR PLENUM ENVIRONMENTS.
  • CMR Cables. Communications Riser. Approved for use in riser spaces.
  • CMP Cables. Communications Plenum. Approved for use in riser and plenum spaces.
  • NFPA. The National Fire Protection Association. This is the organization that created the National Electrical Code.
  • NEC. The National Electrical Code (NFPA 70). This document is designed to create a baseline for electrical safety in residential, commercial, and industrial buildings.

Need help determining whether you need plenum vs riser cables for your next low-voltage system installation? Reach out to the experts at Blue Wave Communications for assistance.

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