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Understanding Plenum and other NEC Cable Ratings

By Trent Crawford

plenum space

When you run cable through any building you first need to consider plenum spaces. Plenum spaces are the open spaces above the ceiling or below the floor that are used for air circulation. Technically, any ductwork is considered a plenum space too. These spaces are important for air circulation in any building, but they also bring some problems if you ever have a fire because of the high oxygen content and lack of fire barriers. Using untreated cables in plenum spaces can spread the fire to other areas very quickly and spread noxious smoke throughout the building.

Plenum cables are coated with flame retardant and made using special plastics that don’t smoke nearly as much as other plastics to help prevent this problem. Any cable that you run through plenum spaces must be plenum rated, even wiring used for information transfer, like Cat5 wiring.

Also worth mentioning are ‘accidental’ plenum spaces—all it takes is a leaky duct to make any open space a plenum, and it happens more than you might think. Some HVAC installers really do top notch work, but unfortunately sometimes sloppy systems can be treated as “good enough.” All duct systems should be air tight, however empty screw holes, slipped joints and misaligned vents can all create airways that allow smoke, fumes or anything else to get sucked into the air system and pushed through an entire building.

Plenum spaces provide two problems if there is ever a fire. First, by necessity plenum spaces are loaded with fresh oxygen. A small, smoldering fire can really take off if it breaks into the plenum space. The second issue is that the air in the plenum space is forced throughout the whole building. Any smoke or fumes that get released into the plenum are pumped all over the rest of the building.

What is Plenum Cable?


Burning cables


If you have ever seen regular plastics burn, you understand why the cables in a plenum need special treatment. Most plastic burns quickly and lets off plumes of thick dirty toxic black smoke. Unrated cables should never be used in any kind of construction, especially in a plenum. If they catch fire, they can actually spread the fire to other parts of the building. Any cables installed in a plenum space must be plenum rated. Even if a cable is only used temporarily, only plenum cables should be used above ceiling tiles or below dropped floors… it is surprising how permanent “temporary” fixes can be.

Plenum cables use treated jackets, usually low-smoke PVC (polyvinyl chloride) or FEP (fluorinated ethylene polymer). The jacket is designed to 1) hinder the progression of flame and 2) not release the harmful smoke that normally accompanies any type of burning plastic. We use Teflon (or PTFE) coated wires in our plenum network cables to make sure that the cables are safe and heat-resistant.

Plenum cables are the highest standard and can safely be substituted for any other cable type such as riser, CM, CL2 or CL3 cables.

Guide to Cable Ratings

What is CMP rated Cable? Or CMR rated cable? What do these acronyms mean and why do they matter? Did you know that there are at least 16 ratings or levels used to designate what a cable is suitable for? Whether you're planning out a basic in-wall residential installation or a commercial job that involves plenum spaces, it pays to know what different acronyms mean and where you should use them.

This guide is intended to serve as a quick reference to the different levels and classifications of cables and their associated acronyms. It is worth noting that cables of a higher rating can generally be used as substitutes for cables of a lower rating, however the reverse is not true. Generally it goes up from residential cable to commercial cable, riser cable, and plenum cable, with different levels of cable within each tier. More about NEC cable ratings.

  • CMP : Communications, Plenum - can be installed in any space
  • CATVP : Cable TV Plenum
  • CL3P : Class 3 Plenum - for in-wall installation in plenum, riser and general spaces
  • CL2P : Class 2 Plenum - for in-wall installation in plenum, riser and general spaces
  • CMR : Communications, Riser
  • CATVR : Cable TV, Riser
  • CL3R : Class 3, Riser - for in-wall installation in riser and non-riser spaces
  • CL2R : Class 2, Riser - for in-wall installation in riser and non-riser spaces
  • CM or CMG : Communications
  • CATV : Cable TV
  • CL3 : Class 3 - for in-wall installation in non-riser, non-plenum spaces
  • CL2 : Class 2 - for in-wall installation in non-riser, non-plenum spaces
  • CMX : Communications, Residential
  • CATVX : Cable TV, Residential
  • CL3X : Class 3, Residential - for in-wall installation in non-riser, non-plenum spaces
  • CL2X : Class 2, Residential - for in-wall installation in non-riser, non-plenum spaces

Who is in Charge of Ratings?


The NFPA (National Fire Protection Agency) is the body in charge of setting the code requirements for protecting plenum air spaces (as well as other fire concerns) and the National Electric Code or NEC is the standard they provide for handling all cables including power, network and video cabling. In the NEC Section 800 it describes the properties of cables used for network and AV cabling. Any Nationally Recognized Testing Laboratory (NRTL) can certify NEC compatibility. Underwriter Laboratories (UL) is the de facto standard for making sure that your cables meet or exceed all of the required specifications. UL can simply test your cable, or they can certify the factory where it is made and issue a UL number that can be printed on the cable with the UL trademark.

Get UL Certified Plenum Cable

Unfortunately there are some companies that have been advertising cables as plenum without meeting the requirements. Many times they just mark their inferior Riser rated cables as “Plenum” and hike the price way up. The best way to protect against this ‘fake’ plenum cable is to check for the UL number and look it up on UL’s website This will tell you who manufacturers the cable and what types of cable they can produce. See our UL listing. Real plenum cable will usually have the number printed right on the cable jacket. If the seller doesn’t have it or won’t give you the UL number, don’t risk it.

We offer a variety of UL certified plenum cables. From the data center to the conference room we can provide the network, data, video and audio cables that you need to ensure that your installations are safe and up to code. For all the details check our CAT5, HDMI, and RG59 plenum cables.

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