What is Plenum?
When talking about cables, you may hear the word “Plenum” being used, but what does it mean really? If you want to know all about plenum cable – let’s get started. While “Plenum” may sound like something that may belong on a faraway planet, it actually refers to a space (see the far planet connotation?!) that is usually above a ceiling or below a floor, serving as a receiving chamber for heated or cooled air to be distributed to living spaces.
In simpler terms, plenum is the space below a raised floor or above a dropped ceiling. Also known as the space used for the HVAC (Heating, Ventilation & Air Conditioning) system in a home or building.
Cabling & Plenum
Other than being used for routing air, plenum spaces make for the perfect space in a structure to house network communication cables as well as telephone cables. The difference between plenum cables and other cables has so do with fire safety. Plenum cables are required to comply with specific fire-safety standards.
What standards you ask? And who sets them? Well, Article 800 of the National Electrical Code (NEC) requires that plenum cables comply with the specifications for flammability and smoke density outlined in Underwriters Laboratories (UL) as well as National Fire Protection Association (NFPA) testing methods.
This fire resistant rating is achieved by using specialty jacket plastics. Flame-retardant, low smoke materials for example polyvinyl chloride (PVC), fluorinated ethylene polymer (FEP), or a polyolefin offer a heightened resistance against fire AND low emission of harmful fumes in the event of a fire.
In simpler terms, plenum refers to a rating – plenum-rated cables are fire-rated.
Do We Really Need Plenum?
Why do we need plenum rated cables? After all, they are placed out of sight in uninhabited spaces. Well, even though that is true, network cables runs are designed to reach most spaces in a home or building in order to connect them all to one another. In the event of fire, if the cables are not fire-rated, the fire will spread faster and to more spaces with the help of the cabling network.
What About Raiser Cable?
Just to make things clear, while riser cables are also fire-rated, they are NOT intended for plenum spaces. Riser cables are only designed for non-plenum, vertical applications. Their fire rating is not suitable for plenum spaces; therefore raiser cables should not be used instead of plenum cables. They each have a unique use, and for safety reasons it is crucial to use each type of cable for its intended purpose.