WHAT DOES "PLENUM" MEAN?
The word “plenum” gets used a lot in cabling circles but what does it mean? Let’s look up some definitions, shall we? One definition of plenum is “a full assembly, as a joint legislative assembly.” That can't be right, here’s another definition with the meaning of plenum that we need:
Plenum — a space, usually above a ceiling or below a floor, that can serve as a receiving chamber for air that has been heated or cooled, to be distributed to inhabited areas.
At its root, plenum refers to “full space” and most of its varying definitions pertain to a separate enclosed space that is set apart from surrounding areas. It’s the space above a building's dropped ceilings or below a raised flooring system, that gets used for heating, ventilation or air conditioning (often shortened to “HVAC”).
WHAT DOES THAT HAVE TO DO WITH CABLING?
How do cables factor into all of this? In addition to being utilized for routing air, plenum spaces are often used to house telephone and network communication cables being run from one location to another. These cables are appropriately referred to as plenum cables. What sets these wires and cords apart from regular cables? The answer is they need to adhere to special fire-safety standards.
According to Article 800 of the National Electrical Code (NEC), plenum cables must comply with the specifications for flammability and smoke density outlined in Underwriters Laboratories (UL) and National Fire Protection Association (NFPA) testing methods.
To accomplish this greater resistance to fire, plenum-rated cables use special types of plastics in their jacket coverings. Flame-retardant, low smoke materials such as polyvinyl chloride (PVC), fluorinated ethylene polymer (FEP) or polyolefin, offer good resistance against fire. In the event they do begin to burn, they will not emit large quantities of harmful fumes.
WHY USE PLENUM?
Since plenum cables are routed through air circulation spaces which often contain very few fire barriers, they need to be coated in material that won't contribute to the spreading of flames. As network and communications cabling runs also typically spread throughout buildings, that increases the chances that if one ignited, flames and smoke could spread quicker.
If you have heard of riser cables, you may have heard that these are also flame retardant, but they cannot be used in plenum spaces. Riser cables are intended for non-plenum, vertical applications (between floors of a building). Though it is true they need to be fire resistant, riser cables typically incorporate load-bearing strengtheners since they need to stay upright without being stressed too hard. However, the specifications are much stricter for plenum cables; and these two cable types are not interchangeable. While plenum cables can be used in riser applications, the opposite is not true.
The bottom line is if your cabling application requires materials that are flame-retardant or compliant with strict safety standards, you should always opt for plenum-rated products.