What Does "Plenum" Mean, Anyway?
The word “plenum” gets thrown around a lot in cabling circles. But what does it mean? It kind of sounds like some obscure anatomical term. Some kind of a membrane, maybe, or bodily fluid. But thankfully, “plenum” is none of those gross-sounding things. So what is it, then? Let’s look up some definitions, shall we? Let's see here…A plenum is “a full assembly, as a joint legislative assembly.” Wait, what? That can't be right, I don't even know what that means. No, wait, here it is:
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.
Ahh, that's better. 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. To use terminology that our likely readership will understand: 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 (oft shortened to “HVAC”).
What's That Got to Do with Cabling?
So how do cables factor into all of this? Excellent query, my friend. 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, appropriately, are referred to as plenum cables. What sets these wires and cords apart from regular old cables? Another fantastic question. The answer is that 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 a polyolefin offer good resistance against fire, and in the event that they do begin to burn, they will not emit large quantities of harmful fumes. Which is a win-win!
Why is this all necessary? 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. Also, as network and communications cabling runs typically spread throughout buildings (to make sure everyone is connected to everyone else), having them ignite would be a surefire (no pun intended) way for the flames and smoke to spread that much quicker. So seriously, this is not the place you want to cut corners.
Now, perhaps you've also heard of a thing called riser cables. You might have heard that these are also flame retardant, so maybe you think you can get away with using riser cables in plenum spaces. Well, if I had a ruler I'd rap your knuckles with it for even thinking such an obscenity, because DON'T DO IT.
Riser cables are intended for non-plenum, vertical applications (ie. between floors of a building). They typically incorporate load-bearing strengtheners since they need to stay upright without being stressed too hard, and it's true that they do have to be fire resistant. However, the specifications are much stricter for plenum cables, so they are 100% not interchangeable. While you can use plenum cables in riser applications, the opposite is definitely not true.
Bottom line? If your cabling application requires materials that are flame-retardant or compliant with strict safety standards, always opt for plenum-rated products. As it happens, CableOrganizer.com offers a full line of plenum-grade cable, raceway, wire connectors and more to meet all of your fire safety requirements!