How to choose and use PVC and plenum cable

Deciding between PVC and plenum cable is very important because the type of cable you choose can have critical consequences.

The difference between PVC and plenum is the type of jacket that surrounds the cable. Whether you choose PVC- or plenum-jacketed cable depends on where you are going to use the cable. Most of the time, the type of cable depends on your local building codes and/or the age and design of the building.

What’s plenum?
First, let’s define plenum. The term plenum is an HVAC term. A plenum space is the part of a building, or pathway, designed for circulating heated and cooled environmental air and for return airflows. In most buildings, the space above the ceiling or below a raised floor is used for HVAC air. Duct work is also considered a plenum. A plenum ceiling is where the air is forced through the ceiling rather than being ducted.

Plenum spaces are air tight and usually have a greater atmospheric pressure and a greater oxygen content. Plenums can be particularly dangerous in case of fire. The oxygen can turn a small spark into an out-of-control fire. And because the air is forced through the plenum, smoke and fire can very quickly travel throughout the building. If cable is run through a plenum, it must be a plenum-rated cable if no conduit is used.

Building_Plenum_NoPlenum

Building_Plenum_Normal

Plenum cable
Plenum cable has a flame-resistant jacket and is usually made of fluorinated ethylene propylene (FEP), such as Teflon® by DuPont. The coating is designed to lower the emission of toxic fumes or smoke when burned. Plenum cable is designed to have a low fire-spread index. It must self extinguish and not reignite. Plenum cable is designated CMP for data communication cable.

Low Smoke, Zero Halogen (LS0H, LSZH)
LS0H is a type of plenum cable with a thermoplastic or thermoset compound in the jacket. It limits the emission of smoke and corrosive gases and permits no halogens to be released when exposed to high heat or flames. Halogen in cable jackets is considered to be a good flame retardant, but it emits toxic fumes and smoke when exposed to flame and it can create acid when exposed to moisture. LS0H cables are ideal for use in areas with poor ventilation, such as aircraft and rail cars. They are commonly used in Europe.

One of the differences between plenum cable and LS0H cable is that plenum construction is designed to reduce the amount of smoke. Also plenum cable releases halogen when burned, while LS0H does not.

PVC cable
PVC cable features an outer polyvinyl chloride jacket that gives off smoke and toxic fumes when it burns. It’s most commonly used between the wallplate and workstation. It can be used for horizontal and vertical runs, but only if the building features a contained ventilation system. If PVC cable is used in a plenum, it will spread fire and noxious, black smoke throughout the building. PVC cable is designated CM or CMR.

If you’re not sure if you need to use a plenum cable or not, check with the NFPA (National Fire Protection Agency) the National Electric Code (NEC), or your local city department.

Insurance and cable
Because plenum cable is less toxic than PVC cable, it is often specified for use throughout schools and hospitals, often due to insurance requirements. A fire in these facilities could prove to be catastrophic because of the large number of people.

Installation ease
PVC cable is the most flexible cable and thus easier to install. Plenum and LS0H jackets are slightly thicker and heavier making installation a little more difficult.

The expense factor
Generally, PVC cable is less expensive than plenum or LS0H cable. If plenum cable is not required or specified in a project, PVC cable will most likely be used. If you’re having a contractor install cable, make sure you specify what type of cable you want to use and where. Some unscrupulous contractors will use PVC cable in plenum spaces just to cut costs. Some organizations, to be on the safe side, are stipulating the use of plenum cable throughout the building.

Alphabet soup
CM, CMR, and CMP. You see these printed on cable jackets, but what do they mean? In the US, the NEC specifies the environment where each cable is used. There are actually 16 ratings, but only the most common are listed here. You can substitute a higher level of cable for a lower one, but not the reverse.

CMP: Communications (Plenum). This cable can be used in any spaces, ducts, plenums, and spaces used for environmental air. Cables that are plenum rated meet the NFPA-262 safety standard. No substitutions. (Canada: CSA FT6)

CMR: (Riser). This cable can be use in vertical runs in a shaft or from floor to floor. Not for use in environmental air spaces. Cable must self-extinguish and prevent the flame from traveling in a vertical burn test. Cables that are riser rated meet the UL-1666 safety standard. Substitution: CMP.

CM or CMG: Communications (General Purpose). This cable is for use in locations other than risers or plenums. It is often used for workstations and patching. It has less strict burn test requirements than CMR cable, but still has to self extinguish. Cables that are general purpose meet the UL-1685 safety standard. Substitution: CMP, CMR (Canada: CSA FT4).

Burn testing
To ensure consistent quality, legitimate cable companies put their cable through burn testing at national recognized testing labs, such as UL or ETL. Four times a year, Black Box sends random samples of cable from our warehouse out for testing.

Graphics courtesy of D Mahalko

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