Plenum cable meets fire safety standards for electrical and communication cables in building plenums. Plenums are spaces above suspended ceilings where infrastructure is installed. Plenum-rated cable is necessary for larger buildings, and abandoned old cable poses a fire and smoke threat.
Plenum cable has been manufactured to meet the fire safety standards set by modern building codes for cables, both electrical and communications. It is run through a building’s plenums, the spaces usually above the suspended ceiling where air can circulate. In most locations, including the United States, safety standards require cable to be highly flame resistant and, when burning, to emit a relatively low level of smoke and fumes. Plenum cable is commonly Teflon® sheathed, while cable that runs in floor-to-floor uprights is manufactured to a less stringent specification and is usually sheathed in a cheaper PVC.
In modern construction, plenums are an integral element of a facility’s design. Most American buildings are built with a standard floor-to-ceiling height of 8 feet (2.43 meters), but the actual height is usually between 10 and 12 feet (3.05-3.66 meters), with a suspended ceiling below the actual ceiling to create the 8 foot ceiling. The space above the false ceiling is the plenum. Critical components of a building’s infrastructure, such as ventilation and return ducting, plumbing, and cables for electricity and communications, are installed in the plenums. In some cases, a raised floor will create an underlying plenum through which the plenum cables pass; this is commonly found in rooms built primarily to house computers.
The use of plenum-rated cable is a problem primarily in larger buildings where thousands of feet (meters) of cable run in plenums. While not a significant factor when building small residential structures, the cost differential justifies purchasing both types of cable and using them where appropriate in a larger structure, such as an office tower or apartment building. Higher quality plenum wire can be used in risers, but riser wire cannot be used in plenums. The cost differential, however, justifies the use of riser cable for vertical cables, with splices at each floor to connect plenum cables to distribute power or signal to different points on each floor. Care must be taken in the design of the risers and where the plenums meet to ensure they do not take on the characteristics of the plenums, thus requiring plenum cable.
A growing concern among fire safety professionals is the abandonment of old cable within the risers and plenums of a structure. Updating a facility’s infrastructure or a tenant’s IT systems often requires new cabling. The equipment to be replaced is disconnected from its wiring and replaced, but the wiring itself remains in the plenums and risers. The concern is that older cable, manufactured to outdated fire resistance standards, could pose a fire and smoke threat even if it is no longer in use.
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