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Coax and Fire Code Classifications
The space above a drop ceiling (between the ceiling and the floor of a building’s next level) is extremely significant to both network administrators and fire marshals. This space (called the plenum—see Figure 3.10) is a convenient place to run network cables around a building. The plenum, however, is typically an open space in which air circulates freely, and, consequently, fire marshals pay special attention to it.

The most common outer covering for coaxial cabling is polyvinyl chloride (PVC). PVC cabling gives off poisonous fumes when it burns. For that reason, fire codes prohibit PVC cabling in the plenum because poisonous fumes in the plenum can circulate freely throughout the building.

Plenum-grade coaxial cabling is specially designed to be used without conduit in plenums, walls, and other areas where fire codes prohibit PVC cabling. Plenum-grade cabling is less flexible and more expensive than PVC cabling, so it is used primarily where PVC cabling can’t be used.