What is plenum?
In building construction, a plenum (pronounced PLEH-nuhm, from Latin meaning full) is a separate space provided for air circulation for heating, ventilation, and air-conditioning (sometimes referred to as HVAC) and typically provided in the space between the structural ceiling and a drop-down ceiling. A plenum may also be under a raised floor. In buildings with computer installations, the plenum space is often used to house connecting communication cables. Because ordinary cable introduces a toxic hazard in the event of fire, special plenum cabling is required in plenum areas.
In the U.S., typical plenum cable sizes are American Wire Gauge (AWG) sizes 22 and 24. Plenum cabling is often made of Teflon and is more expensive than ordinary cabling. In the event of fire, its outer material is more resistant to flames and, when burning, produces less smoke than ordinary cabling. Both twisted pair and coaxial cable are made in plenum cable versions.