Plenum is defined as what?

• A. A type of cable insulation.
• B. A ground fault protection device.
• C. A surface-mounted conduit system.
• D. An open space above a building's dropped ceilings used for air circulation.

Answer: (D)

Plenum refers to the air-handling space in a building. It’s the open area used for circulating environmental air, typically the space above a suspended (dropped) ceiling or, in some designs, under a raised floor. This isn’t about cable insulation, protection devices, or conduit systems—the term specifically describes the air circulation path. Because cables running through a plenum can affect fire and smoke spread, codes require special plenum-rated cabling for safety.