A plenum ceiling is one that uses the space between the top of the suspended ceiling and the bottom of the floor above to handle air for ventilation. All suspended ceilings are not plenums; some may use HVAC ductwork to move air to returns and diffusers located in the ceiling tiles (a ‘dead’ ceiling). Consult the local code authority to confirm that a suspended ceiling is a plenum. The NEC requires the use of plenum-rated cable (or cable in EMT, rigid or intermediate metal conduit) for plenum spaces but permits general purpose-rated cable in non-air handling ceilings and walls. However, this requirement may be superseded by local codes; for example, conduit may be required even with plenum cable. Know the local code before installing, or even ordering, the cable.