<Li> NFPA 13 - Standard for the Installation of Sprinkler Systems </Li> <P> In older buildings the space above the dropped ceiling was often used as a plenum space for ventilation systems, requiring only enclosed ducts that deliver fresh air into the room below, with return air entering the ceiling space through open grilles across the ceiling . This practice is now used less frequently in new construction . </P> <P> In the event that the dropped ceiling is used as a plenum, low - voltage cables and wiring not installed inside conduit need to use a special low - smoke and low - toxicity wire insulation which will tend to char and stop burning on its own . This helps to protect building occupants so that they are not poisoned with toxic chemicals sucked through the ventilation system in the event of a fire, and helps to prevent fires from spreading inside the hidden plenum space . This special low - smoke cable is typically referred to as plenum cable or Low Smoke Zero Halogen (LSZH or LS0H) cable . While twisted pair cable for networking and telephone service is the most common form of plenum cable, coaxial cable also needs to be plenum - rated for safety . </P> <P> High - voltage electrical equipment (generally regarded as being over 50 volts) is not permitted to be exposed in the plenum space above a drop ceiling . High voltage wiring must be enclosed in conduit or raceways, and must be physically isolated from low - voltage wiring . High voltage electrical devices similarly must be enclosed in a plenum space, inside a metallic container . Similarly, electrical outlets for domestic powered devices are not permitted inside the plenum space, though outlets can be installed on ceiling tiles inside electrical boxes, with the sockets exposed on the exterior bottom face of the drop ceiling . The purpose of these restrictions is to limit flame spread inside the unseen plenum space, in the event of high voltage equipment or wiring failure . Low voltage cabling is permitted because current flow is typically negligible so the risk of overheating and fire is limited . </P>

Which type of cable might be required for installation in a drop ceiling