Alarm cables installed in a plenum rated ceiling must be _____ rated.

Alarm cables that are installed in a plenum rated ceiling must be plenum rated. This requirement is essential because plenum spaces, which are used for air circulation in heating and air conditioning systems, can facilitate the spread of fire and smoke if appropriate materials are not used.

Plenum rated cables are designed to emit less smoke and have a lower flame spread when burned, thus maintaining a safer environment within the space. Building codes and safety regulations stipulate the use of plenum rated cables in such areas to minimize hazards; therefore, using cables that are not certified for plenum use could lead to serious risks, including increased smoke in the event of a fire.

In contrast, options like water, AIR, and low smoke are not specific designations for the requirements governing cables in plenum spaces. While low smoke is a desirable characteristic for many cable types, specifically for plenum applications, it is the plenum rating that specifically meets the safety regulations.