Lobby smoke detectors shall be located on the ceiling within how many feet of the centerline of each elevator door?

The correct answer is based on safety standards and regulations concerning smoke detection in areas near elevators. Lobby smoke detectors need to be positioned strategically to ensure they effectively detect smoke and other potential hazards in the vicinity of elevator doors.

Placing smoke detectors within 21 feet of the centerline of each elevator door ensures that any smoke generated in the lobby area is detected promptly. This is crucial because smoke can accumulate and spread quickly in enclosed spaces like lobbies, and an early warning is essential for the safety of occupants and for initiating emergency procedures.

The specified distance provides adequate coverage, ensuring that the detector is close enough to sense possible smoke from an elevator malfunction or fire, but also respects guidelines that might relate to air movement and other factors that can influence smoke flow in a building's design.