If return air risers serve two or more stories, at what design capacity must smoke detectors be installed at each story?

Smoke detectors play a crucial role in ensuring safety within buildings that have return air risers extending over multiple stories. According to the North Carolina Mechanical Code, the installation of smoke detectors is mandated when the return air risers have a specific design capacity.

When the design capacity of the return air risers is 15,000 cubic feet per minute (cfm) or more, the code requires that smoke detectors be installed at each story where the risers are situated. This requirement is in place to enhance safety measures, as return air risers can potentially act as conduits for smoke and heat during a fire, allowing it to travel quickly throughout multiple levels of a building.

Therefore, selecting 15,000 cfm as the correct answer aligns with the code's emphasis on ensuring that adequate detection systems are in place to alert occupants to the presence of smoke, thereby facilitating timely evacuation and potentially reducing fire damage or casualties.