Not fewer than one smoke detector listed for the intended purpose shall be installed in all of the following areas:
- Mechanical equipment, electrical, transformer, telephone equipment, elevator machine or similar rooms.
- Elevator lobbies.
- The main return and exhaust air plenum of each air-conditioning system serving more than one story and located in a serviceable area downstream of the last duct inlet.
- Each connection to a vertical duct or riser serving two or more floors from return air ducts or plenums of heating, ventilating and air-conditioning systems, except that in Group R occupancies, a listed smoke detector is allowed to be used in each return air riser carrying not more than 5,000 cfm (2.4 m3/s) and serving not more than 10 air inlet openings.