System Layout, Design, and Documentation
Coverage and spacing
Start with the coverage decision: complete (total), partial, or selective. Then apply spacing. For spot smoke detectors on a smooth ceiling 10 ft or less, the classic grid is 30 ft between detectors with no point of the protected area more than 15 ft from a detector - effectively a 30 ft by 30 ft square per detector. Heat detectors use the manufacturer's listed spacing for a 10 ft ceiling, reduced by the height-adjustment factors as the ceiling rises; wall distance is half the listed spacing. Spot detectors mounted on a ceiling must be at least 4 in. from a sidewall; wall-mounted units go in the top 4 to 12 in. Keep detectors away from HVAC supply diffusers (36 in.) and out of dead-air pockets.
Notification design
Public-mode audible signals must be 15 dBA above average ambient (or 5 dBA above maximum ambient, whichever is greater), measured about 5 ft above the floor. Sleeping areas are stricter: 75 dBA at the pillow, with low-frequency (520 Hz) appliances where required. Visible appliances are selected by candela for room size and ceiling height - a small room allows low candela; a large or high room requires much more.
Documentation
Deliverables a Level II technician must recognize: shop/as-built drawings with legend and scale, battery and voltage-drop calculations, a sequence-of-operations matrix, manufacturer data sheets, the record of completion, and the record of inspection and testing.
Worked example
A 60 ft by 40 ft room with a smooth 10 ft ceiling. Along 60 ft: two detectors at 15 ft and 45 ft give 15 ft to each wall and 30 ft between - compliant; one centered detector would leave 30 ft to the wall and fail. Along 40 ft: two detectors at 10 ft and 30 ft give 10 ft edges and 20 ft spacing - compliant. Total: four detectors in a 2 by 2 grid.