NFPA 72 requires that all points on the ceiling have a detector within a distance of 0.7 times the listed spacing of the detector. This ensure that when detection is used, the entire space on the ceiling is covered by detection. This requirement is commonly called the Point 7 (0.7) Rule.
The most common application of the Point 7 Rule is smoke detectors in corridor applications. Applying the Point 7 Rule allows spacing between smoke detectors to be extended beyond 30 feet in corridors that are less than 30 feet wide.
Figure A.126.96.36.199.1(g) illustrates the permitted spacing for smoke detectors based on corridor width. However, it is important to note that the spacing permitted in the illustration is intended for smoke detectors. Smoke detectors are not listed for spacing, but rather have an accepted spacing that is 30 feet.
The illustration states below the table of coverage areas based on corridor width that “listed spacing for heat detectors only” must be used. Since heat detector spacing typically ranges anywhere from 15 feet to 50 feet, this illustration cannot be applied.
While the Point 7 Rule can be mathematically calculated for heat detectors and applied to rectangular area and corridor applications, thankfully, the math has already been done for us in NFPA 72.
Figure A.188.8.131.52.1(e ) illustrates Typical Rectangles for Detector Curves of 15 ft to 50 ft.
Unlike Figure A.184.108.40.206.1(g), which is based on a single number, 30 feet, the accepted spacing of smoke detectors, Figure A.220.127.116.11.1(e ) provides separate illustrations for different heat detector listed spacings from 15 feet to 50 feet.
Each curve illustrates points on the curve for different rectangle widths (5, 10, 15, 20, etc) and the permitted spacing allowed for that width. For example, the 15 Ft Detector Curve shows permitted spacing of 20.6 feet in a 5 foot wide rectangle, 18.7 for a 10 foot wide, and 15 for a 15 foot wide. The 50 Ft Detector Curve shows 70.5 foot spacing for a 5 foot wide rectangle, 69.1 foot spacing for a 15 foot wide, and 58.3 for a 40 foot wide.
As with Figure A.18.104.22.168.1(g), the permitted increased spacings may be applied to corridor applications, reducing the number of detectors required in the space. However, the spacing should be pre-approved by all parties if used in a fire alarm design.