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 ensures 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.188.8.131.52.1(g) illustrates the permitted spacing for smoke detectors based on corridor width.
Figure A.184.108.40.206.1(g) illustrates how rectangular areas (which can be sections of a length of corridor) that are less than 30 feet wide can cover a length greater than 30 feet and still remain within the same coverage area of a smoke detector.
The table of rectangles below the image illustrates the spacing that may be applied. The rectangles, labeled A, B, C, and D show the permitted extended spacing for rectangular areas with a width of 10, 15, 20, and 25 feet. The first number is the width of the rectangle, the second number is the permitted spacing. For example, row A shows that a rectangle 10 feet wide permits a spacing of 41 feet between smoke detectors. Row D shows that a rectangle 25 feet wide permits spacing of 34 feet between smoke detectors.
Why This Works
Although we discuss smoke detectors covering an area that is 30’ x 30’, smoke detectors actually have a circular coverage area. The coverage area of a smoke detector is a circle with a 21 foot radius (from the center of the circle to the edge). We use 30’ x 30’ coverage since it is easier to layout detectors using square instead of circles.
A circle with a 21 foot radius will entirely encompass a square that is 30’ x 30’. This also illustrates application of the requirement that all points on the ceiling be within 0.7 times the listed spacing (0.7 x 30 feet = 21 feet).
The Point 7 Rule is an important concept to remember in the NICET exam. Questions that ask for the “minimum number of smoke detectors in a corridor” are typically Point 7 Rule questions and should be answered using Figure A.220.127.116.11.1.1(g) in NFPA 72.
However, application of this concept is less common in the real world. Because this concept appears in Annex A, it is a “should” not a “shall.” It is technically a recommendation, not a requirement. Many code officials do not allow it to be applied in system design applications. While it can give a competitive advantage in bidding by reducing the number of detectors required, it should only be used with approval of all code enforcement officials involved in the project.