Visible notification design uses prescriptive design. The location and placement of visible notification appliances are entirely dictated by NFPA 72, Chapter 18. While the code permits performance-based design options to be used with visible notification, most applications follow the tables in Chapter 18. For example, the intensity requirements for strobes are based on the mounting location and room size.
Wall Mount Visible Notification Appliances
Wall mount strobe requirements are based on the number of strobes (1 or 4) and the room size. If a single strobe is used, it can be mounted on the center of any wall in the room. If four strobes are used, they are mounted on each wall of the room.
The decision to use one or four strobes for wall-mount visible notification depends upon the room’s layout. Installing a single strobe in a room is more economical since the wiring is only brought to one location, and the overall current requirements are lower than multiple appliances.
However, the ability of the occupants to see the strobe is also important. Although NFPA 72 does not require a direct line of sight to visible notification appliances and recognizes indirect signaling from light reflected from the surface in the room, the fact that parts of the room may not receive visible notification signals should be considered during design. In addition, in rooms with multiple obstructions to viewing the strobe, multiple strobes may provide better coverage.
The location of the strobe (or strobes) on the wall is also an important consideration. Rooms with single strobes require that the strobe is centered on the wall to effectively cover the entire room with the required light intensity. A strobe delivers a specific amount of light over a specific size area at a specific intensity. Therefore, the higher the intensity of the strobe, the larger the area that light can be delivered to.
If the strobe is not centered or the room is not square, the required intensity shall be determined by the maximum room size dimensions. These dimensions are determined by measuring the distance to the farthest wall or doubling the distance to the farthest adjacent wall, whichever is greater.
Rooms with four wall-mount strobes require offsetting of the strobes on the walls. Remember that strobes have specific coverage areas based on intensity, which allows them to deliver a specific amount of light to the coverage area. If a room with four strobes has each of the strobes mounted on the center of the wall, there are areas in the room that do not receive the required light intensity. By offsetting the strobes, as shown below, the room can be properly covered with visible notification.
