Visible Notification – One Strobe or Four?
NFPA 72, National Fire Alarm and Signaling Code, covers notification requirements in Chapter 18. Audible notification (horns, speakers, chimes) are performance based. NFPA 72 requires 15dBA above average ambient for public mode notification but does not state specific device spacing or sound level requirements. As long as the minimum sound level requirements are met, the code is not concerned with how many devices are used or how they are spaced to meet the requirements.
Visible notification requirements in Chapter 18 are prescriptive. Using the tables in Chapter 18, the design determines strobe candela intensity requirements based the location of the strobe (wall or ceiling) and size of the room. If wall mount strobes are used, candela requirements are based on whether one or four strobes are used. If ceiling mount strobes are used, candela requirements are based on ceiling height.
Examples of the tables are below:
Since the Code permits multiple methods of achieving the visible notification requirements, it becomes the designer’s decision of what method to use.
Let’s take a sample room – 50’ x 50’. According to the tables, our options are using 1 or 4 wall mount strobes, or one ceiling mount strobe. If we use wall mount, we can use 4 – 30cd strobes, or 1-94cd strobe. If we use ceiling mount, we 1-95cd strobe for a ceiling up to 30 feet.
If the designer considers labor costs, it basically costs four times the labor to install 4 wall mount strobes, versus 1 wall or ceiling mount. The counter-argument may be that 30cd strobes have a lower current draw than a 94/95cd strobe.
However, a single strobe (wall or ceiling mount) may be the most economical option. Here’s why:
A typical 30cd strobe draws around 75ma, and since four are required, the total draw to cover the room is 300ma (4 x 75ma). However, a single 95cd strobe draws around 150ma. Single wall mount strobe at 95cd draws half the current of 4 – 30cd strobes and requires one-quarter the labor (1 device vs. 4) to install.
The above example also works for a single ceiling mount strobe as well. A 50’ x 50’ room requires a 95cd ceiling mount strobe for a ceiling up to 30’. The typical current draw of a 95cd ceiling strobe is around 175ma. More than a wall mount, but still significantly less than 4-30cd wall mount strobes.
To learn more about NFPA 72, check out our Fire Alarms Systems online training.