Fire Alarm System Design and Installation
This domain covers how a required system actually gets built: documentation, power, circuit integrity, and the calculations an examiner can check on paper.
Documents and sequence
NFPA 72 Chapter 7 requires a documented design: specifications, working plans with floor plans, riser diagram, device and appliance locations, and wiring. Approval comes from the AHJ before installation, and changes require revised drawings. At completion the installing contractor and owner sign the Record of Completion, which becomes the baseline for later inspections.
Power and circuits
The secondary supply must carry the system through 24 hours of standby plus 5 minutes of alarm. Battery capacity is calculated with a 20 percent aging factor: capacity equals 1.2 times (standby current times 24 hours, plus alarm current times alarm duration).
Class B circuits tolerate a single open only by means of an end-of-line resistor, and a break disables everything beyond it. Class A circuits return the path to the panel so a single open does not disable the circuit. Addressable notification may use Class X, which adds isolators so a short affects only one segment. Pathway survivability levels 0 through 4 describe how much protection the wiring receives, from none to 2-hour rated cable or raceway.
Notification circuits must deliver at least the listed minimum voltage to the last appliance; voltage-drop calculations, not rules of thumb, prove it.
Worked example
Standby current 0.50 A, alarm current 2.0 A. Capacity equals 1.2 times [(0.50 x 24) + (2.0 x 5/60)] = 1.2 x (12 + 0.167) = 14.6 Ah. Select the next standard battery size above 14.6 Ah, such as 18 Ah.
Traps
Omitting the 20 percent factor; using nominal rather than alarm currents from device data sheets; treating Class B as fault tolerant; submitting a riser diagram without device counts or battery calculations.