NFPA 72 is the National Fire Alarm and Signaling Code used by most fire alarm professionals for the design, installation, and maintenance of alarm panels and emergency communication systems. Fire Marshal certification of a system usually requires written documentation that the system meets and complies with NFPA 72 Code. One important component of certification is power. NFPA 72 states that the panel shall have a primary and secondary source of power. In the majority of panels, the primary power is 120 VAC. The secondary source is usually a sealed lead acid (SLA) rechargeable battery.
“What size batteries should I use in the panel?” is a question that could have many different answers based on the type of system, power supply, panel components, and configuration. Battery size is computed by adding the power requirements of each component in the alarm system. Often, manufacturers include a battery calculation worksheet in the power supply manual that simplifies the calculations. The fire alarm system installer enters the correct count of equipment on a worksheet. And from that, two important pieces of information can be determined: the standby power requirement and the alarm power requirement.
Standby power is any power that is needed for the panel equipment to operate under normal conditions. The display screen, the main CPU board, and additional specialty boards or cards are all examples of items requiring standby power in a fire alarm system. “Standby” sounds basic, but it is the main contributor to battery size. Alarm power is power that is needed in an alarm condition to operate components, such as horns, strobes, speakers, emergency panel lights, and relays. Standby and alarm power are also measured in amperes.
Once standby and alarm power have been calculated, they are multiplied by the time component given in the initial construction specification. Standby times are usually 24 hours. This allows a full day to address the reason for the main power loss, which shows as a trouble condition on the fire alarm control panel (FACP). More stringent standby times call for 60 hours of backup. This amount of time allows the panel to operate without primary power through a normal weekend but results in larger batteries, more batteries, and more equipment to charge the batteries. If a technician calculated standby power to be 1.5 amperes, a 24-hour standby time would result in a minimum battery size of 36 Amp Hours (1.5 Amperes x 24 hours). When including the 20% spare capacity, required by NFPA 72, in the previous example, a minimum battery size of 43.2 Amp Hours is required. That same standby power on a 60-hour standby system results in a minimum battery size of 90 Amp Hours (1.5 Amperes x 60 hours) and a minimum battery size of 108 Amp Hours when including the 20% spare capacity!
Alarm power has very little effect on battery size. Alarm times are given to be 5 minutes (.083 hours) for horn/strobe systems and 15 minutes (.25 hours) for voice (speaker) systems. If a technician determined that 10 amperes of alarm power were needed, it would only add roughly 1 Amp Hour for a horn/strobe system (10 amperes x 0.083 hours) and 2.5 Amp Hours for a voice (speaker) system (10 amperes x 0.25 hours).
Sizing the correct batteries for a fire alarm system requires proper calculation of the standby and alarm power of all equipment connected to a power supply or FACP. It is also important to make sure that the panel or FACP is able to fully charge the selected batteries. Correct battery sizing and maintenance will ensure reliable fire alarm system response, even if the power is out.
In some configurations, the secondary source of power is a generator. When primary power fails, the generator turns on to deliver power to critical systems. In this case, NFPA states that the standby time is 4 hours. The batteries provide a power bridge from when the primary source fails to when the secondary (generator) source is delivered to the panel. The fuel level for the generator must be monitored by the fire alarm panel, providing a warning if low level is reached.
