How to Calculate Fire Alarm Battery Size (NFPA 72)

What NFPA 72 requires of the batteries

A fire alarm control unit has to keep working when the building loses power. NFPA 72 (2022), in 10.6.7, requires a secondary power supply with enough capacity to run the system in its normal, quiescent condition for 24 hours and then operate every notification appliance for 5 minutes at the end of that period. For an emergency voice/alarm communications system the alarm period is 15 minutes at the maximum connected load. Some system types and some jurisdictions require longer standby, and the adopted edition of NFPA 72 and the AHJ govern, so confirm the hours before you start. The batteries that meet the requirement are almost always two 12 V sealed lead-acid batteries in series for a 24 V panel, and the amp-hour rating of the set is the rating of one battery, not the two added together.

The arithmetic is short. What takes time is collecting the currents, and that is where most sheets go wrong.

Step 1: list every device's standby and alarm current

Pull the data sheet for the panel and for every device drawing power from it, and write down two numbers each: the standby (supervisory) current and the alarm current, in amps. Divide milliamps by 1,000. The list includes:

• The control unit itself, which has its own standby and alarm draw.
• Every addressable detector and module on the signaling line circuit, or the end-of-line current of each conventional zone.
• Notification appliances: horns, strobes and horn/strobes at the candela setting installed, and speakers at the tap setting. These draw almost nothing in standby and everything in alarm.
• Annunciators, relays, the communicator, door holders and any other auxiliary device powered by the panel. Door holders draw in standby and drop out in alarm; some sheets carry them in both columns to be safe.

Multiply each device's current by its quantity and total the two columns. Anything fed from a separate NAC power supply with its own batteries goes on that supply's sheet, not this one.

Step 2: amp-hours, derating and the next standard size

The formula the fire alarm battery calculator uses is:

Ah = [(standby amps × 24) + (alarm amps × 5 ÷ 60)] × 1.2

Standby amp-hours are the standby current times 24 hours. Alarm amp-hours are the alarm current times 5 minutes expressed in hours (5 ÷ 60 = 0.0833; use 15 ÷ 60 = 0.25 for voice). Add them and multiply by 1.2, a 20% derating factor for the capacity batteries lose to age and temperature. NFPA 72 sets the durations; the 20% comes from the panel manufacturers' installation instructions, which are part of the listing, and it is what nearly every AHJ expects to see. A few manufacturers and AHJs call for 25%. Then round up to the next standard battery: 7, 12, 18, 26, 33, 40 or 55 Ah. Never round down.

Check two more things in the panel manual. First, the largest battery the panel can charge; a typical control unit charges up to 18 or 26 Ah inside the cabinet and perhaps 55 Ah in an external battery cabinet. Second, whether the batteries physically fit. If the calculation exceeds the panel's maximum, you do not buy bigger batteries. You move load to a listed NAC power supply with its own batteries and its own calculation, or you reduce the load.

Worked example

A conventional panel with its detectors, a dozen horn/strobes and a communicator totals 0.350 A standby and 2.8 A alarm.

• Standby: 0.350 A × 24 h = 8.4 Ah
• Alarm: 2.8 A × 0.0833 h = 0.233 Ah
• Subtotal: 8.633 Ah
• Derated: 8.633 × 1.2 = 10.36 Ah

The next standard size up is a 12 Ah set: two 12 V, 12 Ah batteries. Notice what drives the answer. The 5-minute alarm period adds a few tenths of an amp-hour, while 350 mA of standby load over 24 hours accounts for 8.4 Ah. Cutting standby current, by moving auxiliary loads off the panel, does far more for battery size than trimming a strobe or two. If this panel's charger topped out at 7 Ah, the job would need a bigger panel or a separate power supply. For a voice system the alarm term becomes 2.8 × 0.25 = 0.7 Ah and the derated result is 10.92 Ah, still a 12 Ah set. The alarm current on this sheet should match the number on your NAC voltage drop calculation, since both come from the same appliance list.

Document it, then test it every year

NFPA 72 Chapter 7 lists battery calculations among the required system documentation, and the AHJ will want them in the submittal, with the record of completion, and in the document box at the panel. Show the device list with quantities, the standby and alarm current for each, the standby hours and alarm minutes used, the derating factor, the result and the batteries installed. NFPA 72 also requires batteries to be marked with their month and year of manufacture; write it on if the manufacturer did not. Chapter 14 then calls for annual battery testing: a charger test, a discharge test and a load voltage test for sealed lead-acid batteries, per Table 14.4.3.2 in the 2022 edition. Batteries are replaced on the manufacturer's recommended schedule or when they fail a test, and sealed lead-acid sets have traditionally been replaced within about five years of manufacture, sooner in a hot location. Check the table in the adopted edition and the panel manual for the specifics. Fireforge carries this battery calculator, the NAC voltage drop calculator and the candela tables offline, and battery sizing is a dependable question on the fire alarm practice exam.