When the power goes out, your fire alarm system must keep working. That reliability depends on correctly sized batteries. It may sound simple—just put batteries in the panel—but the actual calculation is guided by strict codes like NFPA 72 and the IRR of RA 9514.
For developers and owners, this shows why fire alarm design isn’t just about devices on the wall. For engineers, it’s about precise computation, compliance, and safety.
What the Codes Require
Both NFPA 72 and RA 9514 require that fire alarm batteries must support:
- 24 hours of standby operation (system powered but no alarms)
- 5 minutes of alarm operation (sirens, strobes, modules active)
This ensures the system can still detect and notify during extended outages.
How Battery Sizing Is Computed
The calculation is straightforward in concept:
- Add up standby current (all devices in normal mode).
- Multiply by 24 hours to get standby ampere‑hours (Ah).
- Add up alarm current (horns, strobes, activated modules).
- Multiply by 5 minutes (0.083 hours) to get alarm Ah.
- Add standby and alarm Ah for total load.
- Apply a 25% safety margin for battery aging and real‑world conditions.
The result tells you the minimum battery capacity required.
Typical Device Current Draws
Here are common values used for estimates (actual numbers vary by brand):
- Control Panel: 150 mA standby, 300 mA alarm
- Smoke Detector: 0.5 mA standby, 3 mA alarm
- Modules: 1 mA standby, 3 mA alarm
- Horn/Strobe: 0 mA standby, 120 mA alarm
Example Calculation
Imagine a system with:
- 1 control panel
- 30 detectors
- 10 modules
- 12 horn/strobes
Standby load = 190 mA → 4.56 Ah over 24 hours
Alarm load = 1770 mA → 0.15 Ah over 5 minutes
Total = 4.71 Ah
With 25% margin = ~5.9 Ah
Recommended battery size: 7 Ah (two 12V batteries in series for 24V)
Common Battery Sizes
Fire alarm systems typically use sealed lead‑acid batteries in sizes such as:
- 7 Ah
- 12 Ah
- 18 Ah
- 26 Ah
- 40 Ah
- 65 Ah
- 100 Ah
The right size depends on your device count and system design.
Why This Matters
For electronics engineers, this is a clear design exercise. For developers and owners, it shows that fire alarm reliability depends on careful engineering—not guesswork.
At Buildlink Solutions, we prepare the full battery sizing, device load calculations, and FSIC‑ready documentation. If you’d like a consultation or quotation for your project, we’d be glad to help ensure your fire alarm system is compliant and dependable