🔥 FDAS / Fire · 6 min read

FACP Standby Battery Calculation — 24 Hours Plus 5 Minutes

The battery calculation is a required submittal for every FDAS. It is simple arithmetic, but every device on the system has to be in the table.

Build the current table

List every current draw in two columns — standby (quiescent) and alarm:

Control panel and each expansion/loop card
Every initiating device (addressable detectors, modules)
Every notification appliance (alarm column only, at rated candela/dBA setting)
Auxiliary relays, communicators, door holders released on alarm

The formula

AH = [ (I_standby × T_standby) + (I_alarm × T_alarm) ] × 1.2

T_standby = 24 h for a protected-premises system (60 or 72 h for some remote/central-station and voice systems — check the AHJ).
T_alarm = 5 min (0.0833 h); 15 min for voice evacuation.
1.2 = 20% ageing/derating margin.

Worked example

I_standby = 0.45 A, I_alarm = 3.8 A:

AH = [ (0.45 × 24) + (3.8 × 0.0833) ] × 1.2
= [ 10.8 + 0.317 ] × 1.2 = 13.3 AH → select 18 AH batteries

Also check

Battery charger capacity: must recharge the battery to full within 48 h while carrying standby load.
Battery cabinet space and the panel’s maximum battery size.
Notification booster power supplies each get their own battery calculation.

🔥 Using EngEst Pro

EngEst Pro’s FDAS Calculator totals standby and alarm current from the device schedule, applies the 24 h + 5 min + 1.2 formula, and sizes the FACP battery — the result and the devices flow into the BOM and the BFP submittal.

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