Power Supply Requirements
Calculating the Battery Size
40
FCPS-24S PN 51977:C1 08/06/03
6.4 Calculating the Battery Size
Use Table 6.4 to calculate the total Standby and Alarm load in ampere hours (AH). This total load
determines the battery size (in AH) required to support the power supply under the loss of AC
power. Complete Table 6.4 as follows:
1. Enter the totals from Table 6.3 on page 39, Calculation Columns 2 and 3 where shown
2. Enter the NFPA Standby and Alarm times. Refer to “NFPA Battery Requirements”
3. Calculate the ampere hours for Standby and Alarm, then sum the Standby and Alarm ampere
hours
4. Multiply the sum by the derating factor of 1.2 to calculate the proper battery size (in AH)
5. Write the ampere hour requirements on the Protected Premises label located inside the
cabinet door
6.4.1 NFPA Battery Requirements
• NFPA 72 Local and Proprietary Fire Alarm Systems require 24 hours of standby power
followed by 5 minutes in alarm
• NFPA 72 Central, Auxiliary and Remote Station Fire Alarm Systems require 60 hours of
standby followed by 5 minutes in alarm. Batteries installed in a system powered by a
generator need to provide at least 4 hours of standby power
6.4.2 Selecting and Locating Batteries
Select batteries that meet or exceed the total ampere hours calculated in Table 6.4 . The power
supply can charge batteries in the 7 AH to 18 AH range. The power supply cabinet is capable of
housing batteries up to 7 AH. Batteries larger than 7 AH require the BB-17 or other UL listed
external battery cabinet.
Table 6.4 Total Secondary Power Requirements at 24 VDC
Secondary Standby Load
(total from Table 6.3
Calculation Column 3)
[ ]
Required Standby Time
(24 or 60 hours)
X [ ]
=
AH
Primary Alarm Load
(total from Table 6.3
Calculation Column 2)
[ ]
Required Alarm Time
(for 5 min., enter 0.084,
for 10 min., enter 0.168)
X [
=
AH
Sum of Standby and Alarm Ampere Hours
=
AH
Multiply by the Derating Factor
X 1.2
Battery Size, Total Ampere Hours Required
=
AH
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