AMPS-24 Manual —
P/N 51907:J2 02/26/2014
41
Section 4: Power Supply Calculations
Calculations must be done to determine standby and alarm DC current loads. Ampere-hour
requirements must be calculated as well to determine battery size.
The AMPS-24 provides filtered 24VDC (nominal) power that may be used for operating external
devices. The power for operating external devices is limited. Use Table 4.1 to determine if external
loading is within the capabilities of the power supply.
4.1 Calculating the System Current Draws
When used as the main power supply, the AMPS-24 must be able to power all internal system
devices (and several types of external devices) continuously during non-fire alarm conditions. Use
column 1 in Table 4.1 to calculate the Non-Fire Alarm Load on the power supply when applying
primary power. The main power supply must also provide a finite amount of additional current
during a fire alarm condition. Use column 2 in Table 4.1 to calculate the additional current needed
during fire alarms. The requirements for non-fire alarm and fire alarm current loads cannot exceed
the capabilities of the power supply as listed below.
4.1.1 How to Use the Calculating Tables
In the following section, the term “secondary” refers to the AMPS-24’s backup batteries: the term
“primary” is reserved for the AMPS-24’s primary source of power, 120 VAC 50/60 Hz power
(240 VAC 50/60 Hz for the AMPS-24E). The term “standby” refers to the output current required
when no fire alarm is present. The term “alarm” refers to the output current required when a fire
alarm is present.
The Primary Non-Fire Alarm Current and Fire Alarm Current columns are DC current calculations.
Use these calculations to confirm that the AMPS-24 can provide enough current to support the
system during Primary Non-Fire Alarm and Fire Alarm conditions.
Quantities
List the number of devices powered by the MAIN 24V Main Output (TB1), AUX 24
Output (TB3), and the AUX 24V Accessories Output (TB2 on the CPS-24). Devices powered by
the Accessories Outputs draw current directly from the AMPS-24. Devices powered by the Main
Output draw current through the fire panel’s connection to the power supply. Use these quantities to
calculate total current draw of each set of devices in calculation columns 1, 2, and 3.
Calculation Column 1 (Primary, Non-Fire Alarm Current in amps)
Add the contents of
Column 1 to get the current drawn from the AMPS-24 during a non-fire alarm condition, with AC
power applied. See Section 2.7.1.
Calculation Column 2 (Primary, Fire Alarm Current in amps)
Column 2 lets the system
designer determine the current load that the AMPS-24 must support during a fire alarm. See
Section 2.7.1.
Typically, a system should contain capacity to activate all output circuits and relays, and support
fire alarms on no less than 10% of Initiating Device Circuits, subject to the requirements of the
Authority Having Jurisdiction (AHJ).
If the system provides power for Notification Appliance Circuits, refer to the Device Compatibility
Document for 24 VDC notification appliances that are UL/ULC-listed for fire alarm systems.
Calculation Column 3 [Secondary (Battery) Non-Fire Alarm Current]
Column 3 lets
the system designer calculate the non-fire alarm current drawn from the secondary source in a non-
fire alarm condition during AC power loss. The non-fire alarm current is required to complete the
standby battery calculations. After summing all current draws, insert the total in Table 4.2.
