Johnson Controls, Inc. SLC Wiring Manual —
P/N 51870:G 04/23/2009
53
Appendix A: Power Considerations
A.1 Supplying Power to 24 VDC Detectors and NACs
Resistance and Size
To determine the maximum allowable resistance that can be tolerated in supplying power to
24 VDC four-wire devices and NACs, use the calculations below. These simplified equations
assume that the devices are at the end of a long wire run. With the computed resistance and using
the manufacturers specifications for the desired wire, select the proper gauge wire for the power
run.
For Four-Wire Detectors:
For NACs:
Where:
R
max
= maximum resistance of the 24 VDC wires
V
ms
= minimum supply voltage (see Table A.1 below)
V
om
= minimum operating voltage of the detector or end-of-line relay, whichever is greater, in volts
N
= total number of detectors on the 24 VDC supply circuit
I
s
= detector current in standby
N
a
= number of detectors on the 24 VDC power circuit which must function at the same time in
alarm
I
a
= detector current in alarm
I
r
= end-of-line relay current
N
b
= number of Notification Appliance Devices
I
b
= Notification Appliance current when activated
The minimum supply voltages produced by Johnson Controls power supplies are listed below:
Table A.1 Minimum Supply Voltage
R
max
=
(V
ms
- V
om
)
(N)(I
s
) + (N
a
)(I
a
) + (I
r
)
R
max
=
(V
ms
- V
om
)
(N
b
)(I
b
)
NOTE:
This simplified equation assumes that the devices are at the end of a long wire run.
FACP
Vms
Power Supply
Vms
IFC-200
19.4
FCPS-24/E
19.1
IFC-640/E
19.15
FCPS-24S6/FCPS-24S8 19.1
KAPS-24 on IFC2-640 or
IFC-320
20.16V
MPS-24A/E
19.6
MPS-24B/E
20.1
MPS-400
19.23
ACPS-2406/E
19.8
AMPS-24/E
20.27
ACPS-610/E
19.4
