User's Manual

Versus Technology, Inc.
Hardware and Wiring Installation Guide 35
4.8.1 Power Requirements
The relay board requires a 24VAC power supply. The watts needed varies with the type of relay
controller device being used.
To calculate watts needed:
If you know the number of watts each device needs, you can calculate total watts needed by
multiplying the number of devices times the watts needed by each.
If you do not know the watts that each device needs, but you know the amps needed by each
device, you can calculate the number of watts needed as follows:
The number of amps required by each device multiplied by the number of devices
multiplied by the volts (24V).
Example:
If using lights, and each light requires .04 amps, there are 48 lights on the relay board,
and the power supply is 24V, the calculation is:
.04 amps x 48 lights x 24V = 46.08 watts or more required.
4.8.2 Wire Size Determination
The designer must be sure the last device on the circuit has sufficient voltage to operate the device
within its rated voltage. When calculating the voltage available to the last device, it is necessary to
consider the voltage drop due to the resistance of the wire. The thicker the wire, the less the
voltage drop. Generally, for purposes of determining the wire size necessary for the system, it is
best to consider all of the devices as “lumped” on the end of the supply circuit (simulates “worst
case”).
Typical wire size resistance:
18 AWG solid Approximately 8 ohms/1000 ft.
16 AWG solid Approximately 5 ohms/1000 ft.
14 AWG solid Approximately 3 ohms/1000 ft.
12 AWG solid Approximately 2 ohms/1000 ft.
Example: Assume you have 10 devices on a zone and each requires 50 mA average and 2000 ft.
of 14 AWG wiring (total length = outgoing + return). The voltage at the end of the loop is 0.050
amps per device x 10 devices x 3 ohms/1000 ft. x 2000 ft. = 3 volt drop.
If you have a power supply that is 20 volts, each device runs at 18 volts, according to this
calculation you are under voltage by 1 volt.