The size of the cable is not important here. The main reason to use relays would be if each system used a different voltage. For eample, if the fire alarm signal was 24 volts dc and the security system expected 12 volts ac, or perhaps just a switch closer, then a relay could be used. For this example you would pick a relay with a 24 volt dc coil and then connect the relays contact to the security system input. If the input expected 12 v ac input and had a 12 v ac "hot" connection, then connect the "hot" wire to the relay common contact and a wire from the relay normally open (no) contact to the security input. Just match the relay coil voltage to the fire alarm signal output. |