It's not the wire size that's important; it's getting enough power to all the horn or strobe devices that's important. The biggest problem, though, is even though the thinnest wire in a cable is under 15 ohms per 1,000 feet of 2 conductor cable (55 ohms per kilometer of 2 conductor cable), the resistance is a real problem.
This is an Ohm's Law issue; any current through the wire means that some of the voltage is lost into the wire.
Another point of view is that the horns and strobes are resistors, and so is the wire. Any voltage spend on the resistance of the wires won't reach the resistance of the horns and strobes.
To make sure that the wires won't soak up too much voltage, each NAC circuit has to be checked as it's being designed to make sure that enough voltage gets to the horns and strobes. This calculation is called a "Voltage Loss Calculation". Use the total resistance (in ohms) of both wires in the circuit, multiply that resistance by the total current (in amps) of all the horns and strobes on the circuit to find the voltage loss when there's an alarm.
The wire size and length make a difference in resistance, and the number of horns and strobes make a difference. The voltage from the panel also makes a difference. The panel may say "24 volts", but that's a nominal voltage (the name of the power supply, not its voltage). Usually, the voltage from the panel used for voltage loss calculations is more like 21 or 22 volts. Contact the manufacturer's technical support team to obtain the correct voltage.
Also, don't use the 24-volt current rating on the horn or strobe, use the current rating for the lowest voltage that can be applied as the amount of current. It seems counter-intuitive, but especially with strobes, the current goes up as the voltage goes down.
There is an easier way of determining wire size.
The easy way to find out what wire that should be used is to contact the manufacturer's technical support team. Ask them for design software for the Notification Appliance Circuit (NAC) for the exact model of fire alarm panel. Each manufacturer will have different requirements, and often each model of panel for each fire alarm system will have different requirements.
The software takes into account the voltage of the fire alarm panel, the resistance of the wire size being used, and the current being used by the horns and strobes. By using the manufacturer's software, you can make sure that the distance for the wire is kept short enough, and there are few enough horns or strobes on the NAC circuit.
If the wires are too long and/or there are too many horns and strobes on the circuit, there won't be enough voltage at the end of the circuit to power anything. The software will show that.
The software assumes you plan on using that company's horns and strobes. Also, when calling technical support, check to make sure the horns and strobes you are using are "compatible".
If the horns and strobes aren't compatible, they may not work correctly, or they may not work at all. Save yourself some time and money, make sure the horns and strobes are compatible with the panel.
What size wire should be used between the Fire Alarm Control Panel and the Notification devices like the horns and strobes? You will have to perform calculations, or use the manufacturer's software.