There are two issues here. One is a wire-break issue, and the other is a voltage-loss over the wires issue. Both of them are a life safety, fire alarm issue.
Wire Break Issue
Because people depend on a fire alarm system to detect fires and warn them to take action, if a wire breaks, or comes loose from a screw terminal, some or all horns and strobes on a circuit may not work. If that happens, some people may not hear the alarm and won't take quick enough action.
Yes, the wires may break or come loose from a connection. That kind of thing often happens. The idea for life-safety, though, is that the fire alarm control panel detects the broken connection and announces to the building owner that there is a problem with the fire alarm system. If the owner knows about the problem, the problem can be fixed before there is a fire.
In a fire alarm system, to detect that a wire breaks connection, the control panel performs a continuity check of the wires. If the continuity is broken anywhere, the current stops, and the panel turns on its trouble light and buzzer.
In order to perform the continuity check of the wires (supervise the wires), the panel passes current through one wire, all the way to the end of the circuit, through a end-of-line resistor, through the other wire all the way back to the panel.
Any wire that is t-tapped onto the circuit will not be included in the continuity check, so the panel can't supervise the t-tapped wire or the t-tapped connections.
Also, when installing the wires, make sure they are not twisted together as they go under the screw terminals. If the wires are twisted together and then come loose from a screw terminal, the panel is lied to and assumes the horn or strobe is still connected.
This information is in the installation manual for the control panel, and this information is in the installation instructions for the horns and strobes.
Read the installation manual for the control panel and the installation sheets that come with the horns and strobes. Both of these have specific instructions on wiring. Follow these instructions.
Also, remember that each manufacturer has slight variations on wiring, so read and follow the instructions for each manufacture.
Many times, you can even find this information on the web.
Voltage Loss Over Wires Issue
Fires occur when the utility company is providing power to the building; fires occur when power lines are down and the utility company isn't providing power. That's why there is a battery backup in the control panel, so the fire alarm system will work even during a power blackout.
A 24-volt panel, however, doesn't have a 24-volt power supply; 24 volts is the name of panel (Nominal). The true power supply voltage is the varying voltage on the battery.
When the battery is fully charged, it's really about 27.5 volts. While it's being used, the battery voltage slowly goes down, so that when the battery is about dead, the voltage is more like 20 volts. All horns and strobes still have to be working when the power supply in the panel is down to 20 volts.
OK. Some fire alarm control panels only go down to 22 volts. However, the only way you can use the 22-volt assumption is to locate this information in the panel's installation manual, or by calling the manufacturer's technical support team.
When you read the installation sheets that come with the horns and strobes, you will see that the horns and strobes require at least 16 volts to actually work. When you subtract the 16 volts needed by the horns and strobes, from the 20 volts provided by the panel during a power blackout, you will find that any voltage lost in the wires has to be less than 4 volts.
Because there are 2 conductors in a circuit, that's 2 volts per conductor.
In a courthouse, I've had to find out why the strobes didn't work on a horn/strobe circuit. What I found is that in the past, "just 4 more horns and strobes were added", and then later, "just 4 horns and strobes were added", and then still later, "just 4 horns and strobes were added". The true number was actually an added 12 horns and strobes, and a whole lot of added wire. To compound the added wire issue, much of the added wires were a thin 18 AWG wire (about 0.75mm2 wire).
Even while the panel's power supply was at 27.5 volts, because of the added current in the horns and strobes, and the added length of all of the added wires, near the end of the circuit, the strobes flashed at a very slow rate. The last one only received 13.5 volts and didn't flash at all.
Bottom line, even though the panel was providing 27.5 volts, by the end of the circuit, the voltage lost in the wiring was measured to be 13.5 volts. Not good for life safety purposes, especially if there's a power blackout.
Just adding the horns and strobes, and even using the proper "In and Out Wiring" is risky. The fire marshal may even require official "Voltage Loss Calculations" on the whole circuit, including the old horns, strobes, and with the added wiring included in the circuit. If the current in the added horns and strobes, and the voltage loss in the added wiring fails the calculations, you may be required to rewire the whole building.
There's a very good chance the circuit will fail the voltage loss calculations, I wouldn't suggest even trying that.
Instead, since the wires are already installed, the easy alternative is to make each horn and strobe a separate NAC circuit. NAC stands for Notification Appliance Circuit. Because each added horn/strobe will be at the end of the circuit, an end-of-line resistor has to be added to each new horn/strobe location.
To add new NAC circuits, you have to add a separate "Listed to be Used for Fire Alarm NAC Circuits" power supply. Most of them come with the ability to power and supervise up to four complete NAC circuits.
Contact the fire alarm company that services the system to find out which NAC power supply to use, and how to wire it in.
To Make It Work
There are other ways to make the added horns and strobes work, but right now, this is the quickest and cheapest way I can think of.