Horn Strobes on Fire Alarm Panels

calculating how many is important

calculating how many is important

How many Horn Strobes can I put on a circuit is a question that I often get from Electrical contractors and the answer is “it depends.” They really don’t want to hear that answer so my normal answer is 8 Horn/Strobes per loop. If you have some that are set to the lower settings and some set on 75cd then a mix will put you close to that figure. If they are all ceiling mounted high current horn/strobes then the answer will be different.

Not figuring in a volatage line drop if you have a 1.5 amp circuit that is 1500 mA and if your using Horn/Strobes set at 75cd you could put 7 on a circuit and if the circuit is a 2amp circuit 2000mA you could put 9 and if the circuit is a 2.5amp Circuit you could put 12 although now you need to consider 2 more factors. The distance of your wire and if you have a requirement to have any extra capacity on your circuit for additions at a later date. Many Electrical Engineers will require that you have a 25% buffer so on a 1.5amp circuit you could only put 5 and on a 2amp circuit you could only put 7 and on a 2.5amp circuit you could only install 9.

On a 500 foot run of 14 gauge wire you will have just over a 3.3 volt line drop so if you started at 24 you will now be at 20.7 volts and now you need to figure all the combined devices and their draw. So if you have a 1000 foot run you cannot put 10 horn/strobe on that circuit because with the mA draw and the voltage line drop combined you will not have enough voltage/current left to power all the appliances.

So now you have your answer as long as you know what power supply you are using and what gauge wire and how long your loop or circuit is.

Now I will explain how I get to that figure.

1. What gauge wire are you using? FPL, FPLR or FPLP solid 14 gauge 2 conductor wire is the most common so doing calculations as to how far you can run the wire and still have power at the last device is very important. If you use 18 gauge cable the distance you can go will be less.

2. What power supply are you using to power the horn/strobes? Normally Horn/Strobes are 24 volt DC although you can get them in 12 volt for very small projects although I will talk about 24 VDC models. Many installation use 6amp UL listed Fire Alarm supplies or 4 –1.5amp circuits although I prefer the 10amp power supplies with four of the 2.5 amp circuits. (Batteries are a whole different topic but remember your batteries need to run the Horn/Strobes for 5 min after the power has been out for 24 hours or for 15 min if the building is a larger building so choosing the right size batteries is important)

3. What cd or candela (candle power) setting and what setting will you use for the sound are other questions that will help you determine how many you can put on a circuit. There are 3 main brands that I will use for comparison (Gentex, System Sensor and Cooper Wheelock) and the appliances set to 15cd for a small area like a restroom would draw between 105 and 158mA depending on the unit you decided on. Most areas would have the cd set to 75 so the horn/strobes set to 75 would draw between 162 and 253mA or an average of just over 205mA. If you are in larger rooms and you have to have the higher settings then the mA is higher. Do your battery calculations using the exact models that you are dealing with. For this example we will an average of 205mA draw per appliance. Remember when checking the mA draw to include the horn draw and the strobe draw for the cd that you will need. Many people overlook the horn db mA draw.

4. Voltage line drop means simply that the further you run a wire know that you will loose some of your voltage just because of your wire run and the length of it.There are spread sheets just to calculate these figures.


Load X Ohms Resistance = Voltage drop.

Example: A 24 Volt Horn/Strobe will still sound 16 Volts
The example above of 12 Horn/Strobes on 1000 feet would not allow the last horn to sound
whereas the 10 on 500 feet would work just fine (within acceptable limits) as long as you have 2.2 avail amps on the circuit

Standby load (amps) x standby time (hrs)=standby AH
convert mA into amps for this chart

Battery calculations should be done for all circuits on a project and most Horn/Strobe manufactures have those sheets on their web sites.

Standby Load (Amps)
Multiply by Required Standby Time(24 or 60 hrs)
Total Standby AH =

Alarm load(amps) x alarm time (.084hrs)=Alarm AH
Standby AH + Alarm AH = Total AH

This will assist you so you can purchase the appropriate batteries.

Ohm’s law: E=IxR (E=Volts, I=amps, R= ohms)
Power law: P=ExI (P=watts, E=volts, I=amps)

If you don’t want to mess with any calculations make sure you hire an experienced Fire Alarm Contractor such as PEAK ALARM who has been in business for over 45 years and has the ability to do any size project.

Call Larry Love at Peak Alarm 801 486 7231 x 384 or email any suggestions or corrections to Larry@peakalarm.com