Speaker strobes and strobe voltage line drop

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Speaker Strobes are installed on VOICE EVACUATION and MASS NOTIFICATION SYSTEMS to inform people of different options. Sometimes the option is to evacuate the building and other times it may be to stay put or move to another area.

Speaker Strobes have a different wiring configuration than Horn/Strobes do. Horn/Strobes are meant solely to evacuate a building. Speaker Strobes have two wires that power the strobes and two wires that power the speaker and allow the messages to be heard. Many times the wire on the speakers is shielded meaning it has like a foil type protective cover or shield around the wire to protect the cable from interference that could cause the speakers to have extra noise.

There is a power supply to power the strobes and another one to power the speaker which will have a watt draw. Most of the speakers will have an adjustment to allow them to be set at 1/4, 1/2, 1 watt etc... A 25 Watt voice evac system could handle 25 speakers set at 1 watt considering wattage affected by wire distances and also the mA draw of the strobes.

Strobes draw anywhere from 66mA to just over 200 mA on normal settings and higher when the cd (candela settings) are higher. An example of a strobe that draws 200mA would allow you to run 5 of them on a circuit with 1 amp. Many power supplies have 1.5 amp or 2 or even 2.5 amps available. Let’s say it has 2 amps available. Since each amp is 1000mA you could install ten of the 200mA strobes or so you would think. The other consideration is how far the strobe is from the power supply and what size wire you are using.

Now let’s look at how voltage line drop is figured out.

Load X Resistance = Voltage drop

(With our example of 10 strobes that have a load or mA draw of 200mA each or 2 amps together the Load is 2 so we times that by the resistance of 3.19 which I got from the NEC (NFPA 70) electrical code for 14 gauge wire and 2 x 3.19 is a voltage line drop of 6.38 volts so if we start with 24 we would end up with 17.62 and since the strobe will work from 16 VDC to 33VDC we should be fine as long as we have a 2amp circuit. Many Engineers will require a 20% buffer when they design the system so they tell you to only put 80% or in this case 8 strobes on a circuit.

When you have a power supply with 4 circuits 2 amps each then you could only put 32 strobes on one power supply and then you would need an additional power supply. The limit would be 40 so with only 32 on the power supply you can add up to 8 at a later time. These examples are using 24 Volts DC not 12.

Going back to our example of the drop there is another item to consider. Instead of counting on a voltage of 24 we start with 20.4 and then subtract our voltage line drop giving us only 14.02 which is outside the limits so then we do another calculation and find out that when we run a cable that is only 689 feet on the circuit then the voltage line drop is only 4.4 volts so we end up with over 16 volts DC at the last strobe even with a safety factor.

So you can put less strobes on a circuit to help your calculations


you can shorten the cable runs to help put your calculations into the proper limits


You can use a larger power supply


You can find strobes that have a lower current draw

When you run 18 gauge wire (cable) normally these cables are 2 conductor and the cable is normally FPL, FPLR or FPLP and is normally SOLID copper.

18 gauge wire has a resistance of 8.08 Ohms resistance per 1000′ so you could install up to 5 strobes on a circuit that was 545 feet and have a voltage line drop of 4.4 volts. So you can see that installing 18 gauge cable for strobes is not the best option. The 14 gauge wire is a better choice.

Many people say then let’s use 10 gauge or 12 gauge cable and the figures will be better and that sounds reasonable although 10 gauge cable does not fit under the strobe terminals and 12 gauge is very difficult to work with. Sometimes you can make pigtails from the higher gauge cable to a lower size although this can cause issues at a later date.

Keep in mind when you have to install class A circuits that most of the time you would loose 2 of the 4 circuits on the power supply so you would need double the power supplies. Some manufacturers allow you to keep all four circuits. FireLite has some power supplies where you don’t loose the other two circuits when you add special modules.


Call Larry Love in Salt Lake City Utah for more information (801) 428 1384 Larry@peakalarm.com