Fire Alarm Speakers and Voltage Drop

[Shujinko]

Is there a limit to voltage drop allowed on a fire alarm speaker? What is the maximum voltage drop or minimum voltage allowed on a 25V speaker? Is there anything in NFPA 72 that addresses this or does the fire alarm speaker manufacturer set a limit on the minimum voltage required for the FA speaker to function correctly?

 

[LarryFine]

I've never heard of a voltage drop concern on a constant-voltage audio line, which is what this sounds like.

The important thing is to not exceed the wattage rating with the total of the individual speakers.

 

[GoldDigger]

I've never heard of a voltage drop concern on a constant-voltage audio line, which is what this sounds like.

The important thing is to not exceed the wattage rating with the total of the individual speakers.

Indeed, the voltage drop is likely to be low enough to be ignored at the low line currents involved. A 70 volt line installation will be less susceptible to voltage drop than a 25V line setup. But the higher voltage limits some of the things you can do running the wire.
Usually there is enough available power from the amplifier that, if the voltage drop is large compared to 25V you can still get the required sound level at the speaker by using a higher wattage tap.
The sound output is linearly proportional to the electrical power into the speaker, so you can calculate the reduction in sound level from knowing the voltage drop. It is not something that the manufacturer normally specifies. Instead they give a sound output for a given voltage and wattage tap and let you decide whether that meets the sound level requirements of the applicable codes.

 

[gadfly56]

I have also never heard of, nor done, voltage drop calculations for speaker circuits. Each speaker has its own transformer which is why you can tap them at different wattage levels.

As an aside, why is someone using 25 volt speaker lines? Most of the big boys let you do 25 or 70.7 volts, and the higher voltage will more likely keep you out of trouble.

 

[Shujinko]

Indeed, the voltage drop is likely to be low enough to be ignored at the low line currents involved. A 70 volt line installation will be less susceptible to voltage drop than a 25V line setup. But the higher voltage limits some of the things you can do running the wire.
Usually there is enough available power from the amplifier that, if the voltage drop is large compared to 25V you can still get the required sound level at the speaker by using a higher wattage tap.
The sound output is linearly proportional to the electrical power into the speaker, so you can calculate the reduction in sound level from knowing the voltage drop. It is not something that the manufacturer normally specifies. Instead they give a sound output for a given voltage and wattage tap and let you decide whether that meets the sound level requirements of the applicable codes.

The voltage readings in the field were measured at 9V at the last FA speaker in the circuit and at 14V at the circuit terminals at the amplifier side. This is on a 25V system. That's why I am asking the question. This why I am asking the question in the original post.

 

[LarryFine]

The voltage readings in the field were measured at 9V at the last FA speaker in the circuit and at 14V at the circuit terminals at the amplifier side. This is on a 25V system. That's why I am asking the question. This why I am asking the question in the original post.

That sounds overloaded. I'd suspect the transformer tap wattages add up to more than the amp's capacity.

 

[juliekile]

Calculating voltage drop is crucial for deciding whether or not your notification appliances can operate with the supplied head end equipment. Almost all fire alarm control units operate on a 24 volt DC power supply. There are a few 12 volt DC fire alarm control panels. The calculations for NAC voltage drops are the same for these systems, however the cut off voltage for a 12 volt system will be roughly half that of a 24 volt system.

 

[GoldDigger]

Calculating voltage drop is crucial for deciding whether or not your notification appliances can operate with the supplied head end equipment. Almost all fire alarm control units operate on a 24 volt DC power supply. There are a few 12 volt DC fire alarm control panels. The calculations for NAC voltage drops are the same for these systems, however the cut off voltage for a 12 volt system will be roughly half that of a 24 volt system.

A valid point, but the OP's situation is specifically audio notification elements (i.e. speakers). The two likely system voltages will be 70v and 25V. And a significant voltage drop will not cause a complete failure but may instead drop the sound level in some spots below the statutory minimum.

 

[ryant35]

I do calculate dB loss with wire distance for all speaker circuits. Wire distance doesn't affect speaker circuits nearly as much as strobe circuits, but the calc allows you to keep tract of your wattage usage per circuit and per amp.

 

[wilkin250r]

The voltage readings in the field were measured at 9V at the last FA speaker in the circuit and at 14V at the circuit terminals at the amplifier side. This is on a 25V system. That's why I am asking the question. This why I am asking the question in the original post.

What kind of meter are you testing with? It's not going to be a constant sinusoidal waveform like an AC power source, and I'm pretty sure the AC measurements of your typical multimeter are only accurate within a certain frequency range (obviously around 60Hz), and might not be capable of measuring at 3000Hz+. So unless you're measuring with an oscilloscope, your readings might not be accurate.

But to answer the original question, I just finished a job installing a fire alarm in a high rise building with lots of speaker circuits, and the design engineer used -3dB as a reference point for max allowable power loss, but if that guideline exists as an actual specification, I've never come across it.

 

[GoldDigger]

If you are driving an amplifier with a 25V line output to maximum output without clipping and are measuring only 14V, the amp is overloaded. The load resistance is too low, which corresponds to the sum of the output tap wattages being more than the nominal power output of the amp.
If you are not driving the amp to maximum output, you will not be getting the sound power output you calculated.

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