To get an answer to this, we have to look at the purpose of the wiring. The purpose of the wiring, whether it's FPL, FPLP, Communication Wire, THHN wire, etc., is to carry power from the amplifier to the speakers.
The output of the amplifiers, as they are configured for most of your buildings, is at a potential of 25 volts.
The potential output power of the amplifier, though, can be measured in watts.
Remember, the purpose of the system, whether it's a 25-volt system or 70-volt system is to carry power from the amplifier to the speakers. When considering power, electrical power uses the electrical term "Watts". Watt's Law is W = E x I or W (in Watts) equals E (Electromotive Force in Volts) times I (Intensity of Current in Amps)
The question is "If the output of the amplifier is changed from a potential of 25 volts to 70 volts, will the wire carry the power to the end of the circuit?"
Amplifier - Transformer - Distribution Circuit Wires - Transformer - Speakers
There are five parts to a circuit: the amplifier, the amplifier's transformer, the circuit wires, the speakers' transformers, the speakers. The amplifier provides the audio power; the circuit wires carry the power around the building; the speakers convert the electronic power they receive to the sound power that is heard.
Another way of looking at it is that audio distribution system is very similar to the country wide power grid. Both the power grid and the audio distribution system deliver AC power. The only real difference is the power grid has many generators, while the audio distribution system only has one generator (the audio amplifier).
I'm comparing the audio amplifier to the power grid generator because they both generate the power that is eventually used by the speakers. In fact, at least one brand of fire alarm audio amplifier has its own EVAC audio generator built in, which is default if the amplifier is turned on without any incoming audio from the fire alarm panel.
As far as the electronics inside the amplifier goes, whether it's a 20-watt amplifier, a 50-watt amplifier, or a 90-watt amplifier, the total added-up power used by the speakers cannot exceed the amplifier's power rating.
The Amplifier's Transformer
Like the power grid, the voltage sent to be distributed is determined by the voltage tap on the output transformer. In the fire alarm audio distribution system, the voltage tap will be either 25-volts or 70-volts.
A transformer is a 1:1 power conversion device. That means that if 10 watts of power is on its input, then close to 10 watts of power will be on its output. For our purposes, the internal inefficiencies of the transformer won't be considered.
Keep in mind that the maximum possible power taken from the amplifier is fixed and cannot ever be exceeded.
Even though the power used from the amplifier has to be kept below 10 watts, the current sent through the distribution circuit wires is different at 70 volts versus 25 volts.
Watt's Law is W = E x I
10 Watts equals 25 Volts times 0.40 Amps. The wires have to carry 0.4 Amps of current.
10 Watts equals 70 Volts times 0.14 Amps. The wires have to carry 0.4 Amps of current.
From the amplifier's output transformer, a 70-volt system sends far less current (0.14 Amps) through the distribution circuit wires than-25 volt system sends (0.40 Amps). You could say that the current for a 70-volt system is approximately one-third the current for a 25-volt system because 70 volts is approximately three times 25 volts.
Distribution Circuit Wires
The speakers are Notification Appliances. The wires they are attached to are Notification Appliance Circuit wires, or NAC for short. In most cases, the method of wiring is exactly the same for a conventional voice/speaker evacuation system, including the end of line resistor, as it is for a conventional horn/strove NAC circuit.
As far as the current carrying capacity of the wire, the size of the copper wire is the important part. The insulation around the wire, be it FPL, FPLR, FPLP, THHN, or Romex, doesn't affect the current carrying capacity of the wire.
In the copper wire itself, the current is everything.
When converting from a 25-volt audio distribution system to a 70-volt system, the current is cut to approximately one/third. Of course, that's when the power for the system (watts used by the speakers) is the same for both systems.
The answer to your question, then, is that if there wasn't an excess current problem before when the system was 25-volts, there probably won't be an excess current problem now when the system is changed to 70-volts.
Converting the other way, though, is a problem. When converting from a 70-volt audio distribution system to a 25-volt system, the current is approximately tripled. And that's when the when the power for the system (watts used by the speakers) is still the same for both systems.
When changing from a 70-volt system to a 25-volt system, a good probability is that the wires in the circuit won't handle three times the current. If you have to convert from a 70-volt system to a 25-volt system, re-calculate the voltage drop on the wires.
Speaker-Transformers and Speakers
For practical purposes, each combined speaker-transformer and speaker can be considered as a single package. By using the combination, Ohm's Law and Watt's Law are easier to use. For these calculations, you can consider the impedance of a speaker to be equal to its resistance.
Ohm's Law shows the relationship of voltage, current, and resistance: E = I x R or E (Voltage) equals I (Amperage) times R (Resistance)
. A 25-volt speaker will have a certain resistance (impedance), and a 70-volt speaker will have its own resistance (impedance).
When working with a 25-volt or 70-volt system, the transformer is part of the speaker assembly. Its primary winging has many taps, but its secondary is wired directly to the speaker coil. These taps are marked as 25-volts or 70-volts. When there are three wattage settings, there are actually three wattage taps for 25-volts and an additional three wattage taps for 70-volts, making a total of six independent taps on the transformer. Some speakers have more taps.
If the speaker/transformer taps aren't changed when the potential voltage is changed from 25-volts to 70-volts, the overall wattage used by the speaker radically increases.
Remember that when converting from a 25-volt system to a 70-volt system, the voltage approximately triples. Using Ohm's Law with the tripled voltage, E = I x R, the current triples.
According to Watt's Law, W = E x I
, tripling the voltage and tripling the current at the same time will increase the wattage nine times.
Failing to change the taps on the speaker transformers increases the current on the wires approximately three times, and the wattage used from the amplifier approximately nine times.
When converting to 70-volts, if you fail to change the speaker taps to 70-volt taps, or use 25-volt only speakers, you will most probably overload the system.
While the wires in the building's circuit will probably handle the 25-volt to 70-volt conversion, the speaker voltage has to match the amplifier voltage, or the system may overload.