To understand the requirements in the code, the question isn't "Is it mandatory to follow the code?", the question is "Why?". To understand the "Why" of the requirements, we have to backwards-engineer the requirements.
When performing the voltage drop calculations for the Notification Appliance Circuit (NAC), there are two concerns that have to be dealt with before even determining what voltage to use. One is Life-Safety and Reasonableness, and the other is Real Life Battery Voltage.
Life-Safety and Reasonableness
For Life-Safety purposes, even when the utility power is off (power blackout), the fire alarm system should still work as a Detect and Warn system. When there's a power blackout, the only things keeping the fire alarm system alive are the backup batteries.
To be reasonable, though, the length of time the batteries should keep the system working is considered to be 24 hours. That is the point in time that the batteries are considered to be close to exhaustion. Of course, they still have to be able to sound the alarm for either 5 minutes or 15 minutes at the end of that 24 hours.
Real-Life Battery Voltage
Commonly, in fire alarm systems, the name of the battery stamped on its side is 12 volts. There are two of these 12 volt stamped batteries, making a total of 24 volts, at least when adding up the stamped numbers.
Yes, I'm being coy here and not saying that these are 24 volts of battery. The reason is that the 12 volts stamped on the side of the battery isn't the real-life voltage of the battery, the 12 volts stamped on the side of the battery is only the name of the battery (Nominal).
These are sealed lead-acid batteries. A chemical reaction is used to produce the voltage, and the chemicals change and move as the battery power is used up.
In a 12 volt nominal battery, when fully charged, the voltage produced by the chemical reactions is really about 13.6 volts. This voltage changes as the chemicals change and move.
When power is slowly drained from the battery, and its chemical makeup changes, the voltage produced by these chemicals drops. When the voltage gets down too low, the battery is damaged; the amp-hours of power is reduced. Some batteries can be damaged when the voltage drops below 10 volts on a 12 volt nominal battery. The engineers at the NFPA have put the cutoff at 10.2 volts, or 20.4 volts when two 12 volt nominal batteries are wired in series.
Below this voltage point, also, there's not much left of the fresh power-producing chemicals inside the battery, it's considered dead.
Check It Out
Just to check this out, take a voltmeter, go up to a fire alarm panel, and measure the voltage on the batteries. Unless there's a power outage or something's wrong with the power supply or batteries, the voltage on the batteries will be higher than the voltage stamped on the side of the batteries.
Life-Safety and Battery Life
The 20.4 volts is the real-life, worst-case scenario of what is needed to keep the fire alarm system working at the end of a 24 hour power blackout. The assumed voltage has to be that low because, even if the power to the building has been out for 24 hours, the fire alarm system still has to detect fire and warn people. Because the system still has to detect and warn during a power blackout, common sense dictates that 20.4 volts should be used in the Notification Appliance Circuit (NAC) Voltage Drop Calculations.
Yes, when following the complicated network of rules in the NFPA Code, 20.4 volts is mandatory for the calculations, and most governments have said that the NFPA Code is the Law. However, the NFPA Code isn't there to tell us how to follow the law, the NFPA Code is there to tell us how to make a Fire Detection and Alarm System (FDAS) that works. They just have to use legal-ease language to explain real-life applications.