Douglas Krantz - Technical Writer - Describing How It Works

Is it Mandatory to use 20.4 Volts for NAC Calculations?

It's not a question of whether it's mandatory; it's a question of whether it will work. If it's 20.4 volts or if it's 22.0 volts, it's the voltage the manufacturer has designed, and then tested and listed by someone like UL, ULC, FM, CE, CCC, etc.

Is it Mandatory to use 20.4 Volts for NAC Calculations?

Is it Mandatory to use 20.4 Volts for NAC Calculations?

Greetings Douglas,

I have a question regarding voltage drop calculation. Is it mandatory to start with the NAC [Notification Appliance Circuit] supply voltage at 20.4 volts (15% below 24 VDC) as per NFPA? As per System Sensor, it is code complaint.

Please find the below mentioned link of the voltage drop calculation-

Thank you, JM

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.

Douglas Krantz

Mr. Krantz

Thanks for your answer. But I have searched NFPA 72 but not found the rules of 20.4 V. Would you please give the NFPA 2013 article no. that says the voltage drop of 20.4 V?

Thank you, JM


The NFPA Code is Performance Based, that is the fire alarm system to perform to a standard laid out in the Code. To that end, the requirement is that the Fire Detection and Alarm System (FDAS) is going to still detect fires and sound the alarm for 5 minutes at the end of a 24 hour power blackout.


On the other hand, designing the power supply voltages (the Functional Design) is a decision left up to the manufacturers.

Each manufacture's model of equipment is slightly different. The power supply could be a 12 volt nominal system, where the voltage normally runs between 10.2 volts and 13.7 volts. Then again, the power supply could be a 24 volt nominal system, where the voltage normally runs between 20.4 volts and 27.4 volts. From the manufactures point of view, it doesn't matter what voltage is used for the functional design, so long as the equipment and system performs according to the NFPA Code.

When deciding what voltage to use for the Notification Appliance Circuit (NAC) voltage drop, the voltage used has to be in compliance with the manufacture's voltage specifications for the particular make and model of fire alarm system.

If the manufacturer has designed a power supply that produces voltages anywhere from 20.4 volts to 27.4 volts, then the NAC circuit, along with all the horns and strobes have to perform with any of those voltages, or the circuit is out of compliance with the manufacture's specifications. If the NAC can provide enough power to keep the strobes flashing at 24 volts but not at 20.4 volts, then the NAC circuit is out of compliance.

To find out the exact voltages that need to be complied with, you have to talk to the manufacture's technical support. It may be that the maintenance manual or installation manual only shows "nominal" voltages. If so, to find out what "nominal" means, talk to technical support.

Remember, the NFPA Code only shows how a system is supposed to perform during a power blackout; exact voltages aren't in the performance specifications. The manufacture's specifications show the exact voltage to use for the calculations. When calculating the NAC voltage drop, it's the manufacturer's specifications that have to be complied with.

Code Compliance

As far as the "Code Compliance" in the System Sensor White Paper goes, the first suggestion is that you contact System Sensor to get the information. However, that is probably not going to get you very far because the "Code Compliance" is a whole series of indirect codes that eventually say that the Type Acceptance of a manufacturers system has to meet performance standards.

You, when making the NAC voltage drop calculations have to comply with the manufacturer's specifications for the type accepted equipment. To be type accepted, the equipment has to comply with a testing laboratory's specifications. The testing laboratory has to comply with the NFPA Code.

Complicated? Yes. Easy to comply with? No. The rules are there, they're just not found in a single place, there's a whole series of indirect rules being covered by many documents.

Douglas Krantz
Life Safety
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