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Do You have to Calculate Voltage Drop on All NAC Circuits?

By Douglas Krantz | Maintenance

Do You have to Calculate Voltage Drop on All NAC Circuits?

Do You have to Calculate Voltage Drop on All NAC Circuits?

Greetings Douglas,

I have a question regarding the necessity of voltage-drop calculation for NAC in class A circuit. My questions are as follows:
  • Is it allowed to connect conventional sounders in class A?

  • If yes, is voltage-drop calculation is required? Why?

  • In case of addressable sounder, is voltage-drop calculation is required?

Thank You, RA

Does the System Work?

When we're designing or troubleshooting a fire alarm system, we have to remember that a fire alarm system is first and foremost a life safety system. The rules, codes, and laws covering the building's fire alarm system show what has to be in the system, but they don't really show how to make any of the system work.

In order to properly design a fire alarm system, at least from a technical point of view, we have to figure out how to accomplish the requirements in the rules, codes, and laws.

In other words, instead of going at this from an attitude of "The rules, codes, and laws say I have to follow their guidelines, so I'll do as they say." Designers and technicians should have the attitude of "Whatever I do, the bottom line is that the fire alarm system has to detect fires and warn the occupants of the building."

Notification Appliance Circuit Voltage-drop Calculations

Inside the installation manual for the fire alarm control panel or the Supplemental Notification Appliance Circuit (SNAC) power supply, there is often a worksheet to help with the voltage-drop calculations. Using those worksheets on each and every NAC circuit could be all that is needed.

If you don't have access to the worksheets, you have to do your own voltage-drop calculations. Even if the worksheet is available, I suggest you perform the calculations once or twice yourself, just to see the big voltage-drop issues.

When designing a Notification Appliance Circuit (NAC), we have to look at whether or not the horns or strobes receive enough voltage to even work. This is true of the NAC circuit we normally think of, and this is also true of a power circuit to operate the horns and strobes.

Even though voltage-drop calculations aren't normally applied to the door holder circuits, the door holder circuits require a lot of power to operate. Performing the calculations on these circuits is something that should be done.

The calculations take into account the voltage available on the power supply, the wire, and the total amount of current that is used to power the horns or strobes. The voltage-drop calculations use Ohm's Law, and after that, some simple math.

Power Supply: In most cases during a power blackout, the power supply voltage slowly goes down. This is normal and expected. In 24 hours, the voltage goes down to the minimum voltage that the panel can operate with. This power-blackout worst-case voltage is the voltage you have to work with.

You can find this voltage in the panel's installation manual, or you can check with the technical support team for the manufacturer to find out what this lower voltage is going to be.

Wires: Wires have resistance. Think of the wires as long, thin resistors. There are two wires in series: one to carry current to the horns or strobes, one to carry current back from the horns or strobes.

The length of the wire will be the total length of the wire, including length as it takes longer routes through the building, and the length as it drops down and returns for each horn or strobe.

Add up the total length of the wire in the circuit (remember, there are two wires in the cable, and the wires are in series). This gives you the total length you have to work with.

The resistance of the wire for that length can be found on one of the tables in the NEC (National Electrical Code). Each size of wire will have a different resistance. Use the length of the wire and the size of the wire to find the resistance of the wire.

Load: Add up the total amount of current from all of the horns or strobes on the circuit.

You can find the current amounts for the horns or strobes in the installation sheets that come inside the box for each device. The installation sheets show one or several current values for each candela setting for the strobe, or each sound level setting for the horn.

Use the highest current value for the candela level on the strobe, or sound level on the horn.

Once you've added up the currents being used for all the horns or strobes on the circuit, that is the current that is going to pass through the wires in the circuit.

Ohm's Law

You've figured out the resistance of the wire and you've figured out the how much current the horns or strobes will be using. Now, use Ohm's Law, E = I x R, to find out how much voltage will be lost into the wires because of the resistance of the wires.

This is the voltage-drop from the wires in the NAC circuit.


Start with the power supply voltage (after a 24-hour power-blackout), subtract from the power supply voltage the amount of voltage dropped by the wires, and that will give you the voltage given to the horns or strobes.

If the voltage going to the horns or strobes is less than the minimum voltage shown in the installation sheet for the device, some of the devices may not work if the power to the building is blacked out for 24 hours.

Performed to give an idea of whether the circuit will provide power to the devices, these voltage-drop calculations are really guesses. If the calculations show that there isn't enough voltage to power all the horns or strobes - change something. Do one or more of the following:
  • Reduce the number of horns or strobes on the circuit in order to reduce the total current needed, or

  • Reduce the length of the wires in order to reduce the wire's resistance, or

  • Increase the size of the wire in order to reduce the wire's resistance

After reducing the current or the resistance, perform the calculations again.

Show Your Work

Remember that the fire marshal might want to see that you have designed a circuit that works, even during a power-blackout. For each Notification Appliance Circuit (NAC), write down everything you did to come up with the results showing that the horns or strobes will work.

Rule of Thumb: If a circuit carries power to operate anything, in order to make sure the circuit wiring will carry enough voltage to the end, voltage-drop calculations have to be performed on the circuit.

The whole idea behind the voltage-drop calculations is to show you, the technician, the installer, the designer of a fire alarm system that all of the occupants of the building will be warned if there's a fire.

Questions 2 and 3: All power circuits, whether they're conventional NAC circuits, addressable NAC circuits, door holder circuits, or other power carrying circuits, require voltage-drop calculations in order for you to know that the circuit works as designed.

Conventional versus Addressable

If a circuit is conventional, it uses a DC type of signal. On a conventional circuit, the signal is a DC, or continuous relationship between voltage, current, and resistance. An addressable device, like an addressable input device or output device will not work at all on a conventional circuit. An addressable device is not compatible to be used on a conventional circuit.

If a circuit is addressable, it uses a data (electronic words in an on/off format), somewhat similar to computer data to communicate. On an addressable circuit, the signals themselves rapidly turn on and off. A conventional device, like a conventional input device or output device will not work at all on an addressable circuit. A conventional device is not compatible to be used on an addressable circuit.

If it's not compatible, it won't work. That's why the NFPA Code requires that the devices be compatible.

Look up in the installation manual for the fire alarm control panel to find out what devices are compatible with the control panel. You can download an installation manual from the web, or contact the technical support team for the fire alarm system manufacturer to get a manual.

Class B versus Class A

Classification of fire alarm circuits shows what happens to the circuit when things go wrong.

The circuit can be an addressable circuit or a conventional circuit. The circuit can be made up of wires, made up of fiber optics, or other signal carrying methods.

In a Class B circuit, the panel can communicate with the devices between it and a break in the circuit, however, the panel cannot communicate with any devices that are cut off beyond the break.

In a Class A circuit, the panel can still communicate with the devices between its Class B Out screw terminals, and because the end of the circuit is connected to the Class A In terminals at the panel, the circuit provides an alternate (redundant) route to the devices that would otherwise be cut off the panel.

See more information on circuit classification at:

NFPA's 7 Classes of Fire Alarm Paths

Douglas Krantz
Life Safety
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