What Does NAC Stand for in Fire Alarm Systems? (Find Out!)

A lot of people ask about the NAC, which plays a vital role in the Fire Alarm Control Panel system. If you are one of them, this guide will clear all your curiosity.

Well, it is a pathway that helps the FACU to send trigger signals to devices like fire alarms, bells, rings, and so on in order to notify people.

Feeling stuck as it seems unclear? Don’t worry, you’ll learn what NAC stands for in fire alarm systems and get a clear idea about the NAC here. Let’s begin!

What Does NAC Stand for in Fire Alarm

What Is a NAC and How It Works!

The NAC is a short form for Notification Appliance Circuit. You might be thinking what a NAC is, right?

If so, know that the NAC is a special circuit that sends signals to turn on notification appliances. Think of it as a route where signals or currents go from FACP to the notification appliances.

The devices that notify people are usually controlled by the FACP using a NAC. And each device of notification appliance has a Diode or Capacitor in it to block the signals from going in another direction. Due to this, the signals will pass via the NAC in a specific direction.

Image: nfpa.org

In the usual (non-alarm) state, the FACP sends a little supervisory voltage (not less than 6 VDC) via the circuit to monitor its condition. If the FACP finds no supervisory voltage, it means there’s an error (Trouble).

However, in the alarm state, the FACP reverses the polarity of the voltage. It’s a state that causes the devices like horns or bells to turn on and off). Then, the FACP raises the voltage to 24 VDC.

When it occurs, the Diodes or Capacitors will let the signals flow via the notification appliances. Then, it causes the audible and flash lighting alerts to turn on.

What Are the Different Types of NAC Devices?

After learning what a fire alarm NAC panel is, it’s time to get familiar with all devices of NAC. Here are the most common types of NAC devices:

  • Sounding Devices- Bells, buzzers, horns, speakers, and fire alarms.
  • Textual Devices- Signs, visual texts, symbols, and notices.
  • Visual Devices- Lights, strobes, and red lighting.
  • Tactile Devices- Shaking gears, vibrators, or any sensing of touch.

The Circuit Styles of the NAC in Fire Alarm!

In total, there are two types of circuits in the Notification Appliances Circuit system. The NAC system has to use one style of the circuit to make the route for FACP to send messages to the notification appliances. Here’s the detail on these styles:

1. Class B (Style Y)

Class B is one of the common types of pathways in the fire alarm systems used for NAC. It has no redundant route that goes back to the control panel. Due to this, it is more susceptible to wiring issues.

It’s the style where the NAC uses 2 wires to connect the FACP and notification appliances one after another. In this connection, you can see only two cables (negative black wires) are used using parallel lines with no T-taps.

Image: prod-edam.honeywell.com

That means when a wire breaks, the signals can’t go to another route as Class B has no redundant route. Due to this, the device can’t get the signal and won’t alert using the horns or bells.

However, in the normal state, it immediately sends down activation signals to the notification appliances (horns or bells) using the parallel path.

2.   Class A (Style Z)

Being the safest circuit style, Class A is a better and more effective pathway than Class B in the fire safety system. The reason is that it contains a redundant route that goes back to the FACP.

Thanks to this, if a wire breaks, the signals from FACP  will still reach the notification appliances and they’ll turn on the bells to alert all. That means it will never fail to deliver the signals to the destination, unlike the Class B circuit style.

Image: prod-edam.honeywell.com

It’s a style where the NAC uses 4 wires to connect the FACP and notification appliances. In this connection, two cables are (negative) black, and the other 2 wires are red (positive) wires.

The Performance of the NAC!

The NAC usually has 3 voltages to indicate a condition. And these are Normal, Shorted, and Open. Let me explain these states in detail −


It is a state when the voltage (6 – 24 VCD) drops through the end of the line. In this condition, both NAC 1 and NAC 2 will show a similar voltage. FYI, this state can be described in 2 ways and these are:

  • Supervision: It’s a non-alarm condition where the notification devices won’t get signals.
  • Alarm: It’s a condition when notification devices like horns or fire alarms turn on to alert people.


This is the performance of the NAC when no voltage can return to the panel. It’s the state when the LED will show NAC 2 Open. This simply means that NAC 1 has the usual voltage while NAC 2 contains a higher voltage (NAC 2 > NAC 1).


To indicate the NAC trouble in the fire alarm where the 24 VDC has returned to the panel, this is the state that you’ll find.

In this case, the LED will show NAC 2 Trouble or Short to alert you about an issue. This just indicates that NAC 1 has normal voltage while NAC 2 contains lower or zero voltage (NAC 1 > NAC 2).

Both Open and Short mean something is wrong in the NAC system. While the Normal state means the notification appliances (horns or bells) are working fine.

Frequently Asked Questions (FAQs)

1. What is NAC Power Supply?

It is a combination or mixture of Class A or Class B NACs to give notifications to devices like horns or bells.

2. What is the difference between the fire alarm NAC Vs. SLC?

Even if both are circuits, still the NAC and SLC differ in purpose. The SLC is a path that the control panel uses to get signals if initiating devices sense fire or smoke. Then again, the NAC is a path that the control panel uses to send trigger signals to notification appliances.

3. What Is the purpose of the NAC?

Its main goal is to help trigger signals to reach the notification appliances. That way, devices like horns, fire alarms, and so on can turn on to inform all people about fire or smoke.

Ending Note

The NAC is an essential thing that works as a path when the FACP sends signals to the devices that notify folks about the fire. Hope you now understand what the NAC stands for in fire alarm and its inner details. Thank you for reading this guide!