✓ Checked 17 Oct 2026 against NFPA's Fire Alarm Basics blog series, a System Sensor notification appliance installation manual and the Notifier SLC Wiring Manual.
Two circuits come up constantly on fire alarm drawings, in panel manuals and on the NICET exams: the notification appliance circuit (NAC) and the signaling line circuit (SLC). In one line: a NAC powers the horns, strobes and speakers that warn people, and an SLC carries data between the panel and addressable devices.
What a NAC is
A notification appliance circuit carries the signal from the control unit to the appliances that alert occupants. According to NFPA's guide to notification, those appliances are strobes for visible signals and horns or speakers for audible ones, often combined in one unit.
A conventional NAC is mostly a power circuit: in alarm, every appliance on it turns on. So NAC design is a power question. How much current do the appliances draw, and how much voltage is left at the far end of the wire?
What an SLC is
A signaling line circuit is a data circuit. NFPA's guide to initiation describes addressable devices as ones that can send a unique address to the control unit over an SLC. The panel polls each device in turn, and each one answers with its status and address. That is how the panel can tell you which detector is in alarm, not just which zone.
An SLC typically carries addressable smoke and heat detectors, manual stations, monitor modules for things like waterflow switches, and control modules. The Notifier SLC Wiring Manual notes that addressable control modules on the SLC let the panel switch individual NACs on. So the two circuits often work together: the SLC tells a module to act, and the module drives a NAC.
Conventional systems use an initiating device circuit (IDC) instead of an SLC for detectors and pull stations. On an IDC, one device in alarm puts the whole zone into alarm, and the panel cannot tell which device it was.
How each circuit is supervised
NAC. NFPA's notification blog explains the usual method. In standby, the panel sends a small supervisory voltage through the circuit in the reverse direction, typically around 6 VDC. A diode in each appliance blocks that current, so the appliances stay off. An end-of-line device at the far end of the circuit completes the path, and if the panel loses that path it reports a trouble. In alarm, the panel reverses the polarity and raises the voltage, typically to 24 VDC, and the appliances turn on.
SLC. An addressable SLC does not rely on an end-of-line resistor in the same way. The panel's polling checks the devices themselves: if a device stops answering, the panel knows. The Notifier manual's class table lists loss of communication as a trouble condition on every SLC class. Exactly how a fault is reported depends on the panel, so check the panel manual.
Why voltage drop matters on NACs
Wire has resistance, so some voltage is lost along every run, and the loss grows with the current the appliances draw. A System Sensor horn strobe installation manual puts the designer's job in two checks:
- The total current of the appliances must not exceed what the panel or power supply can deliver.
- The last appliance on the circuit must still get a voltage inside its rated range. ✓ For the models in that manual, the range on a 24 V nominal system is 16 to 33 V.
The same manual makes three points worth remembering. Current draw depends on the model and candela setting, so use the tables for your exact appliance. Thicker wire gives a smaller voltage drop. And Class A wiring can allow a run up to twice as long as a circuit that is not fault tolerant.
Use our fire alarm voltage drop calculator to work a NAC from the panel end to the last appliance. NAC current also feeds the standby battery size, so run the same load through the fire alarm battery calculator.
What wire size to use
There is no single right gauge. Wire size comes out of the voltage drop calculation and the manufacturer's limits, within the wiring rules of NFPA 70.
- For NACs, choose a gauge that keeps the last appliance inside its rated voltage. Check the appliance terminals too: the System Sensor manual above lists ✓ 12 to 18 AWG for those models.
- For SLCs, the panel manufacturer sets the limits, because the circuit has to carry data reliably. ✓ The Notifier manual, for example, recommends twisted unshielded pair from 12 to 18 AWG, and for several panels it caps each branch at 50 ohms and 0.5 microfarads. Other brands use different limits.
- Installation rules for fire alarm circuits, such as cable types and power-limited circuits, are in NFPA 70, Article 760. Look the details up in your own copy.
Circuit classes, and what happened to "styles"
You will see NACs and SLCs labelled Class A, Class B or Class X. Pathway class designations live in NFPA 72 (2022), Chapter 12, mainly Section 12.3, as National Training Center's summary also notes. Older drawings use "styles" instead. The Notifier manual's history table maps NAC Style Y to Class B, Style Z to Class A, and SLC Style 7 to what is now Class X.
In general terms:
- Class B has no redundant path. A single break leaves the devices past it without signal, and the panel reports a trouble.
- Class A returns to the panel on a second path, so devices can keep working through a single open.
- Class X adds protection against a single short as well as a single open, usually with isolator modules or isolator bases.
The chapter defines other classes for other kinds of pathway too. These summaries are our own wording, so read Chapter 12 in your own copy, or in NFPA's free read-only access, before relying on them. The code describes how each class performs. The specification, the designer and the authority having jurisdiction decide which class a circuit needs.
Practise this for the NICET exams
NAC loading and voltage drop sit in the submittal preparation and system layout domain of the Level II exam. Our submittals and layout guide has a practice routine for drawings and power checks. To see where you stand, take the free Level II diagnostic. It has 25 original questions and is never built from real or recalled exam items.
Sources
- A Guide to Fire Alarm Basics: Notification, Shawn Mahoney, NFPA, 21 May 2021. Retrieved 17 Oct 2026.
- A Guide to Fire Alarm Basics: Initiation, Shawn Mahoney, NFPA, 14 Apr 2021. Retrieved 17 Oct 2026.
- Selectable Output Horn Strobes, Chime Strobes and Strobes, Wall Mount, Installation and Maintenance Instructions (I56-5845-002), System Sensor. Retrieved 17 Oct 2026.
- SLC Wiring Manual, Document 51253 Rev U9, Notifier. Retrieved 17 Oct 2026.
- NFPA 72 circuits and pathways, National Training Center. Retrieved 17 Oct 2026.