NAC circuit: what it means on the job

How a conventional DC NAC works

1. Standby: prove the wire is whole. The panel puts a small current through the circuit in one polarity. Each horn/strobe has a blocking diode that keeps it off in that polarity, so the current only flows through the wiring and the end-of-line resistor at the far end.
2. A fault shows as trouble. If a wire opens, that current stops and the panel reports trouble on the NAC. Fire-Lite's MS-9200UDLS manual, §2.5, shows this Class B arrangement and a Class A version.
3. Alarm: reverse and power. When the panel goes into alarm it flips the polarity. The diodes now conduct, full current flows, and every appliance on the circuit operates.
4. Speakers are different. A voice evacuation speaker circuit is also notification, but it carries audio from an amplifier. Do not run a horn/strobe DC calculation on a speaker circuit.

This is the classic conventional arrangement. Some systems put addressable appliances on the loop or use other supervision methods, so the panel's manual for the equipment in front of you is the final word.

Size it with voltage drop, not hope

Each appliance has a listed voltage window. System Sensor's L-Series instructions give 16 to 33 VDC for their 24 V nominal arrangement, as one manufacturer example. The last appliance on the run has to stay inside its window with every appliance drawing alarm current.

A worked example with invented numbers: a radial NAC runs 250 ft out and 250 ft back, so 500 ft of conductor. The conductor is stated as 0.003 Ω per foot, and the appliances draw 1.2 A in alarm. Drop is 500 x 0.003 x 1.2 = 1.8 V. From a stated 20.4 V at the panel under the worst-case condition, the last appliance sees 18.6 V, inside a 16 V minimum. Count both conductors, and don't double a length that already includes the return.

Common mix-ups

• NAC is not a wiring class. NAC says what the circuit does. Class A or Class B says how it is wired and what a fault does to it. Look for the class on the drawing, and read Class A vs Class B wiring if the two blur.
• NAC is not SLC. A signaling line circuit carries data between the panel and addressable devices. A NAC carries power to appliances. An addressable system can still drive its horns from conventional NACs on the panel or a power supply.
• EOL at the panel is not supervision. A resistor across the panel terminals keeps the panel quiet while the field wiring goes unchecked. It belongs where the manufacturer's instructions put it, at the far end of a Class B run.
• Candela is an output setting, not spacing. Pick spacing and candela from room size and mounting with NFPA 72 (2022) Chapter 18, then use that setting's current in the NAC load.

NACs on the NICET Level I exam

NACs show up in both big Level I domains. Installation (44 to 54%) covers termination and cable, and Maintenance (40 to 50%) covers testing and correcting device problems, which is where trouble on a NAC lives. The voltage-drop habit carries straight into Level II layout work.

Practice it

Run your own numbers in the fire alarm voltage drop calculator, which counts both conductors and checks the last appliance against its minimum. Then test what an open, a short or a misplaced EOL does to a NAC in the fire alarm circuit class drill. The fire alarm cable types guide covers what to pull for the run, and NAC vs SLC circuits goes deeper on supervision and pathway classes.

Know your circuits? Check the rest of Level I

Take the free 10-question check. It's weighted to the Level I outline, scored by domain and needs no signup, so you see whether installation, maintenance or layout costs you the most points. The Level I course gives you full timed practice exams, an explained answer for every question and the section number to look up for each one.

Sources

Show the list

• MS-9200UDLS Series Manual, 52750 Rev M, Fire-Lite, §2.5. Retrieved 17 Oct 2026.
• L-Series wall appliance instructions, I56-5845-002, System Sensor. Retrieved 17 Oct 2026; DC voltage window.
• NFPA 72 (2022), NFPA, Chapters 12 and 18, referenced by number only. Retrieved 17 Oct 2026.
• Fire Alarm Systems Level I content outline, NICET. Retrieved 17 Oct 2026.