NAC: Notification Appliance Circuit

A notification appliance circuit, or NAC, is the supervised output circuit that powers horns, bells, strobes and speakers so the building gets an audible and visible signal. It runs at low voltage from the panel or from a booster power supply, is supervised while idle for both opens and shorts, and reverses polarity to activate, which is why polarity has to be right at every appliance. What limits a NAC is not the panel's rating alone but voltage drop: the appliance furthest along the run has to still see its listed minimum operating voltage at full load. Conductors are governed by Section 32 of the Canadian Electrical Code, installation by CAN/ULC-S524, and required sound levels by the Building Code.

What a NAC is and how it activates

A NAC is a two conductor output circuit. In standby the panel applies a small reverse polarity voltage across it and measures the end-of-line device at the far end. Every horn and strobe on the run is polarised, usually by an internal diode, so in standby they are reverse biased, draw nothing, and let the supervisory current pass through to the resistor. On alarm the panel flips polarity, the appliances become forward biased and the whole circuit sounds.

Mircom's own wording for its control panels is that powered output circuits are supervised while they are not active, for both open circuits and shorts. That is the important qualifier: a NAC in alarm is no longer being supervised, because it is delivering full load current. Supervision is a standby function.

The polarity dependence is why a single appliance wired backwards is such a common defect. It will not sound in alarm, and in standby it presents a forward biased path that can mask the end-of-line device or throw a trouble, depending on the panel.

What actually limits a NAC

Not the panel. The binding constraint is almost always voltage drop. Appliances are listed to operate down to a minimum voltage, commonly in the low twenties on a nominal 24 V system, and the panel's own output sags as the battery discharges. You size the circuit for the worst case: the last appliance on the run, with every appliance on that circuit drawing alarm current, at the panel's end of discharge voltage rather than its float voltage.

Get that wrong and the symptom is not a trouble light. It is a strobe that flashes slowly or a horn that sounds weak, only during an actual alarm, only when the batteries are down. It passes every quiet test you can do at the panel. This is a large part of why verification to CAN/ULC-S537 exercises the system under load rather than reading it at rest.

Where the Canadian requirements come from

Three separate documents, and it is worth keeping them apart. The Building Code decides how loud the system has to be and where signals are required, which is the subject of the audibility rules. CAN/ULC-S524 governs how the circuit is installed and how far a single fault may spread across zones. Section 32 of the Canadian Electrical Code governs the conductors: copper, insulation rated not less than 300 V, kept entirely independent of other wiring, and treated as Class 1 even at 24 V because a fire alarm is a safety control device. See fire alarm cable in Canada for the sizes.

The formal circuit class definitions and the exact fault tolerance figures live inside S524, which is copyrighted and paywalled. We do not reproduce them, and the American class definitions are not a substitute.

What goes wrong

Reversed polarity at one appliance, already covered. Overloaded circuits where somebody added strobes to an existing NAC without redoing the calculation. Circuits extended past the drop limit during a renovation. Wire nuts used instead of looping through, so removing an appliance does not report. And booster power supplies whose own batteries are undersized, which shows up in the battery calculation rather than on the NAC itself, but takes the NAC down with it.

Neighbouring terms

A NAC carries outputs. An IDC carries conventional inputs. An SLC carries both, as data, on an addressable system. On a two stage system the NAC is what delivers the alert and alarm patterns.

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