NAC Power Supply
Booster for Notification Appliance Circuits

Why Boosters Exist

Every fire alarm panel has a limited amount of current it can source to its own on-board notification appliance circuits. A typical small panel (Fire-Lite ES-50X, Silent Knight 5820XL) provides 2–3 A total across 2–4 NAC outputs. Modern high-candela horn-strobes draw 150–250 mA each, so a single 2 A NAC saturates at roughly 10 appliances. Any building that needs more than that — essentially every commercial structure over a few thousand square feet — requires additional NAC capacity.

The NAC power supply (often called a booster, transponder, or auxiliary power supply) is a listed UL 864 component that provides that additional capacity. It takes a low-current trigger input from the main FACP (either a reverse-polarity input, a dry relay contact, or an SLC addressable command), and it sources its own supervised NAC output with its own battery backup. Common examples: Notifier AA-30 / AA-100, Fire-Lite FCPS-24S6/FCPS-24S8, System Sensor SPSCWV, Potter PSN-1000, Altronix AL1024ULXR.

Output Ratings and Sizing

Common booster output ratings: 6.5 A, 10 A, 12 A, 20 A at 24 VDC nominal. Higher-current units are rare in fire alarm service because the practical answer to needing more current is multiple smaller supplies distributed closer to the loads — this reduces voltage drop and localizes any failure.

Sizing a booster is a two-step calculation per NAC:

1. Alarm current. Sum the published alarm current of every notification appliance on the circuit at its selected candela setting. Voltage matters — a 110 cd strobe draws 120 mA at 24 V but 145 mA at 20 V.
2. Voltage drop. Calculate end-of-line voltage using conductor size, distance, and total current. NFPA 72 requires appliances to be within their listed voltage window at the end of line under worst-case battery voltage (20.4 V for a standard 24 V system).

If alarm current is within the booster rating but end-of-line voltage drops below 20.4 V, you either up-size the wire, shorten the run, or add another booster closer to the far end. The FCPS-24S8 series from Fire-Lite publishes a voltage-drop spreadsheet that most designers use verbatim; Notifier publishes similar tools for AA-series boosters.

Trigger Inputs

The booster needs a signal from the main FACP telling it when to go into alarm. Three common methods:

• Reverse polarity. The main panel flips polarity on a dedicated 24 V trigger pair. The booster detects the polarity flip and energizes its output. Simple, robust, common on conventional installations.
• Dry contact. A relay on the main panel (either on-board or an add-on relay module) closes on alarm; the booster's trigger input senses the closure. Flexibility: any panel with a relay output can drive any brand of booster.
• Addressable SLC. In addressable systems, the booster has its own SLC address. The panel commands alarm via the SLC digital protocol. This is the most common modern method and allows per-NAC control from the panel programming.

Supervision Requirements

NFPA 72 §10.18 requires every NAC to be continuously supervised — the circuit must detect and annunciate open, short, and ground-fault conditions whether or not an alarm is active. Practically this is done with an end-of-line resistor (EOL) on Class B circuits or by continuous loop current monitoring on Class A.

On the booster itself, the panel must annunciate trouble when:

• AC power to the booster is lost
• Battery power is absent, depleted, or disconnected
• The trigger input wiring from the main panel is open
• Any NAC output circuit has an open, short, or ground fault

These conditions are typically reported back to the main FACP via dedicated trouble-relay output or via the SLC address in addressable systems.

Strobe Sync — The Silent Design Trap

When multiple strobes are visible from the same location, NFPA 72 §18.5.4.4 requires their flashes to be synchronized within 10 ms. The reason is photosensitive epilepsy: asynchronous flashes in the 5–30 Hz range can trigger seizures. The practical method is a sync protocol — a manufacturer-specific modulation pattern on the NAC output power that tells all downstream strobes when to flash.

Every major manufacturer has their own sync protocol: System Sensor / Honeywell, Wheelock (Cooper / Eaton), Gentex, AMSECO. Protocols are not interchangeable. If your booster is running System Sensor sync and your strobe is a Wheelock device expecting Wheelock sync, the strobe either does not flash at all or flashes on its own internal clock — out of sync with everything else.

Design rules: pick one protocol per listening/viewing space and stick to it. When retrofitting an existing building, either match the existing protocol or install a sync module at the boundary. Specify sync protocol on the submittals — "System Sensor sync" is a real, reviewable spec line, not a vague preference.

ITM

• Annual functional test per NFPA 72 Table 14.4.5 — confirm trigger input, output current, supervision, and trouble reporting.
• Semi-annual battery load test on the booster's batteries, same method as the main panel batteries.
• Battery replacement on the same 4–5 year cycle as the main panel.
• Recalculation after any change — adding appliances, changing candela ratings, or extending circuits triggers a fresh voltage-drop and alarm-current verification.

Frequently Asked Questions

References