Fire Alarm Panel Symbol
Definition: The Fire Alarm Panel symbol represents the fire alarm control unit (FACP/FACU) at the heart of a building's fire detection system, drawn as a rectangle labelled 'FACP' with terminals for AC In (primary power), NAC 1 (notification appliance circuit), SLC Loop (signaling line circuit for addressable devices), and Battery (secondary standby power), supervised per NFPA 72.
Also known as: FACP, fire alarm control panel, fire alarm control unit, FACU, fire panel, addressable fire panel, fire alarm system panel.
What the Fire Alarm Panel symbol means
The Fire Alarm Panel symbol denotes the central controller that monitors initiating devices (smoke and heat detectors, pull stations, waterflow switches), makes the alarm decision, and drives notification appliances (horns, strobes, speakers). Its terminal groups map directly to the circuit types defined in NFPA 72: the SLC Loop terminal serves the signaling line circuit — a two-wire addressable loop on which each detector and module has a digital address and reports individually; the NAC 1 terminal serves a notification appliance circuit — a supervised, panel-powered 24 V DC circuit that energizes horns and strobes on alarm.
Power architecture is dual by code: AC In takes the primary supply from a dedicated, mechanically protected branch circuit, and Battery connects the secondary supply — sealed lead-acid batteries sized for at least 24 hours of standby followed by 5 minutes of full alarm load (NFPA 72 requirement for most systems). Every external circuit is supervised: the panel continuously monitors for opens, grounds, and (on NACs) shorts, annunciating a trouble condition distinct from an alarm. On drawings, the panel symbol is the hub from which SLC loops, NAC risers, and monitored auxiliary functions (elevator recall, door holders, HVAC shutdown) radiate.
How to identify the Fire Alarm Panel symbol
The symbol is a rectangle marked 'FACP' (or 'FACU'), often with a row of small dots or squares suggesting zone LEDs, and labelled terminal stubs: AC In on the supply side, Battery below it, and field-circuit terminals (SLC, NAC) on the opposite side. Fire alarm shop drawings follow NFPA 170 symbol conventions in North America; the panel itself is usually a simple labelled block because its internal complexity is documented separately in riser diagrams and battery calculations.
Distinguish it from a burglar/security alarm panel (keypad grid, zone inputs, siren output, no supervised NAC/SLC terminology) and from a releasing panel (adds solenoid/agent-release circuits). On IEC-aligned drawings (EN 54 markets), the equivalent block is the CIE — control and indicating equipment — with loop and sounder circuit terminals; the drawing grammar is the same labelled-rectangle hub.
Function in a circuit
The panel continuously polls its SLC loop — each addressable detector and monitor/control module answers with its status and analog value, so the panel knows a specific device (e.g. 'Detector 23, Level 2 corridor') rather than just a zone. When an initiating device reports alarm, the panel executes its programmed cause-and-effect matrix: energize NAC circuits (horns/strobes in the required temporal-3 evacuation pattern), transmit to the supervising station via its dialer/communicator, and operate auxiliary relays for elevator recall, smoke-control fans, magnetic door-holder release, and fire-suppression interfaces.
Supervision runs constantly in the background. Class B circuits are two-wire with an end-of-line resistor at the last device: the panel measures the standing supervisory current, so a broken wire reads as an open (trouble), while an alarm presents a distinct signature. Class A circuits route a return pair back to the panel, so a single open merely degrades the loop — all devices remain operable from both ends — while still annunciating trouble. NACs reverse polarity on alarm: notification appliances sit behind blocking diodes, passing supervisory current when idle and sounding when the panel flips the circuit polarity and applies full 24 V power.
Standards: IEC vs ANSI
| IEC 60617 | In IEC/European practice the panel is the CIE (control and indicating equipment) per EN 54-2, with power supplies per EN 54-4; system design and wiring rules come from national codes (e.g. BS 5839-1 in the UK), which use loop (addressable) and sounder-circuit terminology analogous to SLC and NAC. |
|---|---|
| ANSI/IEEE 315 | NFPA 72 (National Fire Alarm and Signaling Code) governs circuit classes, supervision, and secondary power; the panel is UL 864 listed; drawing symbols follow NFPA 170. Circuit vocabulary is IDC (initiating device circuit, conventional), SLC (signaling line circuit, addressable), and NAC (notification appliance circuit). |
| Key difference | The device is functionally identical across regimes; the vocabulary differs. NFPA 72 speaks of SLC/NAC/IDC and Class A/B/N pathways with UL 864 listing; EN 54 speaks of CIE, detection loops, and sounder circuits. North American secondary-power sizing (24 h standby + 5 min alarm) and temporal-3 evacuation signal are NFPA 72 specifics; EN 54 markets apply their own standby and sounder requirements. |
Terminals / pins
| Pin | Name |
|---|---|
| ac | AC In |
| nac1 | NAC 1 |
| slc | SLC Loop |
| battery | Battery |
Typical values
Panel operating voltage is 24 V DC (filtered, regulated), fed from 120/240 V AC primary power on a dedicated circuit. NAC circuits are typically rated 1.5–3 A each at 24 V DC (regulated), current-limited and power-limited per UL 864. SLC loops support 127–250+ addressable points depending on protocol, over twisted (often unshielded) 18–14 AWG cable up to roughly 1,000–3,000 m loop length. Standby batteries are sealed lead-acid, 24 V (two 12 V in series), commonly 7–55 Ah, sized by battery calculation for 24 h standby + 5 min alarm (60 min for voice-evacuation systems' amplifiers in some cases). End-of-line resistors for Class B circuits are panel-specific, commonly 2.2 kΩ–10 kΩ.
Where the Fire Alarm Panel symbol is used
- Commercial buildings and offices as the code-required hub for detection, evacuation signalling, and supervising-station transmission
- Schools, hospitals, and care facilities where addressable point identification speeds staff response
- Warehouses and factories integrating waterflow and tamper switches from the sprinkler system as monitored inputs
- Hotels and apartment buildings driving NAC risers of horns/strobes floor by floor
- Buildings with elevators, where the panel's relays execute primary/alternate recall and shunt trip per code
- Data centres and plant rooms interfacing clean-agent releasing panels and HVAC shutdown through monitor/control modules
Example
In a small-building fire alarm riser, the Fire Alarm Panel symbol's AC In terminal is fed from a dedicated 120 V AC branch circuit, the Battery terminal connects two series 12 V 18 Ah sealed lead-acid batteries (24 h standby + 5 min alarm per the battery calc), the SLC Loop terminal runs a Class B addressable loop serving 42 smoke detectors and 6 pull-station modules, and NAC 1 drives a supervised 2 A, 24 V DC horn/strobe circuit with a 4.7 kΩ end-of-line resistor — reversing polarity to sound temporal-3 evacuation when any device alarms.
Key facts
- The FACP's terminal groups map to NFPA 72 circuit types: SLC (addressable signaling line circuit for detectors/modules), NAC (notification appliance circuit for horns/strobes), plus dual power — AC primary and battery secondary.
- NFPA 72 secondary-power rule: batteries must carry the system for 24 hours in standby and then 5 minutes of full alarm (with sizing safety factor — typically 20–25% — applied in the battery calculation).
- All field circuits are supervised: opens, grounds, and NAC shorts annunciate a trouble signal that is distinct from alarm and supervisory signals.
- Class B wiring uses an end-of-line resistor after the last device; Class A adds a return path to the panel so the circuit survives a single open with all devices still operable.
- NACs are polarity-reversing: appliances sit behind blocking diodes and sound only when the panel flips polarity on alarm — which is why notification appliances are polarity-sensitive.
- Addressable SLC loops identify the exact device in alarm (point identification), versus conventional IDC zones that only localise to a zone.
- The panel executes auxiliary fire-safety functions: elevator recall, door-holder release, HVAC/smoke-control interfaces, and supervising-station communication.
- The panel and its major components must be listed (UL 864 in North America; EN 54-2/-4 in Europe), and primary power must come from a dedicated branch circuit.
Frequently asked questions
What is the difference between an SLC and a NAC?
The SLC (signaling line circuit) is the addressable data loop — detectors, pull-station modules, and monitor/control modules communicate digitally on it, each with a unique address. The NAC (notification appliance circuit) is a supervised 24 V DC power circuit that drives horns and strobes; it carries no data, just reversed-polarity power on alarm. Inputs and decisions live on the SLC; audible/visible output lives on the NAC.
What is the difference between Class A and Class B fire alarm wiring?
Class B runs out to the last device and terminates in an end-of-line resistor; a single open circuit disables devices beyond the break (and reports trouble). Class A routes a return pair back to the panel, so after a single open the panel feeds the circuit from both ends and every device keeps working — the fault is annunciated but the system stays fully functional. Class A costs more copper and conduit; NFPA 72 lets the designer/AHJ choose based on required survivability.
How big do fire alarm panel batteries need to be?
NFPA 72 requires secondary power for at least 24 hours of normal standby operation followed by 5 minutes of full alarm load (voice systems have additional requirements). The battery calculation sums every device's standby and alarm currents, applies the time periods, and adds a safety margin (commonly 20–25%). Typical results are 24 V sets built from two 12 V sealed lead-acid batteries of 7–55 Ah.
What is the difference between alarm, trouble, and supervisory signals?
Alarm means a fire condition — a detector, pull station, or waterflow switch has activated — and triggers evacuation signalling. Trouble means a fault in the system itself: open circuit, ground fault, low battery, or AC loss. Supervisory means an off-normal condition in a monitored fire-protection system that is not itself a fire — classically a closed sprinkler valve (tamper switch) or low air pressure in a dry-pipe system. Panels display and transmit all three distinctly.
Why do notification appliances have polarity markings?
Because the NAC is supervised by reverse polarity. In standby the panel applies a small supervisory current at reversed polarity, which blocking diodes in each horn/strobe refuse to pass — letting the end-of-line resistor define the supervision current. On alarm the panel flips polarity and applies full power, the diodes conduct, and the appliances operate. Wire one appliance backwards and it will sound during supervision or break supervision entirely.
Can a fire alarm panel share a circuit with other equipment?
No. NFPA 72 requires the primary power connection to be a dedicated branch circuit, mechanically protected, marked ('FIRE ALARM'), and typically with a red-locked breaker handle — not shared with any other load, and not GFCI/AFCI protected unless specifically permitted. This guarantees another load's fault cannot silently kill the fire alarm's primary supply.
Related symbols
- Buzzer / Audible Alarm symbol
- CO Detector symbol
- Horn / Siren symbol
- Power Supply 24V DC symbol
- Relay NO Contact symbol
- Smoke Detector symbol
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