For 90% of auxiliary fire alarm triggers—such as HVAC shutdown, magnetic door holder release, and elevator recall—your default pick is a Form C (SPDT) supervised relay module like the System Sensor R100, wired with 18 AWG FPL (Fire Power Limited) cable. Fire alarm relay wiring diagrams look complex because they merge two entirely isolated circuits: the 24VDC supervision coil circuit from the Fire Alarm Control Panel (FACP), and the dry-contact load circuit switching your auxiliary equipment. Mixing these up is the most common cause of fried FACP boards and failed inspections.
This guide walks through the exact symbols, physical terminal mappings, and a node-by-node trace to wire, verify, and close out your relay connections to NFPA 72 standards.
Decoding Fire Alarm Relay Diagram Symbols
Before touching a wire stripper, you must translate the schematic symbols into physical hardware. Fire alarm diagrams use standard IEC and NEMA symbols, but they add specific supervision markers.
- The Coil (Circle with diagonal lines or a rectangle): Represents the electromagnet inside the relay. This is the only part of the relay connected to the FACP. It is polarity-sensitive (DC).
- The Switch Blade (Line with a pivot point): Represents the physical dry contacts. The "Common" (C) is the pivot. "Normally Open" (NO) and "Normally Closed" (NC) are the targets.
- EOL Resistor (Zig-zag line at the end of a loop): The End-of-Line resistor. In supervised circuits, this resistor (typically 4.7kΩ or 5.6kΩ depending on the FACP brand) sits at the last device on the loop so the panel can detect open or shorted wires.
- Supervision Loop (Dashed line enclosing the coil and EOL): Indicates that the FACP monitors this specific circuit for wiring faults, not just alarm states.
Terminal Pin Mapping and Physical Layout
Let’s map the schematic to the physical terminals of the industry-standard System Sensor R100 Form C relay module. This module fits in a standard 4-inch square electrical box and features a plug-in base for easy wiring.
| Terminal Label | Physical Location | Circuit Side | Function & Wire Color Standard |
|---|---|---|---|
| IN (+) | Top Left | Coil (FACP) | 24VDC Positive from FACP NAC. (Red FPL wire) |
| IN (-) | Top Right | Coil (FACP) | 24VDC Return to FACP NAC. (Black FPL wire) |
| C (Common) | Middle | Contact (Load) | The moving blade. Connects to the power source of the auxiliary load. |
| NC (Normally Closed) | Bottom Left | Contact (Load) | Connected to C when FACP is normal. Opens during alarm. |
| NO (Normally Open) | Bottom Right | Contact (Load) | Disconnected from C when normal. Closes during alarm. |
Where is the ground terminal? There isn’t one on the relay module itself. The relay is a dry-contact device; it does not reference earth ground. The metal junction box housing the relay must be grounded via the EMT conduit or a green grounding pigtail to the box's grounding screw, but the relay module remains electrically floating.
Node-by-Node Wiring Trace: Source to Load
Here is the exact textual trace for wiring a Form C relay to shut down an HVAC air handler upon fire alarm activation. We are using 18 AWG FPL cable for the coil and 16 AWG THHN for the 24VAC HVAC control circuit.
- Node 1: FACP NAC Output (+) → Run 18 AWG Red FPL wire from the FACP Notification Appliance Circuit positive terminal to the relay IN (+) terminal.
- Node 2: FACP NAC Output (-) → Run 18 AWG Black FPL wire from the FACP NAC negative terminal. If the panel requires supervision, install the manufacturer-specified EOL resistor (e.g., 4.7kΩ) across the IN(+) and IN(-) terminals at the relay. Terminate the black wire to IN (-).
- Node 3: HVAC 24VAC Source (R) → Run 16 AWG wire from the HVAC transformer's 24VAC "Hot" (R terminal) to the relay C (Common) terminal.
- Node 4: Relay NC to HVAC Contactor → Run 16 AWG wire from the relay NC (Normally Closed) terminal to the HVAC contactor coil (Y or G terminal, depending on the shutdown sequence).
Logic: In a normal state, power flows C → NC, keeping the HVAC running. When the alarm triggers, the coil energizes, the blade moves to NO, breaking the C → NC path and killing the HVAC. - Node 5: HVAC Contactor Return (C) → Run 16 AWG wire from the HVAC contactor coil return back to the HVAC transformer's 24VAC "Common" (C terminal) to complete the load circuit.
Decision Tree: Selecting the Right Relay Configuration
Not every auxiliary trigger requires a Form C relay. Use this decision matrix to select the exact module type and contact configuration for your specific load. If you are unsure, the Form C is the universal fallback.
| Auxiliary Load Scenario | Required Action on Alarm | Relay Form Required | Concrete Pick / Part Number |
|---|---|---|---|
| HVAC Air Handler Shutdown | Break the 24VAC thermostat control circuit. | Form C (SPDT) - Use NC to C path. | System Sensor R100 (Default Pick) |
| Magnetic Door Holders | Drop 24VDC power to release the magnet. | Form A (SPST-NO) or Form C (Use NC). | Notifier XPR-8 (Form A) or R100 |
| Elevator Ground Recall | Make a dry contact to the elevator controller. | Form C (SPDT) - Supervised, use NO to C. | System Sensor R100 (Default Pick) |
| Access Control Door Release | Drop power to fail-safe electric strikes. | Form C (SPDT) - Use NC to drop power. | System Sensor R100 (Default Pick) |
The Default Recommendation: Stock System Sensor R100 Form C relays. Because they contain both NO and NC contacts, they can be wired in the field to either make or break a circuit without requiring a return trip to the supply house for a different module. Ensure the FACP NAC output is configured in the panel software for "Supervised Relay" rather than "Notification" to prevent the EOL resistor from triggering a short-circuit trouble.
Verifying Connections with a Multimeter
Before applying power from the FACP or the auxiliary load, you must verify the wiring with a digital multimeter (DMM). Relying on visual inspection will result in failed UL fire alarm inspections.
1. Verifying the Coil Circuit (FACP Side)
- Test: Set DMM to Ohms (Ω). Measure across IN(+) and IN(-) at the relay base.
- Expected Reading: You should read the DC resistance of the coil plus the EOL resistor. For a System Sensor R100 with a 4.7kΩ EOL, expect roughly 4,750 Ω to 4,800 Ω.
- Troubleshooting: If you read OL (Open Loop), your EOL resistor is missing or a wire is broken. If you read < 5 Ω, you have a short circuit in the FPL cable run.
2. Verifying the Contact Circuit (Load Side)
- Test: Set DMM to Continuity (beep mode) or low Ohms. Measure between C and NC, then between C and NO.
- Expected Reading (Unpowered): C to NC should read < 1 Ω (closed). C to NO should read OL (open).
- Test (Energized): Apply a temporary 24VDC source (like a calibrated battery pack) to IN(+) and IN(-). Listen for the mechanical click.
- Expected Reading (Powered): C to NC should now read OL. C to NO should now read < 1 Ω.
3. Verifying Isolation (The Critical Safety Check)
- Test: Set DMM to Megohms or high Ohms. Place one probe on IN(+) and the other on C (Common).
- Expected Reading: OL (Infinite resistance).
- Why this matters: If you read any continuity here, the internal isolation barrier of the relay is compromised, or you have accidentally pinched a wire mixing the FACP loop with the HVAC 24VAC line. Energizing the FACP in this state will send 24VAC back into the 24VDC FACP NAC, destroying the panel's output driver.
By strictly following the node-by-node trace, respecting the galvanic isolation between the coil and the contacts, and verifying the exact ohmic values before power-on, you ensure a reliable, code-compliant fire alarm auxiliary trigger that will pass AHJ inspection on the first attempt.
