A standard wiring diagram for hard wired smoke detectors relies on a 3-wire NM-B (Romex) cable to distribute 120V AC power and a DC interconnect signal across a daisy-chained loop. The direct answer for most residential installations is to use 14/3 or 12/3 NM-B wire, mapping the black wire to the Line (hot) terminal, white to Neutral, red to the Interconnect pin, and bare copper to the equipment ground. This ensures that when one detector senses smoke, the red interconnect wire carries a 9V DC signal to trigger all alarms on the circuit simultaneously.

SAFETY WARNING: Hardwired smoke detectors connect to 120V AC mains. Always de-energize the circuit at the breaker panel, apply a lockout/tagout if possible, and verify the wires are dead with a non-contact voltage tester and a multimeter before touching any terminals. Local codes may require a licensed electrician for new branch circuit installations.

Decoding Diagram Symbols and the Terminal Map

Before tracing the physical wires, you must understand the schematic symbols used in manufacturer diagrams (like those from Kidde or First Alert). A circle with an "S" denotes the smoke sensor chamber, while a wavy line represents the 120V AC source. A straight line with dots or dashes branching off to multiple circles represents the interconnect loop. The piezo horn is usually shown as a semi-circle with radiating sound waves. Unlike simple light fixtures, these diagrams show two distinct circuits sharing one cable: the AC power circuit and the DC signaling circuit.

The physical wiring harness plugged into the back of the detector translates these schematic symbols into physical pins. Below is the definitive terminal mapping for standard 120V interconnected alarms.

Table 1: Hardwired Smoke Detector Terminal and Wire Mapping
Wire Color (NM-B) Harness Wire Terminal / Pin Label Function & Signal Type NEC / Code Requirement
Black Black Line (L) / Hot 120V AC Power Source (Unswitched) Must be fed from an AFCI breaker in dwelling bedrooms/hallways (NEC 2020/2023).
White White Neutral (N) 120V AC Return Path Must be continuous; cannot be switched or shared with unrelated circuits.
Red Red Interconnect (I) 9V DC Signal (Triggers remote alarms) Max 18 devices (Kidde) or 12 devices (First Alert) on a single loop.
Bare Copper Green / Bare Ground (G) 0V Fault Path / Shielding Must bond to metal junction boxes; cap if detector has no ground pin.

Node-by-Node Trace: From Panel to Piezo Horn

Reading a diagram is useless if you cannot trace the physical path of the electrons. Here is the exact node-by-node trace for a standard 3-detector daisy chain, explicitly calling out polarity and ground paths.

Node 1: The Service Panel (Source)

The circuit originates at a 15A or 20A AFCI (Arc-Fault Circuit Interrupter) breaker. The 14/3 NM-B cable exits the panel. The black wire terminates on the breaker's load screw. The white wire terminates on the neutral bus bar. The bare copper wire terminates on the equipment grounding bus bar. The red wire is not connected at the panel; it is capped off with a wire nut inside the panel to prevent accidental contact with the bus bars.

Node 2: The First Detector (Junction Box)

The 14/3 cable enters the first ceiling junction box.

  • Ground Path: The bare copper wire is bonded to the metal junction box via a grounding screw and pigtail. If the detector harness includes a green/bare wire, it is wire-nutted to this bare copper bundle. If the detector is double-insulated (Class II) and lacks a ground wire on the harness, the bare copper is capped with a wire nut and tucked into the box.
  • AC Polarity: The black (hot) wire is wire-nutted to the harness's black wire. The white (neutral) is wire-nutted to the harness's white wire. While the internal transformer of the smoke detector is not strictly polarity-sensitive, NEC 200.4 and 200.5 mandate that white must be used for the grounded (neutral) conductor to maintain branch circuit consistency.
  • Interconnect: The red wire from the incoming cable is wire-nutted to the red wire on the detector harness, plus the red wire of the outgoing 14/3 cable heading to Detector #2.

Node 3: The Daisy Chain (Detectors 2 and 3)

At Detector #2, the incoming 14/3 from Detector #1 and the outgoing 14/3 to Detector #3 meet. All black wires are tied together, all white wires are tied together, all bare grounds are bonded, and all red interconnect wires are tied together. The detector harness pigtails into these respective bundles. Detector #3 (the end of the line) follows the same pattern, but the outgoing red, black, and white wires are omitted. The incoming red wire simply connects to the Detector #3 harness red wire. The 9V DC interconnect signal flows continuously through the red wire bundle; when Detector #1 pulls the interconnect line high, Detectors 2 and 3 sense the voltage change and activate their piezo horns.

Pro-Tip on Wire Nuts: When tying together three or four 14 AWG solid wires plus the stranded harness pigtail, use a red or yellow wing-nut connector. Twist the stranded harness wire slightly longer than the solid NM-B wires before inserting them into the nut to ensure the stranded wire doesn't slip out of the connection.

Verifying the Circuit with a Multimeter

Do not rely solely on the detector's "push-to-test" button to verify your wiring. That button only tests the internal battery and logic board. To verify the hardwired branch circuit and interconnect loop, use a digital multimeter (DMM) following these exact steps.

  1. Verify AC Voltage (Line-to-Neutral): With the breaker ON and the detector unplugged from the harness, set your DMM to AC Voltage (V~). Place the red probe on the black wire and the black probe on the white wire. You should read between 114V and 126V. If you read 0V, check the breaker. If you read ~240V, you have a miswired multi-wire branch circuit or a lost neutral.
  2. Verify Ground Reference: Keep the DMM on AC Voltage. Measure Black-to-Bare (should read ~120V) and White-to-Bare (should read < 2V). If White-to-Bare reads > 5V, you have a floating neutral or a shared neutral carrying return current from another circuit, which is a severe code violation.
  3. Verify Interconnect Continuity: Turn the breaker OFF. Set your DMM to Continuity (the diode/sound wave symbol) or low Ohms (Ω). At the first detector box, place one probe on the red wire heading to Detector #2, and the other probe on the red wire returning from Detector #2 (or at the next box). You should read less than 1 ohm. If you read "OL" (Open Loop), the red wire is broken or disconnected at a downstream wire nut.
  4. Verify DC Signal (Live Test): Reconnect the harnesses and turn the power ON. Set the DMM to DC Voltage (V⎓). At an open harness pigtail, measure between the Red wire and the White (Neutral) wire. Press the test button on a connected detector. You should see the DC voltage spike to approximately 9V DC on the red wire, confirming the signaling circuit is active.

Code Requirements and Common Wiring Mistakes

Even if your multimeter reads perfectly, your installation can fail inspection or fail during a fire if you violate National Fire Alarm and Signaling Code (NFPA 72) or the National Electrical Code (NEC). Understanding these edge cases separates a professional installation from a DIY hazard.

The AFCI Mandate

Under NEC Article 210.12, 120V branch circuits supplying outlets and devices in dwelling unit bedrooms, hallways, and living rooms must be protected by an Arc-Fault Circuit Interrupter (AFCI). Hardwired smoke detectors fall under this rule. If your smoke detector circuit trips the AFCI breaker immediately upon energizing, you likely have a shared neutral with a non-AFCI circuit, or the interconnect wire (red) is running too close to high-interference loads. Ensure the smoke detector circuit is a dedicated home run or shares a neutral only within a properly handled 2-pole AFCI setup.

Mixing Brands and Exceeding Limits

A frequent mistake documented on forums and in field inspections is mixing Kidde and First Alert detectors on the same interconnect loop. While both use a red wire for signaling, their internal DC signaling voltages, pull-up resistors, and communication protocols (especially for smart/CO combo units) differ. Mixing them can result in one brand failing to trigger, or worse, continuous false alarms. Furthermore, exceeding the manufacturer's maximum device limit (typically 18 for Kidde, 12 for First Alert) causes voltage drop on the interconnect line, preventing the furthest detector from receiving the 9V trigger signal. Always count every device on the red wire loop, including combination CO/Smoke units and heat detectors.

Switched Circuits and Receptacles

NEC 210.12 and general fire safety principles dictate that smoke detectors must not be connected to a circuit that is controlled by a wall switch. If a homeowner turns off the switch to a bedroom outlet, they inadvertently kill power to the smoke detector. While the 9V battery backup will keep it alive temporarily, a dead battery combined with a switched-off circuit leaves the home unprotected. Always trace the circuit back to the panel to ensure no dimmers, smart switches, or standard toggles control the hot (black) wire feeding the detector loop.