Hardwired Smoke Detector Wiring: Wire Types and Code Requirements
Hardwired smoke detectors are required by the NEC in all new residential construction and in existing homes whenever work triggers the requirement. Unlike battery-only units that can be placed anywhere, hardwired detectors are permanently wired into the electrical system and are interconnected so that when one alarm sounds, every alarm in the home activates simultaneously. This interconnection requirement drives the wire type selection. Here is exactly what wire you need and how the system is configured.
Two Cables, Two Functions
A hardwired smoke detector circuit uses two different cable runs: a power supply run and an interconnect run. The power supply run delivers 120-volt power from the electrical panel to the first smoke detector in the chain. The interconnect run carries a low-voltage signal between all detectors so they trigger each other. These two functions require different cable configurations.
Power Supply: 14/2 NM-B
The branch circuit that powers the smoke detectors is a standard 15-amp circuit using 14/2 NM-B. The black conductor is the hot supply, the white conductor is the neutral return, and the bare copper ground connects to the detector ground terminal. The circuit originates at a 15-amp single-pole breaker in the panel and runs to the first smoke detector location. Smoke detectors draw less than 1 amp each, so a single 15-amp circuit can supply dozens of detectors without approaching the circuit capacity. Many electricians and code officials prefer a dedicated circuit for smoke detectors so a tripped breaker does not disable the entire alarm system, though shared circuits are generally permitted as long as the circuit is not likely to be de-energized during normal use.
Interconnect Wiring: 14/3 NM-B
The interconnect wire runs from detector to detector through the chain. When one detector senses smoke and activates, it sends a low-voltage signal on the interconnect conductor that triggers every other detector in the system simultaneously. This interconnect function requires the third conductor found in 14/3 NM-B. In 14/3 cable, the black conductor carries the hot supply, the white conductor carries the neutral, the red conductor carries the interconnect signal, and the bare copper conductor is the ground. Running 14/3 between all detector locations enables both power distribution and interconnect signaling in a single cable pull.
How the Circuit Connects
The first detector in the chain receives 14/2 from the panel (or the circuit can begin with 14/3 if the electrician prefers to use one cable type throughout). Between all subsequent detectors, 14/3 NM-B runs from detector to detector. At each detector location, the black connects to the black detector lead (hot), white connects to white (neutral), red connects to red or yellow (interconnect, depending on detector manufacturer), and bare connects to green or bare (ground). The last detector in the chain terminates with no outgoing cable since it has no downstream detectors to signal.
NEC Requirements for Smoke Alarm Placement
NEC Section 760.41 and NFPA 72 (the fire alarm code) require hardwired, interconnected smoke alarms in new one and two-family dwellings in each sleeping room, outside each separate sleeping area within 21 feet of each sleeping room door, on each story of the dwelling including basement, and in the living room or near the stairway to upper levels in some jurisdictions. Alarms must be hardwired with battery backup so they continue to function during a power outage.
Existing Homes and Remodel Triggers
In existing construction, adding hardwired smoke detectors is triggered by certain permits and renovation scopes. Adding sleeping rooms, finishing a basement, pulling an electrical permit for substantial work, or other triggers specified by local codes may require upgrading battery-only alarms to hardwired interconnected alarms. Confirm with your local building department what triggers the hardwired requirement in your jurisdiction before planning the installation.
IB Lighting carries 14/2 and 14/3 NM-B Romex from Southwire and Cerrowire in 250-foot and 1,000-foot spools with free freight insurance. Browse our wire and cable selection or call (800) 674-9019 for contractor pricing on project quantities.