Overview: why interconnection and building power matter
Modern model codes require interconnected smoke alarms when more than one alarm is required in new construction; the International Residential Code (IRC) specifies alarms inside each bedroom, outside sleeping areas, and on every level and mandates interconnection where more than one alarm is needed.
When building wiring is provided for alarms, the IRC also requires those alarms to take primary power from the building wiring while retaining battery backup; the wiring must be permanent and not be on a user‑accessible disconnecting switch (other than overcurrent protection).
Carbon monoxide (CO) alarms follow parallel rules: IRC R315 lists locations that require CO alarms (for example, attached garages and where fuel‑fired appliances are present) and requires interconnection where multiple CO alarms are needed; power‑source rules mirror R314.
Interconnected alarms provide a house‑wide alert and are strongly recommended by NFPA and research because working smoke alarms significantly reduce the risk of death in home fires.
Common retrofit scenarios in older homes without 14/3 interconnect
If your home has existing hardwired alarms but lacks the 3‑conductor interconnect (14/3) conductor, four practical retrofit paths are commonly used: replace all units with a compatible wireless‑interconnect family; install bridge‑capable units that join wired and wireless groups; run new 14/3 interconnect wiring; or use listed relay/interface modules for auxiliary notification—choice depends on budget, access, and local code acceptance.
Replacing with a wireless‑interconnect product family is widely supported by manufacturers as a retrofit option: several manufacturers offer 'wireless' or 'wire‑free' families that interconnect compatible units by radio and publish pairing/setup procedures in their manuals.
Hybrid solutions exist too: some hardwired models include an integrated RF bridge or manufacturers sell bridge‑capable models that allow mixing hardwired units and wireless battery units without running full 14/3 wiring.
Running new 14/3 cable restores the standard hardwired interconnect arrangement and satisfies the primary‑power requirement, but it typically requires cutting ceilings or fishing cable through finished spaces and commonly needs an electrician and permit review for code compliance.
What to watch for: listings, compatibility, and limits
Choose alarms listed to the appropriate standards—UL 217 for smoke alarms and the applicable CO standard (e.g., UL 2034)—and confirm the specific product listing on the manufacturer page or packaging.
Mixing brands or models on a hardwired interconnect is frequently discouraged because signaling arrangements can be manufacturer‑specific; manufacturers advise using the same family or explicitly supported compatible devices for reliable interconnect operation.
Industry guidance and manufacturers commonly state practical limits for non‑supervised interconnect loops: a typical practical limit is up to 18 initiating devices total on an interconnect circuit, with no more than 12 of those being smoke alarms—consult the specific manufacturer's datasheet for exact limits and any supervised/repeater requirements.
Do not place required hardwired alarms on a switched circuit: IRC R314.6 requires permanent wiring without a disconnecting switch (other than overcurrent protection) and alarms intended to be primary powered must be continuously powered by the building wiring when provided.
Relay/interface modules and auxiliary notifications
Relay or interface modules (for example, Kidde's SM120X) can sense an alarm interconnect signal and switch external 120 VAC devices such as lights or bells; manufacturer manuals document wiring diagrams and limitations for these modules.
These modules are auxiliary‑notification or automation devices and do not substitute for required alarm power‑source rules—the primary alarm devices themselves must meet the code's power and listing requirements.
When using such modules, confirm the module is a listed product and follow the manufacturer wiring diagrams and limitations closely.
Costs and practical tradeoffs
Basic battery‑only sealed‑lithium photoelectric smoke alarms commonly retail roughly $15–$40, and AC hardwired alarms with battery backup or combination smoke+CO units commonly range from about $30–$100+ depending on features.
Installed costs vary: replacing a single hardwired alarm where wiring already exists is often in the $70–$200 range for professional labor; running new interconnect wiring in a retrofit through finished ceilings or walls is the most expensive option and can cost several hundred dollars per run or $500–$2,000+ for whole‑house projects depending on access and labor rates.
Because wireless retrofits limit labor to swapping units and commissioning the radio interconnect, they can be less expensive than running new hardwired interconnect when wall/ceiling access is difficult.
Permits, inspectors, and who should do the work
Local code adoption and enforcement vary; the IRC requirements apply only where the IRC is adopted and enforced by the local jurisdiction, so confirm with your local building department before performing a retrofit intended to meet code.
If the work involves any permanent house wiring changes—running new cable, modifying junction boxes, or changing circuit wiring—hire a licensed electrician or confirm local inspector acceptance, since practical safety and code compliance require qualified electrical work.
Also check manufacturer compatibility statements and product manuals before relying on a mixed‑brand or hybrid interconnect; some manufacturers publish explicit compatibility lists or warn that mixing voids listing/compatibility.
Maintenance and useful practices
Test alarms monthly and follow manufacturer guidance for end‑of‑service life; NFPA, CPSC, and manufacturers commonly advise replacing smoke alarms at the end of their service life (commonly 10 years) and testing monthly.
When commissioning wireless or hybrid systems, follow the manufacturer's pairing or bridge setup procedures so all units reliably communicate and respect the manufacturer's stated limits and supported mix of device types.
Bottom line
For older homes without a 3‑wire interconnect, using a listed wireless‑interconnect family or a bridge‑capable hybrid is a practical, code‑aware retrofit path where allowed by the local AHJ; running new 14/3 wiring restores the classic hardwired arrangement but is more invasive and commonly requires an electrician; relay/interface modules can add auxiliary notification but don't change primary alarm power/listing obligations—confirm listings, compatibility, device limits, and local code acceptance before proceeding.