Can I add a smart smoke detector to my existing hardwired interconnected alarms?

The wire carries a signal, not a standard#

Hardwired interconnected alarms share an extra conductor that runs in the same cable as the power. When one alarm trips it signals on that wire and the others sound. Because the wire, the harness and the 120 V supply look alike everywhere, it is easy to conclude that any alarm with a matching plug will join the chain.

That is wrong, and the manufacturers say so. Kidde's page on mixing brands states that when alarms from different manufacturers are interconnected, "the system has not been tested or listed as a complete system", it "may not meet NFPA 72 or local code requirements", and the warranty does not cover system-level performance. Its interconnectivity page warns that a non-specified manufacturer's system "could result in nuisance alarms, failure to alarm, or damage to the devices". Google, explaining why Nest Protect does not interconnect with other brands, says the NFPA "prohibits the connection of alarms from different manufacturers without special testing".

The other answer persists because adapters make plugs fit. Kidde's own RF-SM-ACDC product page says its two universal adapters allow it "to be used with most existing interconnected systems". A plug that fits is a mechanical fact. Whether both alarms read the signal the same way is a listing fact, and the compatibility list in each user guide is what answers it. Kidde says every alarm's user guide "clearly lists which models are approved for interconnection".

Two kinds of smart smoke alarm#

Wire riders replace one alarm in your existing chain. They take power and the interconnect signal from the old alarm's harness, keep the chain sounding together, and add Wi-Fi alerts on top. Kidde's hardwired smart alarms and the hardwired First Alert SC5 work this way.

Radio-only alarms ignore the interconnect wire. A hardwired Nest Protect takes power from the 120 V supply, but Google says "Google Nest Protects speak to each other wirelessly", using Weave, a Nest-developed protocol over 802.15.4 and Wi-Fi (802.15.4 is the radio layer Thread also builds on). Google argues this lets an alarm "convey more safety information in an emergency", such as the type and location of the danger. The cost: a Nest Protect on the landing does not sound when your old hardwired alarm in the basement trips, and the reverse. In a wired chain it is an island.

Google says Protects keep talking "even if your Wi-Fi network goes down", within 50 ft (15 m) of each other. Only the phone notification needs the internet, the same split shown in what still works in an internet outage.

The brand by brand matrix#

These are the pairings each manufacturer's own documents state, as of October 2026. Find your existing alarm's model number on the label on its back, then read across.

The lists are not symmetrical. Kidde's support page names First Alert and BRK model numbers that interconnect with Kidde. None of them appears on the SC5's list or in the P4010ACS-WF guide's list. Brand-level compatibility is not model-level compatibility.

A wire rider reports less about the old alarms than about itself. The P4010ACS-WF guide says that although it hardwire interconnects with the listed models, it will "ONLY transmit smoke and CO alarm signals from these alarms" to your phone. Low battery, end of unit life and faults on the non-Wi-Fi alarms "will NOT be transmitted". The alarms "do not communicate between alarms using Wi-Fi", and "Wi-Fi features do not function without AC power", so in a power cut the phone alerts stop.

How many alarms one chain can hold#

Kidde's hardwired guides set two ceilings. "A maximum of 24 Kidde Safety devices may be interconnected", but the system "should not exceed the NFPA interconnect limit of 12 smoke alarms and/or 18 alarms total (smoke, CO, Smoke/CO Combination, heat, etc.)". With 18 alarms connected, up to 6 remote signaling devices or relay modules can still join, which is where 24 comes from. So the "24 Kidde devices" on Ring's page is a device count, not an alarm count.

The same guides set the wiring limits: interconnect in the same cable or raceway as the AC power, at most 10 ohms of interconnect resistance, no more than 1,000 feet from first to last unit, and every hardwire interconnected unit on a single circuit. A smart alarm swapped in for an existing one changes none of this. One added as an extra unit can push a large house past 12 smoke alarms. First Alert's SC5 page states no maximum.

What the code asks for#

The International Residential Code at R314.4, as carried in Washington's 2021 residential code on UpCodes, says that where more than one smoke alarm is required, "the alarm devices shall be interconnected in such a manner that the actuation of one alarm will activate all of the alarms". Physical interconnection "shall not be required where listed wireless alarms are installed and all alarms sound upon activation". R314.1.1 requires listing to UL 217, and R314.6 requires primary power from the building wiring with battery backup, with exceptions.

Google states that the IRC treats wired and wireless interconnect as equal. That covers a full set of Nest Protects, not a mixed house where one alarm no longer activates all of them. Your state or city adopts its own edition, often amended (Washington's adds an exception for new alarms that cannot interconnect with existing ones), and that adoption is what applies.

The decision tree#

1. Read the label on one existing alarm

   Twist it off its base and note the brand, model number and date code. Kidde's guides state that "the National Fire Protection Association (NFPA) and the manufacturer recommend replacing this alarm after ten years". If your alarms are near that age, buy a full new set and the brand question disappears.

2. Find that model on a smart alarm's compatibility list

   Kidde or Firex model: the Kidde wired smart alarms list it. First Alert or BRK model on the SC5 list: the hardwired SC5. Check the user guide of the exact smart model, not a brand-level support page.

3. Listed: swap one alarm for the smart one

   Replace one alarm on the chain, ideally the one nearest your router. Ring states the Kidde wired smart alarm "can only pair to a 2.4 GHz WiFi network", the usual cause of a device that will not pair. That one alarm then relays smoke and CO alarms from the whole chain to your phone, with the reporting gaps above.

4. Not listed: replace the whole set with one family

   An adapter cannot make unlisted brands compatible. Replace every alarm on the chain with one listed family, at least one of them smart. This is the step most worth handing to an electrician.

5. Not ready to replace anything: add a listener

   Ring's Smoke and CO Listener mounts within 6 inches of an existing alarm (3 inches for best results) and hears the standard T3 smoke and T4 CO patterns. It reports over Z-Wave to a Ring Alarm or Ring Alarm Pro base station, which makes it one of the cases where you do need a hub. Ring states it "does not detect fire, smoke or carbon monoxide". It changes nothing on the wire, which makes it the option for renters.

Nest Protect households#

Google's help page says Nest Protect "is no longer available for sale, but is still supported", and names the First Alert SC5 as the partner device US customers can buy that is compatible with Nest Protect (2nd gen) in the Google Home app. When a smart home company shuts down files this under hardware discontinued, service continuing.

For a full set of Protects, the SC5 keeps the radio interconnect going. For a wired chain with a Nest Protect or two bolted on, the hardwired SC5 is the one device in these documents listed both on a wire chain (First Alert and BRK) and with Nest Protect. That helps only if your wired alarms are on its list. A Kidde chain with a Protect on the landing has no bridge in any document cited here. Google Home's track record is the wider context for buying into that ecosystem.

Matter does not change any of this#

Smoke and CO alarms have been a Matter device type since 1.2, carrying alarm state, test state and battery, per the Matter device types table. That makes a smart alarm visible to more controllers. It does nothing for the wire: alarm-to-alarm interconnect stays inside each manufacturer's listed system, which is why the sensors guide treats a networked smoke alarm as a life-safety device first and a smart device second. Alarm panels and sirens have no device type at all, one of the gaps in what Matter still cannot do.

Where this page stops#

• The brands and models named. Universal Security Instruments, X-Sense and others are not covered, because no document cited here states what their alarms interconnect with.
• US one- and two-family homes. Fire alarm panels and monitored systems follow different rules.
• Your model's own guide wins over any table, including this one. Lists change between guide revisions.

If you are moving into a house with a mix of alarms, the moving house checklist puts them on the first night's list, and reading their labels is the first step of the tree above.