Best Hardwired Smoke Detector 2026

Dead Batteries Cause 25% of All Smoke Alarm Failures — Hardwired Systems Eliminate the Risk

Reviewed by the WC Safety Editorial — independent safety specialists. Last updated: August 16, 2026.

Looking for the full lineup? See our best smoke detectors of 2026 roundup for our top overall picks across every type.

Short answer: The First Alert SM300-AC is the best hardwired smoke detector for most homes — dual-sensor technology (photoelectric + ionization) catches both fast-flaming and slow-smouldering fires, with 120V power plus AA battery backup and interconnect for up to 18 devices. It is First Alert's designated replacement for the discontinued 3120B that previously held this spot. If budget is the priority, the Kidde 20SAR delivers a current hardwired interconnected alarm with AA backup. For smart home integration without sacrificing hardwired reliability, the First Alert SC5 is the current option — it replaced the discontinued Google Nest Protect and works with Google Home.

Why hardwired beats battery-only: The NFPA reports that 25% of smoke alarm failures occur because batteries are dead or missing. Hardwired detectors eliminate this failure mode — they draw constant 120V power with a separate 9V backup for outages. The IRC mandates hardwired, interconnected alarms in all new construction for exactly this reason. A hardwired system that sounds every alarm in the house when a single unit triggers is the most effective residential fire warning system available.

Quick Comparison — 6 Current Hardwired Smoke Detectors

All detectors are hardwired 120V AC with 9V battery backup unless noted. Prices are approximate Amazon list prices at time of publication.

1. First Alert SM300-AC — Best Overall Hardwired Smoke Detector

The First Alert SM300-AC takes this position because it is what First Alert itself names as the replacement for the discontinued 3120B that used to sit here. It keeps the thing that made the 3120B the default pick — a genuine dual-sensor head carrying both photoelectric and ionization sensing, so it covers smouldering and fast-flaming fires in one device — and it is a currently manufactured unit rather than remaining third-party stock.

Power and interconnect. It runs on 120V AC with two AA cells as backup, so the alarm keeps working through an outage. It interconnects with up to 18 First Alert or BRK smoke, CO and heat alarms, of which a maximum of 12 may be smoke alarms — the same NFPA-derived ceiling covered in our interconnect guidance. All interconnected units must share one circuit.

Why dual-sensor matters here. A hardwired chain is usually whole-home, which means one alarm model ends up covering bedrooms, hallways and living space at once. A dual-sensor head avoids having to choose which fire type each room is optimised against. First Alert lists the SM300-AC as conforming to the UL 217 8th Edition requirements.

Specifications: Dual sensor (photoelectric + ionization) | 120V AC, 2x AA battery backup | Interconnect up to 18 devices, max 12 smoke alarms | 85 dB horn | Dual latching alarm and low-battery indicators | End-of-life signal ~10 years after installation | Optipath 360

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2. Kidde 20SA10 — Best Hardwired with Sealed 10-Year Backup

The Kidde 20SA10 removes the one maintenance task a hardwired chain still has: its backup is a sealed 10-year battery, so there is no cell to change and no chirp when someone forgets. Photoelectric sensing (per Kidde’s user guide), wired interconnect, current DETECT line.

Specs: Photoelectric | 120V AC + sealed 10-year backup | Wired interconnect | 85 dB | Self-testing | UL listed

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3. First Alert SMICO100-AC — Best Hardwired Smoke + CO Combo

The SMICO100-AC is the current hardwired combination unit and the named replacement for the SC9120B. The retrofit detail that matters: it ships with an adapter plug, so it can replace an existing hardwired alarm without rewiring the harness.

Specs: Smoke + CO (Precision Detection) | 120V AC + battery backup | Interconnect | Adapter plug included | 85 dB | UL 217 + UL 2034

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4. Kidde 30CUAR — Best Kidde Hardwired Smoke + CO Combo

The Kidde 30CUAR is Kidde’s current hardwired combination alarm and the named replacement for the KN-COSM-IBA. Smoke and CO in one interconnected head with AA backup.

No affiliate link. No Amazon listing naming this exact model could be verified, so we link nothing rather than the wrong product.

Specs: Smoke + CO | 120V AC + AA backup | Wired interconnect | 85 dB | UL 217 + UL 2034

5. Kidde 20SAR — Best Budget Hardwired Smoke Alarm

The Kidde 20SAR is the budget route to a current hardwired chain — DETECT sensing tuned against cooking nuisance, wired interconnect, two AA backup cells that Kidde says to change every 6 months. It replaces the discontinued i4618.

Specs: Enhanced smoke sensing | 120V AC + 2x AA backup | Wired interconnect | 85 dB | UL listed

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6. First Alert SMI100-AC — Best Budget First Alert Hardwired Alarm

The SMI100-AC replaces the 9120B: ionization sensing with Precision Detection tuning, 9V backup, and drop-in replacement of existing detectors. First Alert currently shows it backordered; the Amazon listing is live. Keep ionization at least 10 feet from cooking appliances.

Specs: Ionization (Precision Detection) | 120V AC + 9V backup | Interconnect | 85 dB | UL 217

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Further retired hardwired models

Two further former entries — the Kidde 21026043 (Firex) and the Universal Security MUSST2-BNCA — are older generations we could not verify in any current manufacturer catalogue, so they are no longer recommended.

Legacy Picks — Discontinued Models, Kept for Replacement Research

These alarms are no longer manufactured. They are documented here because a great many homes still have them wired into a chain, and the practical question is what to replace them with. None of them is a current recommendation, and none carries an active buy button.

Legacy Pick — First Alert 3120B (Discontinued)

Status check — verified 15 August 2026: First Alert lists the 3120B as discontinued and names the SM300-AC as its replacement model. It remains widely available from third-party sellers, but the SM300-AC is the currently manufactured equivalent.

The First Alert BRK 3120B held the top spot for years because it does one critical thing no ionization-only detector can: it catches both fire types reliably. The dual-sensor chamber pairs an ionization sensor (fast-flaming fires, such as paper or curtains) with a photoelectric sensor (slow-smoldering fires, such as burning upholstery or electrical insulation). The NFPA and USFA now recommend dual-sensor or photoelectric detectors in bedrooms — the 3120B was the hardwired option that covered both without compromise, and the SM300-AC now fills that role. Add full interconnect capability (up to 18 units, 150-foot wire run) and you have the foundation of a whole-home alarm system.

Specs: Sensing: Ionization + Photoelectric (dual sensor) | Power: 120V AC hardwired + 9V battery backup | Interconnect: Yes, up to 18 units | Alarm: 85 dB at 10 ft | UL 217 listed | Dimensions: 5.9" dia × 1.4" H | Replacement: 10 years

Legacy Pick — First Alert SC9120B (Discontinued)

The First Alert SC9120B is the top choice when you need both smoke and CO detection from a single hardwired unit. It combines an ionization smoke sensor with an electrochemical CO sensor, satisfying both NFPA 72 (smoke) and NFPA 720 (CO) requirements with one device — cutting installation time and the total number of units needed. The alarm clearly differentiates between smoke (Temporal Pattern 3: 3 beeps) and CO (4-beep pattern) so occupants know which hazard they're responding to. An essential choice for bedrooms and hallways near attached garages, gas appliances, and fuel-burning heating systems.

Specs: Sensing: Ionization (smoke) + Electrochemical (CO) | Power: 120V hardwired + 9V battery backup | CO Range: 30–999 ppm | Alarm: 85 dB at 10 ft | Interconnect: Yes | UL 217 + UL 2034 listed | CO Sensor Life: ~7 years | Replacement: 7–10 years

Legacy Pick — Google Nest Protect 2nd Gen, Wired (Discontinued)

The Google Nest Protect (Wired) is the only hardwired smoke detector that delivers genuine smart home integration — WiFi connectivity, app alerts when you're away from home, voice announcements that tell you exactly where the danger is ("Heads up, there's smoke in the upstairs hallway"), automatic monthly self-testing, and app-based hush for nuisance alarms. It uses a split-spectrum photoelectric sensor that distinguishes slow-smoldering fires (blue light) from fast-flaming fires (white light) with exceptional accuracy, paired with an electrochemical CO sensor. Connects to 120V hardwired with 9V battery backup and can trigger existing hardwired detectors on the same interconnect circuit.

Specs: Sensing: Split-spectrum photoelectric + electrochemical CO | Power: 120V hardwired + 9V battery backup | Connectivity: 802.11 b/g/n Wi-Fi (2.4GHz) | Interconnect: Yes — hardwired (triggers all units) + wireless (Nest Protect units) | Smart Home: Google Home, Google Assistant | UL 217 / UL 2034 listed | Replacement: 10 years

Related reference

See also Smoke alarm interconnect compatibility — the published Kidde, Firex, First Alert and BRK model lists before you replace one alarm in a chain.

Neighbouring references on this site: where to install smoke detectors, best first alert smoke detector 2026, hardwired vs battery smoke alarms, kidde vs first alert co alarms, ionization vs photoelectric smoke detectors (2026 guide), and why does my smoke alarm go off for no reason? (2026 guide).

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WC Safety is an independent, affiliate-supported review site. It is not a retailer: it holds no inventory, takes no orders, and earns only from qualifying purchases through clearly marked links — which never changes what a product is rated to do. Standards language is taken from the regulation text directly, and ratings are reported as the manufacturer publishes them. We run no laboratory and perform no testing of our own. Where published sources disagree, we say so and plan on the conservative figure rather than the flattering one.

Our methodology

Why there are no scores on this page. We removed the star ratings in August 2026. They implied a precision we could not justify: WC Safety runs no laboratory and tests no products, so there was no formula, no weighted criteria and no measurement behind the difference between 4.6 and 4.7. What remains is what we can actually stand behind — the manufacturer’s published specification, its current lifecycle status, and a plain statement of what each alarm suits.

Figures come from the regulation and the published specification, in that order. Derated numbers are calculated, not estimated. Nothing here is presented as a measured result, because we measure nothing.

Researched and written by Steven Eaton, editor of WC Safety. Steven holds no safety certification and does not test products; this page compares what manufacturers and regulators publish, with the gaps in that record marked. Last reviewed August 2026.

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