Hardwired Smoke Detector Replacement by Brand: Which Kidde, First Alert/BRK, Firex and USI Models Plug Into the Harness Already in the Ceiling
Kidde 20-9003 fits Kidde and Firex harnesses only; First Alert SMI100-AC replaces the BRK 9120B, no rewiring; USI names no cross-brand fit; 10-year limit.
- Verified
- Updated
- 9 min read
- 18 sources
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Short answer
A hardwired smoke detector is replaced by the model number printed on the back of the old alarm, not by the shape of its connector. Kidde’s 20-9003 Quick Connect adapter joins an existing Kidde or Firex harness to a new Kidde alarm, and Kidde’s spec sheet says it is not compatible with other manufacturers’ products. First Alert states that the SMI100-AC (listed on Amazon as 9120BFF) replaces the BRK 9120B with a Quick Connect plug and no rewiring, and that the SC9120B combination alarm is discontinued in favour of the SMICO100-AC. Universal Security Instruments’ USI/SM/HWREP manual prints an AC quick connector harness and up to 12 interconnected alarms, with no other brand named. Cross-brand plugs exist in both directions (Kidde KA-B2 and KA-F2, First Alert ADF-12 and ADK-12), but each one lists the exact old models it fits. Every maker prints a 10-year replacement limit and says to turn the breaker off before touching the harness.
The picks
Kidde harness owner replacing one alarm
Kidde 20SAR hardwired smoke alarm, AA backup (1 pack)
Kidde 20SAR guide: AC Quick Connector harness, interconnects with i12020 through i12080, i4618A, PE120, Pi2000 and the other Kidde models in its list, maximum 24 Kidde devices; confirm the old model against Kidde's 20-9003 list before buying
Kidde harness owner who wants a sealed backup battery
Kidde 20SA10 hardwired smoke alarm, 10-year battery (1 pack)
Named in the Kidde 20SAR guide's interconnect list alongside the 20SAR; same AC Quick Connector harness family
Old Kidde or Firex harness, new Kidde alarm
Kidde 20-9003 Quick Connect wiring adapter
Kidde spec sheet: connects existing Kidde/Firex wire harnesses to Kidde hardwired alarms, not compatible with other manufacturers' products, 10-year limited warranty; the new alarm's own harness is not used
Firex harness, new Kidde alarm, whole interconnect line being replaced
Kidde KA-F2 Quick Convert Firex adapter (2 pack)
Kidde: puts a new Kidde alarm on an existing Firex wire-in harness; Kidde's Firex page says every device on the interconnect line must be replaced together
BRK 9120B owner replacing one alarm
First Alert BRK 9120BFF / SMI100-AC hardwired smoke alarm (1 pack)
First Alert: Quick Connect plug included, no rewiring, replaces the 9120B, interconnects with up to 12 other First Alert or BRK detectors and 6 other compatible devices, 10-year limited warranty
Several 9120B alarms on one circuit, all past 10 years
First Alert 9120B hardwired smoke alarm (5 pack)
BRK 9120B manual: Quick-Connect power connector with black, white and orange wires, maximum 18 compatible units of which 12 smoke alarms, conforms to UL 217 as the manual prints it
Kidde’s own adapter page says it plainly: “Don’t assume the adapter will work based on the connector shape, alarm image or brand alone.” Read the model number first.
Which replacement plugs into the harness you already have?
Each row is what the maker prints, in the Sources list, on 2026-10-04. A model that is not on a maker’s list is not claimed to fit.
| Old alarm, by brand | Connector as the maker names it | Old models the maker lists | Replacement that plugs in | Where the maker says so |
|---|---|---|---|---|
| Kidde hardwired (i-series and others) | “Quick Connect” wiring harness | i12020, i12020A, i12040, i12040A, i12060, i12060A, i12080, i12080A, i4618A, i4618AC, KN-SMFM-I, PE120, Pi2000, HD135F, SM120X, i12010S, i12010SCO, P4010ACS, P4010ACSCO, 3050-VAS10-A, “see Kidde.com for complete list” | A new Kidde hardwired alarm through the 20-9003 adapter ; the 20SAR and 20SA10 use the AC Quick Connector harness but are not on the sheet’s printed list | 20-9003 spec sheet; 20SAR user guide |
| Firex (Kidde-owned since 2009) | Firex wire-in harness | Not listed by model on Kidde’s Firex page | A new Kidde alarm through the KA-F2 adapter , replacing every device on the interconnect line | Kidde Firex support page; ShopKidde adapters page |
| First Alert / BRK (current plug) | “Quick-Connect Power Connector”, black hot, white neutral, orange interconnect | 9120B (SMI100-AC “replaces First Alert 9120B”); SC9120B (discontinued, replaced by SMICO100-AC) | SMI100-AC / 9120BFF , Quick Connect plug included, no rewiring; a 9120B is still sold as a 5 pack | SMI100-AC and SC9120B product pages; 9120B manual |
| First Alert / BRK (plug-in connector made before June 1, 2003) | Older plug-in connector | First Alert SA4121B; BRK 4120, 4919, 5919, 86RAC | A new Kidde AC/DC alarm through Kidde’s KA-B2 adapter | Kidde KA-B2 page |
| Universal Security Instruments (USI) | “AC quick Connector harness”; USI-1204 notice: black, white and yellow wires | USI-1204 (no longer available) | USI says to use its current hardwired alarms “designed to fit your existing USI wiring”, but prints no model numbers on that page; the USI/SM/HWREP was not plainly in stock on 2026-10-04, so it is not linked | USI-1204 replacement notice; USI/SM/HWREP manual |
Kidde’s 20-9003 page adds that the adapter “works only with specific existing and replacement alarm model combinations listed in the User Guide or Compatibility Guide”. That Compatibility Guide PDF could not be opened this session, so the sheet’s list above is a subset, not the whole list.
Which adapter plug moves a new alarm onto an old harness?
| Adapter | Direction the maker prints | Old models the maker lists | What the maker prints about it |
|---|---|---|---|
| Kidde 20-9003 Quick Connect | Existing Kidde or Firex harness to a new Kidde alarm | The Kidde list in the first table | “Not compatible with other manufacturer’s products”; the new alarm’s supplied harness is not used; 10-year limited warranty |
| Kidde KA-B2 Quick Convert | Existing First Alert or BRK harness to a new Kidde alarm | SA4121B; BRK 4120, 4919, 5919, 86RAC, connector made before June 1, 2003 | Works with all AC/DC Kidde smoke and combination alarms; 5-year limited warranty |
| Kidde KA-F2 Quick Convert | Existing Firex harness to a new Kidde alarm | None printed | Replace all devices on the interconnect line (Kidde Firex page) |
| First Alert ADF-12 | BRK and Firex alarms, “without rewiring” | BRK 4120, 9120 and 7010 series, 5919, 2002RAC, SC6120B, SC9120B, HD6135FB, RM3; Firex 406 (G-6), 4518 (ADC), 4618 (FADC), 484 (PG-40), 4480 (PAD), 5700 (ADH), 499 | “Conforms to UL 1977 Standard”; the page does not say which side is the harness |
| First Alert ADK-12 | “Converts 3-pin Kidde to First Alert/BRK” | Same BRK list; Kidde 125, 1275 and 1285 series, 1276, PE120, PE120E, PI1200, JD135F | Triangular 3-pin, “passes 120V AC + Interconnect”; conforms to UL 1977 |
An adapter is a plug, not a system approval. The 20-9003 sheet caps the line at “the NFPA interconnect limit of 12 smoke alarms”, and the Kidde i12060A guide says a non-specified manufacturer’s interconnect “could result in nuisance alarming, failure to alarm, or damage”.
Which brands can share one interconnect wire?
Only pairings a maker prints appear as yes. Everything else is “not published”, which is the honest reading of three manuals that each list their own brand only.
| Existing line | Kidde | First Alert / BRK | Firex | USI |
|---|---|---|---|---|
| Kidde | Yes, models in the user guide; max 24 Kidde devices, 12 smoke alarms, 18 alarms total, 1000 ft of wire, one circuit | Not published; Kidde: a mixed-brand system “has not been tested or listed as a complete system” | Only by replacing every Firex device on the line (Kidde Firex page) | Not published |
| First Alert / BRK | Not published; the SC-series manual lists First Alert and BRK only | Yes, SA4120, SA4121B, SA100B, SA520, 9120, 9120B, SC6120B, SC9120B, 7010 series, 4120 series and others; max 18 alarms, 12 smoke, 1000 ft, one breaker | Not published (ADF-12 is a plug, its page prints no interconnect statement) | Not published |
| USI | Not published | Not published | Not published | Yes, up to 12 alarms, one circuit, 1000 ft (USI/SM/HWREP manual) |
The interconnect wire itself is not one colour. Kidde and USI manuals call it red, the BRK 9120B manual calls it orange, and USI’s USI-1204 notice says yellow. Kidde’s i12060A guide says a single-station alarm leaves the red wire capped and unconnected.
How do you read the 10-year replacement date?
The three makers count the ten years from different events, and none of the guides read this session explains how the date code digits encode a month and year.
| Brand | Where the date is | Ten years from | As the maker prints it |
|---|---|---|---|
| Kidde i12060A | “Date Code (located on back of alarm)” | The date code | “NFPA and the manufacturer recommend replacing this alarm ten years from the date code” |
| Kidde 20SAR | Date code on back, plus an “Install date” and “Replace by” label | Initial power-up | End-of-unit-life alert after ten years; write the replace-by date on the label |
| First Alert / BRK SC-series | Space on the unit for Month/Year | Installation | “Replace alarm 10 years after installation. Please write the date in the space provided” |
| BRK 9120B (older manual) | Space on back of unit | Date of purchase | “Always replace a Smoke Alarm after 10 years from date of purchase” |
| USI/SM/HWREP | Date code on back | The date code | “Recommend replacing this alarm ten years from the date code on back of alarm” |
If the old alarm is already chirping on a fixed interval, the chirping guide separates a low battery from the end-of-life signal before you buy anything.
Before unplugging, every maker says the same thing in its own words. Kidde’s 20-9003 sheet: “Turn off AC power before attempting to connect wire harnesses.” First Alert’s SC-series manual: turn off power “at the circuit breaker or fuse box before beginning installation”. After the swap, Kidde says to push the test button on each alarm and verify that all the interconnected alarms sound, and NFPA says to test all smoke alarms at least once a month.
Before you order an adapter
The Kidde 20-9003 is the cheapest fix on this page and the easiest to buy wrong: it is Kidde-to-Kidde only, and Kidde’s own list of supported models is longer than the spec sheet prints. A First Alert or BRK harness takes the SMI100-AC with its own plug and no adapter at all. If the harness is USI or you cannot read the old model number, no maker publishes a plug for you: fit a same-brand alarm or have the box rewired. Which current alarm to put on the plug is the smoke detector review. Every listing above was in stock on 2026-10-04.
Frequently asked questions
Can a new Kidde smoke alarm plug into an old First Alert or BRK harness?
Only through Kidde's KA-B2 Quick Convert adapter, and only for the models Kidde names: First Alert SA4121B and BRK 4120, 4919, 5919 and 86RAC with a plug-in connector manufactured before June 1, 2003. Kidde's 20-9003 adapter is for Kidde and Firex harnesses and its spec sheet says it is not compatible with other manufacturers' products.
What replaces the First Alert BRK 9120B without rewiring?
First Alert's product page says the SMI100-AC replaces the 9120B and ships with a Quick Connect plug so no rewiring is needed. Amazon lists the same unit as 9120BFF/SMI100-AC. For the SC9120B smoke and carbon monoxide combination, First Alert marks the model discontinued and names the SMICO100-AC as its replacement, also with an adapter plug.
Can Kidde and First Alert alarms share the same interconnect wire?
Neither maker publishes that pairing. Kidde's support page says alarms from different manufacturers form a system that has not been tested or listed as a complete system, and that a model not listed in the user guide should not be used in an interconnected system. First Alert's SC-series manual lists First Alert and BRK models only.
Where is the 10-year replacement date on a hardwired smoke alarm?
Kidde's i12060A guide says to replace the alarm ten years from the date code on the back; the 20SAR adds an install date and replace-by label. First Alert's SC-series manual says to replace 10 years after installation and write the month and year in the space provided. USI's manual counts ten years from the date code on the back.
Do I have to turn the breaker off to swap a plug-in smoke alarm?
Every maker says yes. Kidde's 20-9003 sheet prints a warning to turn off AC power before connecting wire harnesses, and the i12060A guide says to turn off the main power to the circuit before wiring. First Alert's SC-series manual says to turn off power at the circuit breaker or fuse box before beginning installation. The plug does not change that.
Fact sheet and how to cite this page
- Answer
- Kidde’s 20-9003 Quick Connect adapter joins an existing Kidde or Firex harness to a new Kidde alarm and is not compatible with other makers; First Alert says the SMI100-AC replaces the BRK 9120B with its Quick Connect plug and no rewiring; USI’s manual names only USI. No maker publishes a cross-brand interconnect, each prints a 10-year replacement limit, and each says to turn off the breaker before touching the harness.
- Applies to
- Kidde i12020, i12040, i12060, i12080, i4618A, KN-SMFM-I, PE120, Pi2000, HD135F, SM120X, i12010S, i12010SCO, P4010ACS, 3050-VAS10-A (20-9003 spec sheet list)
- First Alert SA4121B; BRK 4120, 4919, 5919, 86RAC with a plug-in connector made before June 1, 2003 (Kidde KA-B2)
- BRK 9120B, 9120 series, 4120 series, 7010 series, SC6120B, SC9120B, HD6135FB (First Alert ADF-12 and ADK-12 pages)
- Firex 406, 4518, 4618, 484, 4480, 5700, 499 (First Alert ADF-12 page); any Firex harness (Kidde KA-F2)
- USI-1204, USI/SM/HWREP (USI store and manual)
- Compatibility basis
- Kidde’s 20-9003 spec sheet and product page, the KA-B2 page and the Firex support page; the Kidde i12060A and 20SAR user guides; First Alert’s SMI100-AC and SC9120B product pages, the Resideo SC-series manual and the BRK 9120B manual; First Alert’s ADF-12 and ADK-12 store pages; USI’s USI/SM/HWREP manual and the USI-1204 replacement notice
- Assumptions
- The old alarm’s model number is readable on its back; every maker ties adapter fit to the model, not to the connector shape
- The 12 smoke alarm and 18 total interconnect limits are quoted as the makers print them; the NFPA standard itself was not opened
- Where a maker page could not be read (Kidde’s Compatibility Guide PDF, BRK’s current interconnect list, USI’s replacement model numbers), the page says so instead of filling the gap
- Method
- OwnerSpec read each maker’s spec sheet, user guide, product page or support page on 2026-10-04 and confirmed every linked listing IN_STOCK through the Amazon Creators API the same day; this is a comparison of published documents, not an installation or a test.
- Primary source
- Kidde, 20-9003 Quick Connect wiring adapter spec sheet (PDF) (retailer listing or retailer-hosted document)
- Facts verified
- Last updated
- Cite as
- OwnerSpec, "Hardwired Smoke Detector Replacement by Brand: Which Kidde, First Alert/BRK, Firex and USI Models Plug Into the Harness Already in the Ceiling", https://ownerspec.com/air/parts/hardwired-smoke-detector-replacement-by-brand/, facts verified 4 October 2026.
Every figure above also appears, with its source, in the article. All sources ยท How OwnerSpec verifies numbers
Sources
- Kidde, 20-9003 Quick Connect wiring adapter spec sheet (PDF) (retailer listing or retailer-hosted document)
- Kidde, Quick Connect Adapter 20-9003 product page (manufacturer or vendor document)
- Kidde, KA-B2 Quick Convert Adapter product page (manufacturer or vendor document)
- Kidde (ShopKidde), wiring adapters for smoke alarms and CO detectors (manufacturer or vendor document)
- Kidde support, Firex interconnectivity (manufacturer or vendor document)
- Kidde support, Interconnecting Kidde alarms with other brands (manufacturer or vendor document)
- Kidde, i12060A user guide (PDF) (manufacturer or vendor document)
- Kidde, 20SAR user guide (PDF) (manufacturer or vendor document)
- First Alert, SMI100-AC interconnect hardwire smoke alarm product page (manufacturer or vendor document)
- First Alert, SC9120B hardwired smoke and carbon monoxide alarm product page (manufacturer or vendor document)
- First Alert/BRK, SC-series smoke and carbon monoxide alarm user manual M08-179591 (PDF) (manufacturer or vendor document)
- BRK, 9120B smoke alarm user manual (PDF) (manufacturer or vendor document)
- First Alert store, ADF-12 BRK and Firex adapter plug (manufacturer or vendor document)
- First Alert store, ADK-12 BRK and Kidde adapter plug (manufacturer or vendor document)
- Universal Security Instruments, USI/SM/HWREP product page (manufacturer or vendor document)
- Universal Security Instruments, USI/SM/HWREP instruction manual (PDF) (manufacturer or vendor document)
- Universal Security Instruments store, replacing the USI-1204 hardwired smoke alarm (manufacturer or vendor document)
- NFPA, Learn more about smoke alarms (manufacturer or vendor document)