• What is the signalling voltage on hardwire smoke detectors?
  • Is it acceptable to mix brands?
  • Is it acceptable to have detectors on different electrical circuits, connected only with an interconnect wire (will that cause the interconnect to be referenced to the wrong ground level?).

Why? I have an existing hardwire OneLink alarm, and a battery powered slave. But the corresponding hardwire BRK alarms get terrible reviews (see Passive-Aggressive smoke detectors for the type of problem). I'd like to know if I can purchase higher quality units and still interconnect them.

The OneLink alarms transmit extra digital data wirelessly (e.g. the room and alarm type)... do they do that over the interconnect wire also?

The vendor instructions of course tell you to use only matching alarms.

5 Answers 5


It's an old question but I'll still add this. Code requires that devices be used according to their listing, following the manufacturer's instructions. In the instructions for many smoke detectors, specific brands / models are indicated are compatible to interconnect. (I'd assume the manufacturer tests their product with those listed compatible, and UL verifies.)

I believe many use the same voltage on the interconnect, but still it is a code violation to interconnect them if it the combination is not specifically permitted in the instructions.

Even if you connect them, and test them, and they seem to work - that's not a substitute for the kind of testing the manufacturer or test lab would do. You can't be sure what's going on in the electronics, and whether an issue may develop in the future.

If an issue does prevent the device from working properly, it could be undiscovered until the worst possible time. With life safety devices, it's best to follow the rules to the letter.


Hi my name is Josh and I install smoke alarms for a living (licensed electrician), and it is my experience that smoke alarms of different brands can possibly cause false alarms as the interconnect voltage can range between 5v and 9v dc. If interconnected it is best if they are all the same brand. They are now also using RF wireless interconnection, you can even use a combination of both. For example Brooks https://www.brooks.com.au/home-smoke-alarms/battery-powered/wall-mount-test-locate-silence-memory-control-eib450/ even have a wireless remote, very good for high ceilings and the elderly. As to having hardwired alarms on different circuits, not a good idea as it tends to cause RCDs to trip whenever interconnected alarms trigger and sound. Hope that helps.

  • Is it not true the interconnect voltage is whatever the battery is currently putting out? Nov 8, 2022 at 8:12

All hardwired alarms are interoperable. If one alarm goes off it energizes the third wire and all other ones go screaming too. I have quite a few hardwired alarms and many of them are different brands.

  • I've read elsewhere of problems with interchangeability, even within the same brand. What's the type of signal sent on the interconnect wire?
    – Bryce
    Aug 14, 2012 at 23:00
  • 1
    I always thought the third wire just uses simple on/off signal. Probably 9v, since that's the backup battery voltage on the smoke alarm. Now I'm not so sure :) however empirical evidence from bunch of different alarms going off at once suggests that they operate just fine.
    – Vitaliy
    Aug 16, 2012 at 21:09

According to this, this, and this source the initiating detector applies 9 volts DC to the signal line (relative to the neutral line) to indicate an alarm condition.


I just measured a new First Alert hardwired alarm with 2xAA backup, and the interconnect voltage during an event is 7VDC across the interconnect and AC neutral wires. However it seems to have very low amperage support, it's just a voltage to be sensed.

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