We have to replace an old 4 wire smoke alarm, wire colours are: white, black, orange, brown, in our condo with a 3 wire smoke/co2 alarm. My question is which wire from the old system should be capped when I install the new alarm?

  • 2
    It sounds like you're in a building with multiple dwelling units. If that's the case, you should probably contact building maintenance or the building owner. It's likely that the whole building is interconnected, and you shouldn't be messing with the alarm system.
    – Tester101
    Mar 27, 2015 at 13:29
  • It sounds like you bought a standard AC powered alarm (3 wires, L+N, G) but have a home/condo with an interconnected alarm system (4 wires, L+N, G, I). If you don't want it to be interconnected you can leave out the interconnect wire (likely the orange one) but it really does make your home safer and might even be required by local code. If you have a townhome type condo you probably don't have to worry about other units, but if you have a high-rise type of condo it might be connected. As always, sharing the model you are working with improves the odds of us getting the answer right.
    – Jeff Meden
    Mar 27, 2015 at 15:41
  • I think the wiring depends on the system and the detector, as there seems to be quite a few different ways to wire them. Some systems use hot, neutral, and a signal wire. Some use hot, neutral and two signal wires. Some use hot, neutral, signal in, and signal out.
    – Tester101
    Mar 27, 2015 at 17:09

1 Answer 1


I'm assuming you also have a ground wire? You should use the orange wire as the interconnect, and cap the brown. You could even go as far as to use red electrical tape to wrap the orange wire near the marrette so it's properly labeled, but it's not necessary. Just make sure you're using the same wire throughout and it will be fine.

However, do be advised that you will need to replace all your smoke alarms. Interconnected smoke alarms generally do not communicate between brands. You will usually need to buy multiple of the same model for them to work properly. This is especially true switching between 4-wire smoke detectors to a 3-wire model. Is this a condo or apartment building? If so, the smoke detectors are all interconnected, no? In which case you probably should not be replacing just one, as that won't trigger the alarm throughout the building.

Lastly, 4-wire smoke detectors are often tied into a security panel. Do you have a security system installed? If so, firstly, contact your security company before disconnecting your smoke alarm, or you may get a visit from the fire department. Secondly, you may not be able to switch to a 3-wire model. It will prevent your monitored smoke alarm from working.

  • What is (was) the 4th wire used for? Mar 27, 2015 at 19:20
  • 1
    Different systems have different uses for the 4th wire. Some times there are two signal wires, some have signal in and signal out, it varies. Smoke alarms have no standard for interconnecting, so different manufacturers do it differently.
    – amace
    Mar 28, 2015 at 19:00

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