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In my home, I have a hard-wired smoke detector / fire alarm (https://www.lowes.com/pd/First-Alert-Wireless-Interconnect-AC-Hardwired-120-Volt-Photoelectric-Sensor-Smoke-Detector/1000456469) that is linkable to another smoke detector down the line. It is connected with 12-2 or 14-2 but it needs 12-3 or 14-3 normally to link the smoke detector together. I do not have any 12-3 or 14-3 wire on hand and I am wondering if you can use another individual wire to link them together. It is my understanding that the linking wire only sends about 3-9 volts down the wire to the other smoke detector to make it alarm. Could I use a 14 AWG or 16 AWG? Trying to avoid re-wiring the whole thing but if it is the right thing to do I will do it.

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Would wire like this work? https://www.lowes.com/pd/Southwire-100-ft-14-AWG-Stranded-Red-Copper-THHN-Wire-By-the-Roll/1000777296

  • Is there mains voltage on the 14/2? – Harper - Reinstate Monica Feb 14 at 19:29
  • the chosen wire can only be used in conduit (or inside an electrical junction box) – Jasen Feb 15 at 5:25
  • @Harper-ReinstateMonica there will be some sort of signal voltage on the wire relative to live or to neutral, so yes there is mains voltage on the signal wire. – Jasen Feb 15 at 5:26
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Even if the wire ends up only having a low voltage on it, it will be terminated and spliced in the same electrical boxes as the white and black high voltage wires, so it's going to come down to the same rules as any other wire in the circuit.

Using 14/3 is the only right way to do this, unfortunately. There is an exception for running a ground wire along a different path than the rest of the circuit, but that wouldn't apply here.

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  • wouldn't a 14 awg wire meet code? – HoneyDo Feb 14 at 19:04
  • Not always many of the linked systems I have installed and service are powered from a low voltage transformer in the control panel. Most fire alarm systems are power limited and 18 awg fire restive cable can be used where Romex would not meet code. – Ed Beal Feb 14 at 19:15
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    Fire alarm systems are different than smoke detectors and have special wiring requirements. – JACK Feb 14 at 20:37
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    Looks like I read the question the wrong way, but ended up with "smoke detectors" as the OP intended... – JPhi1618 Feb 14 at 20:54
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    @JPhi1618 Great fumble recovery..lol – JACK Feb 14 at 21:16
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The NEC requires fire alarm system(s) and the cable for fire alarms to be listed. Fire resistant cable is required and most of the systems are 18 awg on larger systems 16awg . The cable has a high temp rating that outlasts standard Romex or nmb, I have seen this cable intact after a fire where thhn in conduit melted.

Pulling a separate wire would be a code violation unless in conduit. You probably have a fire alarm circuit that is limited power , but even with power limited system code requires the wires to be in the same raceway or cable.

In some areas of code I might push the limits , Not with “life safety systems” modification of these that is not to code when the unthinkable happens can have you up on involuntary manslaughter charges.

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  • What region does this pertain to? When I installed smoke detectors (in a US home), it looked like regular /3 romex to me (I didn't run the wire). – JPhi1618 Feb 14 at 20:32
  • @JPhi1618 Fire alarm systems are different than smoke detectors and have special wiring requirements. – JACK Feb 14 at 20:38
  • @Ed Beal For clarification, it is a smoke detector, not a fire alarm. I have updated the original question. – Roberts2600 Feb 14 at 20:42
  • A linked smoke detector is part of an alarm system check out article 760 of the NEC. – Ed Beal Feb 14 at 21:52

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