During some renovations I have opened up enough wall for the inspector to note that smoke/CO detectors need to be hardwired throughout the house.
It is much more convenient to power them using the individual circuits I have created than to try to use a whole new circuit and snake it through multiple levels of the house. The detectors are all wirelessly interconnectable, so they do not need to be on one wire.

As far as the local code concerned, this is allowable. (907.2.11.5 Interconnection - Physical interconnection of smoke alarms shall not be required where listed wireless alarms are installed and all alarms sound upon activation of one alarm)

The question: All the circuits I plan to branch the individual alarms out of are 20amp, 12/2 wired circuits. I was planning on using 14/2 wire coming out of those, going solely to one smoke/CO detector. I know that mixing/downsizing gauges in a branch is typically a no-go, but it seems like this isn't hitting the issues one typically has doing so. Any commentary, especially regarding NEC 2017/2020 code sections prohibiting this specific scenario would be much appreciated.

  • 2
    You can't run undersized wire between boxes. Period.
    – isherwood
    Jul 21, 2021 at 12:56
  • 1
    If using 14/2 use 15 amp breaker/s. Quite sure the alarms won't use even 15 amps.
    – crip659
    Jul 21, 2021 at 12:57
  • Looks like i'll be using 12/2 wire for those detectors then, what an unfortunate lack of situational exception in the code, eh.
    – sil80
    Jul 21, 2021 at 13:05
  • 2
    If keeping 20 amp breaker must use 12/2 for whole circuit. 14/2 is big no-no and can cause you to be denied any and all insurance claims and inspector will make you rip it out.
    – crip659
    Jul 21, 2021 at 13:05
  • 2
    Isherwood had it right, but for perspective, try not to think about the device attached, but the proverbial 'weakest link' in the chain. If your run of 14g wire has a nail through it (or any other fault), then the breaker will cheerfully pump 20A until it trips itself, by which time you might already have a fire. Jul 21, 2021 at 14:08

3 Answers 3


The problem is the future:

  • 12/2 20A initially
  • You add a branch 14/2 to a smoke detector
  • Next owner decides they want a light in the ceiling near the smoke detector and adds an LED fixture (in theory, no worse than the smoke detector - very low current)
  • The owner after that decides they want a convenience receptacle for charging phones, etc. (also low current, but now a true 15A duplex receptacle instead of hardwired device) and puts a branch down the wall and installs the receptacle.
  • HVAC goes out in the winter. Owner's relative (we'll pretend owner strictly knows "only use this receptacle for small stuff") plugs in a typical 1500 W heater and some other stuff.
  • Circuit runs for hours at ~ 20A. Overheats that 14/2 weakest link without tripping the 20A breaker.

Where there is likely an exception is that an appliance with factory installed wire can have smaller wire (14, possibly even smaller!) because that wire is guaranteed to only serve that device. But that doesn't work for other wires.

  • 1
    Ah shucks. Yeah. That does make sense. Well, off i go rewiring some lights i had on 14/2 as well. Thanks Manassehkatz. Logic does prevail.
    – sil80
    Jul 21, 2021 at 17:28

Breakers protect wires

And that's the only function it has, on a branch circuit with multiple small loads and receptacles. Since really, the appliances are too small to reliably be protected. A 20A breaker won't protect a smoke detector from destruction.

Honestly, this is fairly obvious when you think about it. How did you miss it? Cognitive bias. You are searching for and noticing rules and logic that support your goal, and disregarding what does not support your goal. You may be at risk of this happening elsewhere in your electrical decisions. That's important a) for safety and b) so you don't get blind-sided at inspection time.

  • Yeah, it is easy to get carried away with looking to validate ones goals, but this is why I ask the question. It does still peeve me that one is left responsible for various future home owners decisions, but I do like erring on the side of caution, so it's certainly better this way. I now have blisters from re-wiring/wire nutting 12/2 wire in dozens of recessed lights (properly, so the wire is twisted outside the nut) and look forward to doing the same for the smoke detectors.
    – sil80
    Jul 22, 2021 at 3:48
  • Oh, you don't need to twist the wires outside the nut. In fact, I wouldn't. I just get all the wires squared off and close enough to all make it into the "funnel". At that point, once you start twisting, the wire nut WILL twist them together tightly, regardless of how they were shaped before. If not, you're not tightening nearly enough. A 20A breaker is incapable of protecting #14 wire today nevermind the future. Jul 22, 2021 at 4:45
  • Yup, squared off for sure, I was also told to keep twisting the nut until i saw the wires outside of it start to twist together as well, indicating that they are good n tight in there. Logic being that when they heat up/cool down during usage cycles, they tend to work themselves loose and the extra twist helps keep em tight
    – sil80
    Jul 22, 2021 at 12:53

You asked for NEC reference. Section 240 is kind of a mess.

NEC 240.4 Protection of Conductors. Conductors, other than flexible cords, flexible cables, and fixture wires, shall be protected against overcurrent in accordance with their ampacities specified in 310.14, unless otherwise permitted or required in 240.4 (A) through (G).

So 310.14 applies unless (A) through (G) requires otherwise. I move through (A) through (C), which don't have provisions that apply, to:

240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.

Many people skip to 240.4(D)(4) which limits #14 copper to 15A, but that skips right over (E), which (D) gives priority to.

240.4(E) Tap Conductors. Tap conductors shall be permitted...(1)210.19(A)(3) and (A)(4)...other loads.

210.19(A)(4) Other Loads. Branch circuit conductors...shall have an ampacity sufficient for the loads served and shall not be less than 14 AWG.

So 310.14 applies, which in turn table 310.16 is applicable. If using NM cable section 334.80 limits NM cable to the 60°C column, so you can't exceed 15A with #14 NM cable.

  • 1
    Right. The tap rules show that NEC did in fact contemplate what OP thinks they neglected unfairly. They contemplated it, and rejected it in residential applications like this. Jul 21, 2021 at 18:37

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