I inherited an installation of several wired AC smoke detectors. They're three-wire units (AC hot/neutral, plus an interconnect wire). At some point a previous installer removed one of the detectors, kept the wires in the ceiling box joined to prevent interruption to the other detectors, and replaced the unit with a 2-wire 12 VDC smoke detector monitored by an alarm panel:

ceiling box

The wires inside the ceiling box are live. The 12V wire goes around the outside of the box directly into the attic -- the DC and AC lines aren't technically in the same box. However they are in front of each other, and I'm not sure if they're too close.

Per the NEC, is there any violation here? The safest thing I can think to do is put a round cover over the box, move the 12 VDC wire a foot or two over, and install the detector in its own separate spot.

  • From the pic yeah I would just suggest a plastic cover between the smoke detector and the cable box, if you don't find an adequately sized one, I'd recommend 3d printing one, since it's a super easy shape, a circle and 2 holes
    – Tofandel
    Sep 10, 2021 at 10:29

3 Answers 3


You're overthinking it

All you need here is some insulating means to separate the 120V circuit from the 12V circuit -- while they don't make box barriers in the configuration you're after, a non-metallic cover plate of the appropriate size will do just as well provided you can route the cable around it. You'll need to use slightly longer screws than what came in the box, though, as you'll have to mount the smoke detector base through the cover plate holes and into the box mounting holes.


Per NEC there should be a partition between the 120v and the 12v since the insulation is not the same voltage rating but that's the only violation that I know about.

  • Unless the fire alarm wire is rated for 300v many are then it could be in the same space.
    – Ed Beal
    Aug 28, 2017 at 20:37
  • Do they make a plate that I can stick into the box, covering up the 120V wires, that would qualify as a "partition" in this context?
    – smitelli
    Aug 28, 2017 at 21:05

I'm probably doubling ThreePhaseEel's answer, but with one change: steel cover plate.

The low voltage wires are already outside the junction box. All you need to do is cover the box.

They make steel junction box covers (intended for steel octagon boxes, but they have the same bolt pattern). Get ones that are round, not octagon. They are so thin they won't make the detector stick out noticeably or need longer screws.

Generally they slot over the screws. Fit them and leave the screws still loose enough to fit the smoke detector also. Just be careful not to dislodge the steel plate when you fit the detector.

  • How do you propose to ground the cover? It appears that the box is nonmetallic (look at the fat molding around the cover-mounting-screw holes)... Nov 4, 2017 at 3:27
  • @ThreePhaseEel hadn't thought about that, given that it's concealed behind the smoke detector and on the ceiling, accidental contact not likely. Is it really Code to ground such a cover, and would the AHJ really put your feet to the fire on it? I suppose one could tap a 10-32 or get a cover that's tapped. Nov 4, 2017 at 17:26
  • 314.25(A) specifies that metal covers shall be grounded IAW 250.110... Nov 4, 2017 at 20:40
  • @ThreePhaseEel to change the subject slightly, wait -- how do you even do that when drywall interference (or paint) keeps you from bottoming the cover down hard on the box? You would need a ground jumper for the cover. Bonus, the cover wouldn't get lost lol. Nov 4, 2017 at 20:43

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