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I am considering adding hard-wired smoke detectors to my house. Due to other in-process remodeling projects and the structure of the house, I have access to get behind the drywall at each location that needs a detector so this would be a relatively easy job.

After watching some videos and reading up on how to do this project I understand what needs to be done except that I am unsure about the electrical boxes for each detector.

The videos and tutorials all used single-gang old work metal boxes. I found a related question here: How can I install a wired smoke detector without compromising the fire rated sheetrock? The accepted answer suggests using metal octagon boxes with a fire protection layer added. However, this house has no fire protection: no OSB layers above the walls, no fire-rated caulk at intrusion points, no fire-rated drywall, etc.

Do I have to use metal boxes or is plastic acceptable? Should they be single-gang or octagon ceiling boxes? Do I need additional fire protection around these boxes? Given that the rest of the house has no fire protection does this even matter?

The house is located in the U.S. in a state that adopts the latest NEC unaltered every three years.

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  • Fire alarms do not need to be protected from fire themselves any more than the rest of the area they are a part of.
    – Billy left SE for Codidact
    Jan 7, 2016 at 22:21

3 Answers 3

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I'd use a round plastic cut-in box even though you have access behind. You'll find this much easier in the long run.

Remember, smokes do not have ground connections, so just splice the grounds with a wire nut or other approved method and fold into the back of the box.

Along the lines of this:

enter image description here

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  • Could you please explain why this is acceptable? Is there something in the NEC that talks about this, or is it because nothing says I can't do it? Also, in my case, I would probably lean toward old-work boxes anyway because they are easier to place. Running the wire is what benefits from having open space above the ceiling.
    – user4302
    Jan 7, 2016 at 22:41
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    Nothing says you can't, so.....there it is.
    – Speedy Petey
    Jan 7, 2016 at 23:10
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    Most boxes like this can only support lightweight luminaires (fixtures), typically up to 6 pounds. So a smoke detector is easily allowable.
    – Speedy Petey
    Jan 7, 2016 at 23:11
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If the surface you are mounting on doesn't have additional fire rating, then you don't need anything more than a plastic or metal old-work box. A single-gang box works fine for most detectors, but an octagon or round box gives you a bit more room.

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  • I've seen a couple where their mounting ring never lined up with the standard spacing of single gang screws. round boxes for luminaries have more compatability in that regard because they have more threaded holes.
    – Billy left SE for Codidact
    Jan 7, 2016 at 23:49
  • That's possible. "Mounts to a standard box" will be a feature to look for when selecting a smoke detector.
    – longneck
    Jan 7, 2016 at 23:51
  • I'm not aware of a junction box 'standard' for smokes. I'd check the smoke's compatibility with specific boxes "single gang box" or "4" round luminary box" for example, or the actual distance between mouting screws. Or just end up securing one side of it to drywall :-p Another nice thing about round boxes. Some wired smokes wiring sticks out from the back quite a bit with a rigid connector that rotates around when attaching to the plate. The larger, round box will reduce the chance it gets damaged or comes loose when mounting.
    – Billy left SE for Codidact
    Jan 8, 2016 at 0:01
  • Not mounting a hard wired smoke detector on a junction box would violate the NEC because you would have splices not in a box. You are being too alarmist about smoke detectors not mounting on a box. If it's hardwired, it will be compatible with standard single gang or light fixture box.
    – longneck
    Jan 8, 2016 at 0:03
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There are a couple of different factors when determining the type of box needed for a particular installation. As far as smoke detectors are concerned the only factors to take into consideration are access to the box for mounting purposes and wire fill. Just as the other answers have said an "old work" box or "cut-in" box work well for this application as it doesn't require any attachment to a structural member of any kind. One thing to remember is if you are worried about adhering to the NEC you need to make sure you secure the wiring for the installation in compliance with the appropriate code.

When it comes to wire fill here is the NEC section to look up in order to find the correct table. Since the wiring itself is the only items that will be taking up space, to include wire nuts, in the box any single gang 18 cubic in. or larger box will have enough capacity to work with this installation.

Part (1) of Section 370-16(a) describes in detail the method of counting wires, as well as clamps, fittings, or devices (i.e., switches, receptacles, combination devices) - by establishing an equivalent conductor-value for each.

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