House Wiring Question, Please


Hello,
Not to sure how to go about this, and would appreciate any thoughts.
Whatever I do, I want to be sure that it is "Code" compliant, fully.
Have a quite old smoke detector on ceiling. Doesn't work reliably
It's AC house wired presently, but I wish to replace it with a regular battery powered one without any 110 V house wiring going to it.
The AC wires going to it just exit the ceiling thru a small hole. No metal wiring box like you always see; just a exit hole for the wires thru the ceiling. Other things on the circuit, so I can't just disable the wiring run at the service box.
Any new smoke detector I put up will be flush against the ceiling, and I am pretty sure there wouldn't be enough room in it's base for me to terminate the wires in, in place of a metal box.
I can of course put a wire-nut on the wire ends, tape it to be even more sure, and push them back thru the hole. But I imagine that this would not be Code compliant. True ?
If I do have to go to a metal box, what kind do I try to find that would enable me to just cut a somewhat larger hole and install it ?
There are no wooden ceiling joists or structure by the hole to fasten the box to. So, the kind of boxes that have that adj. bar that attaches to two ceiling joists does not seem appropriate.
Any new enlarged hole I cut I would, of course, like it to be minimal.
So, how would I anchor the typical metal octagonal wiring box ?
Is there perhaps another type that is made for the problem I have tried to outline ? Sure would be great if so. If there is, who makes and what is it called, please ?
Any thoughts on how to go about this would be much appreciated.
Thanks, Bob
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Robert11 wrote:

If your house is newer than a certain age, code requires 120VAC smoke detectors. All 120VAC devices require a box. So you need to install a box. There are old work boxes both metal and plastic suitable for your situation, although I don't understand the comment about "no wooden ceiling joists or structure" - there'd have to be, I'd think?
But anyway, just go down to your local building supply and ask for an old work ceiling box and they should be able to hook you up. There's kinds with expandable supports that dig into the rafters, and also there's kinds with ears that just hold the box to the plaster or drywall ceiling itself. Either will be fine, use whichever is easier for you.
Oh, and if you ever run into the knucklehead that installed your old smoke detector, give him a dope slap for me.
nate
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I am fairly sure AC powered with battery back-up is the minimum requirement anywhere smokes are required and for sure all insurance companies are looking for that.
To correct the wiring problem you need to use an "old work" box. Plastic is the norm now. Those can be bought at the BORG in round or 3x2 rectangles. Either will work with a smoke as the base plate will attach to and fully cover either. Rectangles are a bit easier to cut.
These boxes have wings that grip the back of the wall board. You will quickly grasp the concept when you see one.
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*In NJ 120 volt smoke alarms with battery backup are required for bedroom areas and one in the basement. The battery backup requirement is new due to code adaptation of AFI circuit breakers recently.
You could can use a fan brace and box. The adjustable brace fits through a four inch hole. If you have a lot of wires get one with a deep octagonal box. There is also a plastic old work round box which will work on drywall.
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Robert11 wrote:

Much better to replace it with a line powered unit that has a backup battery.

Cut in a box which is the way your current detector should have been installed and then connect the replacement detector.

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Smoke detector rules vary wildly from one location to another, and by what was required when the building was built, so you'll need to check with your locality to find out what you need for code compliance. Assuming the cable coming out of the ceiling is a proper cable, and not some jury rig piece of lamp cord, you can easily fasten a surface mounted round wiremold box to the sheetrock using toggle bolts or sheetrock anchors, and bring the cable through a knockout in the back of the box. You can almost as easily install a switchbox supported by Madison bars, plaster tins, switchbox supports, or whatever else they're called.

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In addition to the other advice, many AC powered smoke detectors are wired together so that if one trips on one floor, they all go off. So, you should determine if there are any others and if they are wired that way.
Then as others have said, the solution is to get a replacement that is compatible and install a simple old work plastic box. All you need to do is cut out an opening with a key hole saw. The box has ears that swing out and grab from behind.
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