You are required to have a box at the location.

You can use round remodel boxes or the regular single gang remodel boxes. For existing construction I like to use the round remodel boxes as it is easier and quicker to use a hole saw to cut out the drywall for a round box.

The hole saw will give you a perfect fit and you dont have to worry about an accidental slip with a utility knife damaging the finished ceiling - not that I have ever done that personally though ;)

you can as long as it is not a switch leg , otherwise the smoke will shut off with the light, so you need to have a constant feed at the ceiling junction box ... good luck

And be sure that the location complies with all the applicable codes regarding location. Our State building code permits smoke detectors on ceiling or wall so long as the wall location is not more than 12" from the ceiling. But at least one city in the State requires ceiling location only- and then at least 12" from any wall. Get all these specifications in advance and then cut the box hole only once.

Thanks for the hints. I will install box as you suggestÂ

Two more related questions if you have the patienceÂ

I could tap into the AC supply at the closest ceiling light fixture (I checked and the light switch connection is inside the ceiling light box itself, so there is a non switched supply inside). However, that ceiling light box is a little too crowded to add any additional wires. IÂd rather cut the AC supply wire somewhere in a convenient location up in the attic and add a metal junction box to take power to the smoke alarm. Is there any problem with doing that?

The other question is that , since IÂll be up in the attic branching off with a new wire, I thought this would be a good time to also install an additional AC powered smoke alarm in the attic itself. I was thinking of installing this additional AC smoke alarm somewhere on a rafter close to the roof ridge. I assume that this smoke alarm will also need a box right?

It will need a surface mount box and in some locations they might argue it needs conduit or armoured cable if it's 'prone to damage' but ordinarily a surface mount box with a suitable clamp for transitioning to surface nm type to a surface mount box. You will need a special wire because they will need to be interconnected...you may also want to consider one or more detectors in other levels of the house if that's appropriate.

As for the first detector, yes to the box - I am in canada but it's pretty much a universal requirement here in north america. Any join in the wires is ok provided it's in an accessible junction box. In the attic would probably be fine, you could always run the wire up a joist or queen post to keep it out of the insulation and visible. Keep in mind rules about where wires can run, usually not across joists unless you can't walk there, usually a max headroom of 3' or less.

I'm curious why you'd want one up there but think it's not a bad idea, especially if you store anything up there or have celluose (paper) blown in insulation, over time, the fire retardant added can lose its effectiveness. Also if you are in a bushfire-prone area, again, a good idea. Certainly no harm.

Keep in mind smoke detectors have a finite working life and they will have a 'replace by' date on them...and you'll need to replace all of them.

You may also want to consider a combo smoke/carbon monoxide detector for the one in the house, provided it can be linked to the one in the attic, the linking is mandatory (you probably wouldn't hear the one in the attic all that well.)

Be mindful of placement, by your local rules, and also rules of thumb. Mounting underneath the ridge might seem the best place but usually that's a dead air spot and the last to get the smoke, further down one of the rafters might be better- if you have a ridge vent, it will naturally draw the smoke so it might be a good place.

Keep in mind avoiding exposure to the weather, if it's near a vent, also, check what the operating temperatures of the unit are, you might need one made for low temperature use if you get low temps in winter, since your attic is probably pretty close to the outside temperature.

There are usually garage models available but they usually are heat detectors only, because of the risk of false triggering by vehicle exhaust. A heat detector might actually be a good option for up there for above reasons, some units are combo heat and smoke.

HTH

Depending upon the area of the country you live in you will want either a heat detector or a rate of rise detctor for the attic. Some attics are very prone to exceed the maximum temperature that smoke detectors are rated to operate effectively in. Also, depending upon what type of detection system the actual alarm has in it you can get numerous false triggers in an attic.

Heat detectors are common to about 120-135 degrees which could be reached in an attic without a fire present, that is why you may want to consider a rate of rise unit for the attic - even though these are not very common, especially on the interconnected residential systems.

It is a very good idea to have a detector installed in an attic. Fires that originate in an attic are some of the most dangerous ones to both life and property due to how quickly they can spread, and also how large they can become prior to triggering a detection unit.

jmvd20 has made some good points, I'd forgotten about rate-of-rise heat detectors...One of the factors in false alarming in the attic could be dust, if it's a photoelectric alarm. In an enclosed space the heat detector would operate quite quickly, and the rate of rise would overcome issues with the high ambient temperatures as mentioned.

Positioning of these probably would be at the highest central point (not right next to a heat source of course) but the enclosed literature would tell you how to set it up.

If you have a security system, or are contemplating one, you could consider adding low voltage smoke/heat detectors that would connect to/report to the panel, and would give the option of reporting to a monitoring station or your cellphone, as well as an audible alarm at home.

These could operate independently of any mains-wired smokies or be tied to them with a relay, again, you probably need at least one mains wired unit, but local codes rule.

I hear what you say about heat in the attic becoming a problem for the detector. I had once placed a wireless thermometer in the attic (was doing work up there and used the thermometer to see when it would finally get cool enough in the evening to go up and do work). The thermometer would show a 30F degree temp differential in the early afternoon on a sunny day (it was spring when I did that). Given the fact that I live in San Jose California where temps can typically get to 100-105 for a few days every summer, I can see how attic temperature could climb to 135F.

As a matter of fact, I had a battery operated smoke alarm up there, close to the attic access trap door, and it did not last long. I had thought it was the dust that doomed it but now I think it may actually have been the heat.

The wire that is used to gang the smoke detectors together is yellow. I suspect that is a color convention for smoke detectors only? If so, do they make a 14/3 wire with white,black and yellow wires? (I could not find any at the chain hardware stores). Or is this supposed to be a completely separate yellow wire connection?

Just use the regular 14-3 with black/white/red conductors. Use the red wire for the interconnect wire from alarm to alarm - of course just make sure it is not connected to any other conductors except the interconnect wire on each alarm.