How do I mount smoke alarm to suspended ceiling?

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  #1  
Old 06-04-06, 11:29 AM
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How do I mount smoke alarm to suspended ceiling?

Hi, I'm in the process of replacing the tiles in my suspended ceiling in the basement. The previous owners mounted the hard-wired smoke alarm to an electrical box attached directly to a ceiling joist. They had cut a hole in one of the ceiling tiles so you could see the smoke alarm.

I'd like to mount the alarm directly to the new tiles if possible. What do I need to do to accomplish this?

Thanks!
 
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Old 06-04-06, 01:05 PM
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Make hole in the ceiling tile and use an old work box.
 
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Old 06-04-06, 01:14 PM
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If it is not a low voltage system, I like to use a 4" square box with a round plaster ring and a t-bar hanger. This prevents the tile from sagging and it prevents the gap between the detector and the tile that sometimes happens.

If this is low voltage I would use a couple of screws from the bottom and some washers on the top of the tile to support it if it is light. If there is much weight to it, I use the method I described above
 
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Old 06-05-06, 10:42 AM
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It is line voltage with a transformer.

Do I leave the existing electrical box attached to the joist? The only thing I can think of is detaching the alarm from the electrical box and move it down 3" or so to hang it on the suspended ceiling. This seems like an obvious no-no to me, because you essentially have an open electrical box above the alarm. I attached a picture to show what I'm talking about.

Is there a solution to this problem? Thanks!

<img src="http://i82.photobucket.com/albums/j277/maizegoblue/alarm.jpg">
 
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Old 06-05-06, 10:45 AM
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Leave the box with the transformer where it is. Place a blank cover over the box. Then run a piece of NM cable from the box to the new box you will install for the smoke alarm.
 
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Old 06-05-06, 05:21 PM
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The transformer is most likely for your doorbell. It has nothing to do with the smoke detector.
 
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Old 06-05-06, 05:31 PM
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Exactly what racraft said.
*edit - and pcboss too.*

I like to mount them within 6-8" of a corner of the tile so sag is not an issue. Plastic boxes and smokes do NOT weight much.
 
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Old 06-08-06, 12:23 PM
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Thanks for the suggestions. pcboss--you are absolutely correct--that is for the doorbell. I would have thought that transformer would be attached to the main panel, but there's no transformer there.

Here's my plan for the alarm:
Open knock-out on existing metal electrical box
Insert plastic NM wire holder in knockout
Run 14/2 NM from metal box to plastic old-work box
Cap the metal electrical box with metal cap
Hook up alarm connections in plastic box
Attach alarm to plastic box attached to ceiling panel

Please advise if I'm doing anything wrong. A couple things I'm still not sure about: How much extra NM should go into each box? Does there need to be any kind of clip to hold the NM wire going into the plastic box (I know I need one on the metal box)?

Thanks again!
 
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Old 06-08-06, 12:31 PM
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That particular metal box looks to be the kind with built in cable clamps.

You need six inches of wire inside the box.
 
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Old 06-08-06, 12:36 PM
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Thanks racraft. How are the six inches divided up? Four inches with cable removed and two inches with the exterior cable intact?
 
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Old 06-08-06, 01:12 PM
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No, about 1/4"-1/2" of the sheathing inside the box. The rest is just the conductors.
Six inches total from where it enters the box.
 
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Old 06-08-06, 03:37 PM
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Well, I got everything installed nicely. Unfortunately, I now can hear the doorbell transformer "buzzing." I don't remember if I could hear it before, but it is surely much louder now. When I was working with the metal box attached to the joist, I noticed it was loose. I tightened up these screws so the box was snug against the joist. My guess is that this has caused the transformer buzzing to worsen.

Do I need a new transformer? The doorbell works fine, but I have to get rid of the buzzing noise. What else can I do? I figure that loosening the box on the joist again isn't a very good idea.
 
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Old 06-08-06, 04:46 PM
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Originally Posted by jumpyg
Hi, I'm in the process of replacing the tiles in my suspended ceiling in the basement. The previous owners mounted the hard-wired smoke alarm to an electrical box attached directly to a ceiling joist. They had cut a hole in one of the ceiling tiles so you could see the smoke alarm.

I'd like to mount the alarm directly to the new tiles if possible. What do I need to do to accomplish this?

Thanks!
Go to the box store and buy a box with a support for suspended ceilings (boxes in suspended ceilings MUST be supported). Put in a listed connector into the existing box. Run a new peice of rx (/or low voltage cable depending on what type of smoke you have)to the new box from the old box. Leave the transformer where it is. make the splice in the old box and put a listed cover on it. mount the detector on the new box (cut into the ceiling tile). Do this with the power disconnected to that ckt.
 
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