Adding Smoke Detectors - Box Fill Question

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  #1  
Old 03-31-16, 09:16 AM
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Adding Smoke Detectors - Box Fill Question

I would like to add hardwired smoke detectors to the bedrooms in my upstairs. There is an existing smoke detector in the hallway. It is installed in an 18 in[SUP]3[/SUP] box. There are two wires terminated there - one 12/2 and one 12/3. It appears that this box is filled to capacity. If I wanted to add an additional 12/3, my understanding is that the box would need to be 25 in[SUP]3[/SUP]. Is that correct? Do they actually make one that large?

Are there any good workarounds? I assume that I could add a junction box and splice the 12/3 from that. I would terminate the incoming 12/3 at this new box, run a 12/3 to the existing hall smoke detector, and also run a 12/3 to the first bedroom. Does that make sense, or is there a better way? I appreciate any suggestions.
 
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Old 03-31-16, 09:44 AM
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Is this an exiting circuit or are you creating a new one ? #12 makes things difficult to work with.
I use #14 wiring for smoke detector circuits.
 
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Old 03-31-16, 10:42 AM
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This is an existing circuit. My entire house is wired with #12 unfortunately. It would be so much nicer if #14 had been used.
 
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Old 03-31-16, 10:44 AM
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If it's a 15 amp circuit, you could add # 14 instead of # 12.
 
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Old 03-31-16, 10:55 AM
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It is on a 15 amp circuit. I though I read somewhere on this forum that it's better not to mix 14 and 12. If I remember correctly, the NEC doesn't prohibit it. It's more like a best practice, in the event that someone changed the breaker to a 20 amp. I'm not opposed to running #12...I'm just not clear on the box fill, and whether the use of an additional j-box would be required.
 
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Old 03-31-16, 11:27 AM
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Your box is currently at 13.5 cubice inches. Adding the new 12-3 would put you at 20.25. I would pull the 12-3 out of that box and add a junction. Run a new 12-3 into the original box. Run new cable from new junction.
 
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Old 03-31-16, 12:15 PM
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Can someone help me with choosing a junction box? I know this is probably a stupid question. I've installed plenty of devices/switches/outlets, but I've never actually installed a standalone junction box. Can I use something like a Carlon 2-gang new work, or is there something more appropriate? I know that I will also need to buy a cover. Does it need to be secured to a joist?
 
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Old 03-31-16, 12:31 PM
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Most used is a metal octagon ceiling box such as used for ceiling lights. However other boxes can be used. So long as there are enough cubic inches the choice is yours. Yes, it must be fastened to a joist or other framing member. If it would be covered by insulation you might want to use a rafter or vertical roof framing member.
 
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Old 03-31-16, 12:50 PM
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I would use a deep 1900 box that is 4 inches square and a blank cover. More room than the 8B box.

The box needs to be secured. Cables should be secured within 12" if cable clamps are used. Metal boxes need to be grounded.
 
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