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Box size calculation question

936 Views 5 Replies 4 Participants Last post by  TomU
I currently have a hard wires smoke alarm box, right now two 14/3 wires enter the box. I'd like to add another hard wire smoke alarm and interconnect it, so I'd be adding another 14/3 wire. Is my following calculation correct?

3 x 3 x 2 cubic inches for conducts entering the box
+ 3 x 2 cubic inches for the three conducts that are being spliced (or do I need to count 9 here?)
+ 2 cubic inches for the device (nothing for the alarm's pigtails)
= 26 cubic inches minimum

Is this correct or am I overlooking something?
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Oh yeah I forgot the ground. And yes, I'm in the US.

So the fact that I'm splicing off doesn't add to the count at all? In what cases would it count?

And devices count only if they're mounted inside of the box (like a receptacle or switch/dimmer)? Does this mean a light fixture would also not count as a device?

Thanks guys! Darn, the box is just 18 cu inches, so I guess I'll have to replace it with a bigger one...
Thanks for the explanation!

I have one more question, though: When do I have to account for splicing? Only if the wire nut of the splice doesn't connect to anything like a pigtail or a device because it would be an additional wire nut in the box?
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