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Old 02-17-2013, 05:28 PM   #1
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Box size calculation question


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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Old 02-17-2013, 05:46 PM   #2
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Box size calculation question


device count is 0.

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Old 02-17-2013, 05:52 PM   #3
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Box size calculation question


9 current carrying conductors plus 1 ground; total 10

2 cu. in. per conductor times 10 conductors is 20 cu in.

This assumes you are in the US.

You have no device in the box. Pigtails do not count. All ground equal 1 conductor.
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Old 02-17-2013, 08:15 PM   #4
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Box size calculation question


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...
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Old 02-17-2013, 11:14 PM   #5
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Box size calculation question


Wiring devices on a mounting strap, such as is common with wall switches and receptacles count as 2 conductors. Otherwise, fixtures and such do not require any deduction.
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Old 02-18-2013, 08:28 AM   #6
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Box size calculation question


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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