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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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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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