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Help with box fill calculation

#1
10-23-15, 02:05 PM
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Join Date: Jun 2015
Posts: 280
Help with box fill calculation

I was wondering if someone could help me with a box fill calculation. I have a 1-gang NM box (Carlon 18 cu. in.). There are 4 - 12 gauge cables that terminate in it. There is also a 3-way switch. I am concerned that it is overfilled. From my calculation, it appears that there should be at least a 22.5 cu. in. box. Is that correct? Do they even make a 1-gang box that large? I appreciate any input.

#2
10-23-15, 02:37 PM
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You have:
8 conductors entering and exiting (maybe more if any of those cables are 12-3)
1 conductor to represent all grounds
2 conductors to represent the device (switch)
-------
11 conductors * 2.25 cu/in for #12 wire = 24.75 cu in minimum box size

There may be some speciality boxes out there greater than 22.5, but that is the largest "deep" single gang box commonly available. I'd say you need a 2 gang box in that spot.

#3
10-23-15, 02:49 PM
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Join Date: Mar 2006
Location: USA
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If it is a new installation they make a two gang box where half is covered by the Sheetrock.

Edit: More research thanks to PCBoss shows the box I was refering to is only 17 cubic inches so... never mind.

Last edited by ray2047; 10-23-15 at 10:07 PM. Reason: Removed possibly incorrect information.
#4
10-23-15, 09:53 PM
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I believe the box ray linked to is a 17 inch box based on the part number.

#5
10-23-15, 09:58 PM
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Join Date: Mar 2006
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I believe the box ray linked to is a 17 inch box
You are correct. I missed that.

#6
10-23-15, 10:15 PM
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B 117 = 1 gang, 17 cubic inch.

#7
10-28-15, 05:58 AM
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Join Date: Mar 2014
Posts: 560
This program may be of help also for figuring box fill calculations.

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