Calculating Box Fill

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JFletcher

Senior Member
Location
Williamsburg, VA
I don't have my copy of the NEC nearby, but I need to know how to calculate box fill. From memory, each 14ga wire counts as 2cu in, each 12 as 2.25, all grounds count as 1, each 1 gang device as 2, internal clamps as 1, wire nuts and jumpers/pigtails are not counted. Is this basically correct?

For a 22 cu in j-box, would my max # of 14 awg cables (14/2 w/ ground NM) be 5?

5 hot, 5 neutral, for 10 wires x 2 cu in each, plus 1 for the grounds = 21cu in?

If it were 18cu in, then would that limit me to 4 14/2 cables? 8 conductors x 2 = 16 + 1 for grounds = 17.

I think this is right but I've seen plenty of 18 cu in boxes with 4 12/2 and a receptacle, so I'm not sure I'm doing the math right re: conductor counts.

If there is a good weblink for box calculations, tia for providing it.
 

ActionDave

Chief Moderator
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Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
For a 22 cu in j-box, would my max # of 14 awg cables (14/2 w/ ground NM) be 5?

5 hot, 5 neutral, for 10 wires x 2 cu in each, plus 1 for the grounds = 21cu in?
plus 1 x 2 for grounds = 22cu in.
If it were 18cu in, then would that limit me to 4 14/2 cables? 8 conductors x 2 = 16 + 1 for grounds = 17.
18
I think this is right but I've seen plenty of 18 cu in boxes with 4 12/2 and a receptacle, so I'm not sure I'm doing the math right re: conductor counts.
The boxes are overfilled. It gets overlooked a lot. You are doing the math right except you needed to times the egc by the volume allowance in table 314.16(B), it's one times two for 14AWG, same for device fill it's two times two.
If there is a good weblink for box calculations, tia for providing it.
There's gotta be an app for that.
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
If the boxes have receptacles then your 17 cu in is wrong. As Dave said ---2 for the equipment grounding conductor but then 2 more for the device
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
For a 22 cu in j-box, would my max # of 14 awg cables (14/2 w/ ground NM) be 5?

5 hot, 5 neutral, for 10 wires x 2 cu in each, plus 1 for the grounds = 21cu in?

If it were 18cu in, then would that limit me to 4 14/2 cables? 8 conductors x 2 = 16 + 1 for grounds = 17.

For the EGC's it counts as one conductor which is a 2 cubic inch in the calculation.
 

JFletcher

Senior Member
Location
Williamsburg, VA
ty for the replies. I knew something was off - the single ground counts as 2 cu in. Dennis, no receptacles here, just j-boxes. Seen a ton of single gangs with 5 and 6 14/2 NM and 4/5 12/2 NM this week, got to thinking about box fill, and yeah, none of that is right (none were 22+ ci).
 
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