# Thread: Nonmetallic box fill question

1. Senior Member
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Ok, since I am learning and this is one of the things I am learning, let’s see how bad I screw up..12 wire is 2.25... three gang box 44 cubic inches..lol...
grounds...2.25
clamps.....2.25
three yokes..3*2.25 or 6.75
so now left with 44-10.25=33.75 / 2.25... doing this in my head so.. 4 wires is 9 so 12 wires is 27, plus another two wires is 4.5 or 31.5 ... so can fit 7 lives and 7 neutrals into the box...

not using MWBCs..lol...

2. Originally Posted by Adamjamma
Ok, since I am learning and this is one of the things I am learning, let’s see how bad I screw up..12 wire is 2.25... three gang box 44 cubic inches..lol...
grounds...2.25
clamps.....2.25
three yokes..3*2.25 or 6.75
so now left with 44-10.25=33.75 / 2.25... doing this in my head so.. 4 wires is 9 so 12 wires is 27, plus another two wires is 4.5 or 31.5 ... so can fit 7 lives and 7 neutrals into the box...

not using MWBCs..lol...
Each yoke counts as two conductors. . .

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Ok..two not one. Thanks. So six conductors in a three gang for yokes, one conductor for all grounds and one conductor for all clamps, then figure out how much is left to figure number of conductors

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Originally Posted by al hildenbrand
The question, as written in the OP, does not say if the cable clamp is part of the manufactured box, or if the cable clamp is a snap in clamp in a K.O.
Are you talking about the flap that holds a cable that is pushed in to the box? If so, I would not consider this to be a clamp per 314.16(B)(2) Now if it is a brown box with actual clamps that is different.

5. Originally Posted by Strathead
Are you talking about the flap that holds a cable that is pushed in to the box? If so, I would not consider this to be a clamp per 314.16(B)(2) Now if it is a brown box with actual clamps that is different.
I agree with Jumper on this one. I can't see a way to avoid a clamp fill deduction when the clamp is factory molded into the wall of the box, because of one of the two meanings of "in the box". "Internal cable clamps" are factory made "in the box."

In the past, I've interpreted "internal" to mean inside the air volume of the box, which allows the box wall to neither be inside, or outside. But this language highlighted in red below doesn't allow me, in my opinion, to make that interpretation anymore.

"In the box" means BOTH in the material that is the box itself, AND the air volume inside the material that the box is made of, in my opinion.
2011 NEC 314.16(B)
(2) Clamp Fill.
Where one or more internal cable clamps, whether factory or field supplied, are present in the box, a single volume allowance in accordance with Table 314.16(B) shall be made based on the largest conductor present in the box. No allowance shall be required for a cable connector with its clamping mechanism outside the box.
2017 NEC 314.16(B)
(2) Clamp Fill.
Where one or more internal cable clamps, whether factory or field supplied, are present in the box, a single volume allowance in accordance with Table 314.16(B) shall be made based on the largest conductor present in the box. No allowance shall be required for a cable connector with its clamping mechanism outside the box.

A clamp assembly that incorporates a cable termination for the cable conductors shall be listed and marked for use with specific nonmetallic boxes. Conductors that originate within the clamp assembly shall be included in conductor fill calculations covered in 314.16(B)(1) as though they entered from outside the box. The clamp assembly shall not require a fill allowance, but the volume of the portion of the assembly that remains within the box after installation shall be excluded from the box volume as marked 314.16(A)(2)

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Originally Posted by al hildenbrand
I agree with Jumper on this one. I can't see a way to avoid a clamp fill deduction when the clamp is factory molded into the wall of the box, because of one of the two meanings of "in the box". "Internal cable clamps" are factory made "in the box."

In the past, I've interpreted "internal" to mean inside the air volume of the box, which allows the box wall to neither be inside, or outside. But this language highlighted in red below doesn't allow me, in my opinion, to make that interpretation anymore.

"In the box" means BOTH in the material that is the box itself, AND the air volume inside the material that the box is made of, in my opinion.
I don't consider those clamps.

7. Originally Posted by Strathead
I don't consider those clamps.
To me, if the box manufacturer describes the "flap that holds a cable that is pushed in to the box" (to quote your words) as a cable clamp, then, regardless of how badly I abuse the flap, the flap is a cable clamp.

8. That would be easy enough to determine by looking at a Carlon box with the conductor allowance molded in.

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So, Mr Hildenbrand, does section 2 actually say that if you are using a non metallic box, even if you are using cable clamps inside the box, they are not counted, but they have to be designed for use in the non metallic box? Thus, only metallic boxes using internal clamps lose conductor space for the clamps now?
At least, that is how I read that second section from the 2017 NEC...

10. Originally Posted by Adamjamma
So, Mr Hildenbrand, does section 2 actually say that if you are using a non metallic box, even if you are using cable clamps inside the box, they are not counted, but they have to be designed for use in the non metallic box? Thus, only metallic boxes using internal clamps lose conductor space for the clamps now?
At least, that is how I read that second section from the 2017 NEC...
The first sentence of the second paragraph in 2017 NEC 314.16(B)(2) that begins with: "A clamp assembly that incorporates a cable termination for the cable conductors shall be listed and marked for use with specific nonmetallic boxes." I believe this is describing a new "connector fitting" that is not just a "cable clamp" because the cable conductors terminate within the connector fitting.

The first appearance of this "second paragraph" language is in the 2014 NEC stemming from work that two Code Making Panels did crafting Code with respect to new product coming from manufacturers. I personally have not seen this new product.

So, that second paragraph doesn't apply, in my opinion, to plain old cable clamps and their clamp fill calculation.

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