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Old 11-16-2010, 05:34 PM   #1
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Box fill questions

Single gang plastic boxes don't have any built-in clamps but multi-gang do along with ceiling boxes. When calculating box fill, do you count those integrated clamps or is that figured into the wire count that is embossed in the box?
On ceiling boxes, if the fixture has a dome and less than four wires and a ground, you don't have to count this. Just out of curiosity, if I attach a porcelain fixture, you know those things that hold one light bulb, do I have to allow for it?


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Old 11-17-2010, 07:34 AM   #2
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I will go out on a limb and say that if wire counts are embossed inside the box then you don't have to subtract one for the built in springy fingers.

For a flat back fixture including a porcelain fixture, the box count itself is used and the fixture counts zero points itself. If there is a dome (canopy) where the fixture attaches to the box then you have a few more cubic inches and the fixture counts zero points. Normally very shallow boxes (called pancake boxes) do not alow you to have a cable coming in, wire nutting to the fixture, and another cable daisy chaining on to something else.


The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.

Last edited by AllanJ; 11-17-2010 at 07:42 AM.
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Old 11-17-2010, 08:34 AM   #3
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From the Carlon FAQs.

All your blue boxes come stamped with a wire volume and corresponding
conductor count. Do the volume and/or wire count account for the integral
clamps that are present in many of your boxes?
For instance, your B520A-UPC ceiling box has a 20 cubic inch volume and
therefore can receive ten (10) #14 gauge wires per the NEC. However, it
also has six integral clamps. Per the Article 370-16(b)(2), 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 370-16(b)
shall be made based on the largest conductor present in the box.
Therefore, does the installer need to take into account a volume reduction?
Is the true available volume in the example above really 18 assuming
the use of #14 gauge wire?
Other boxes, such as some of your single gang models, have knock-outs in
lieu of integral clamps; so, the actual wire count is not an issue.
The volume inside an outlet box does not include any deductions for integral
clamps. The clamps are removed from the box when the volume is determined
by UL. Therefore a single deduction in the NEC is required for the space the
clamps take up inside the box. A B520A is 18 cubic inches after the clamp
deduction for #14 wire.
This procedure was determined by IAEI and UL. Not everybody agrees to this.
The 2002 NEC (314.16(B)(2)) still has the deduction requirement. Keep an eye
on the 2005 NEC because it may be changed.
Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.
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Old 11-17-2010, 11:19 AM   #4
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Now it makes sense why 18.0 cu in and 22.5 cu in are the common sizes for outlet boxes. Two cables (12-2 & 12-3) and a receptacle or three cables (two 12-2 & 12-3) and a receptacle.
I think I'd better do some counting on my new house b4 you know who comes out next Tuesday
Update, I redid the above numbers.

Last edited by a7ecorsair; 11-17-2010 at 12:37 PM.
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