Electrical - box fill question

Started by Jeff922, May 18, 2011, 06:43:29 AM

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Jeff922

I have a good understanding on how to do box fill calculations, but there is one small detail I can't get a straight answer on.  When using NM (plastic) boxes, are the integral cable clamps (the little doorway-like punch through openings that hinge on one side) considered to be "internal cable clamps"?  I don't know if they are even considered "clamps" at all.  They make pulling NM cable back out very difficult.  Or are they just considered to be a "knock-out" (hence the staple within 8" rule)?  Perhaps this is left up to the AHJ.  What do you guys think?
"They don't grow trees so close together that you can't ski between them"

dug

The tabs on the plastic boxes are supposed to secure the wire so staples are not required within 8 inches, but I always staple the wire anyway.


Jeff922

But does that mean they are considered "internal clamps"?  If so that adds 2.25 cu.in. volume to each box.  Not a big deal, but a 18cu.in. box is about 25 cents and a 20.3 cu.in. is like 43 cents.  I need to know which to buy...
"They don't grow trees so close together that you can't ski between them"

MountainDon

The local inspector here told me that he did not consider those push thru things to be clamps and simply used the boxes rated size. But boxes that had screw operated clamp devices inside did need to have that accounted for. That may be one of those things that varies from inspector to inspector. I have no idea what would be considered "correct".
Just because something has been done and has not failed, doesn't mean it is good design.

MountainDon

Ammendment to the above...

Thinking back some more ....  I believe he said something like the integral molded in poke thru things were not clamps because he had seen many that were broken when wired and he required a staple within 8 inches outside the box with that box type. But a box with a screw driver operated clamp did not need the staple right close by but also needed to have the volume correction applied. Sounded very much like a personal matter as I think back. No help in the present case I suppose.   

I always like to use the smallest box because I can get more insulation behind it.
Just because something has been done and has not failed, doesn't mean it is good design.


Jeff922

Thanks Don, I think this is just one of those things that is left up to the AHJ to decide.  Your comment about insulation got me thinking; what's the best way to prevent air infiltration at my electrical boxes?  I know they sell those little gaskets that go between the cover plate and box, but it seems like keeping cold air out of the box to begin with would be preferable...hummm...
"They don't grow trees so close together that you can't ski between them"

Alan Gage

http://www.lessco-airtight.com/

I know nothing about them but hear them mentioned often.

Alan

Native_NM

Roofing supply companies will sell you a small, one-time spray insulation system (icylene or similar) pretty cheap. They have a built in spray nozzle. Builders are using them in conjunction with traditional insulation to meet the Energy Star criteria.  Spray foam for the whole house is expensive.  Most of the heat loss/gain occurs in areas like electrical that are hard to insulate with batts.  They spray the foam in those areas, completely sealing them, and use the batts in the large areas.  It does not add that much to the total cost, but greatly increases efficiency.

New Mexico.  Better than regular Mexico.

Jeff922

Thanks, I'll check that out.  I would assume the spray foam must be listed for direct contact with NM cable.  One time at my current home during a wickid cold Maine wintah, a few of my receptacles iced up on the interior of my house.  Yikes! :o
"They don't grow trees so close together that you can't ski between them"


Native_NM

The foam is rated to spray over wire, plumbing, etc.  There are several brands and trade names, but most are similar chemistry.
New Mexico.  Better than regular Mexico.