Box Fill

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evandriel

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Hi,

I'm working on getting my license and have a real simple question that I don't see answered anywhere in the 2002 Code or Handbook.

What is the formula used to calculate box volume? I had always believed Cu In volume was LxWxH but this is apparently not the case. I've asked the few gentlemen I know that have passed the exam and they just said pretty much the same thing, "Don't worry about it, it's all listed in Table 314.16".

Yeah, well that doesn't give me the 'why'. Any help would be appreciated.
 
Re: Box Fill

The way I look at this is a (3x2x2) box would be 12 in. cubed, the table will say 10.0 cu. In
and the reason could be that you will never be putting conductors in the corners, or square to speak of.
I am guessing in the design they have compensated for this as conductors will be on a radius when tucked in a box and not be using the corner areas.

Again, this is just now I see it.......... have nothing to back this up other than what I have said and will not mind a better explanation when someone has one.

John
 
Re: Box Fill

Some adjustment is likely being made for the thickness of the metal and KO's that are dimpled into the interior space.

Years ago when boxes weren't quite as flimsy and made from materiel that was close to 1/16", the metal on the back on a 4-square would subtract a full cubic inch from the totals (1/16" * (4 * 4)). If its a 2-1/4" deep 4-square, then there's another 2+ cubic inches gone due to the metal on the sides.

Even with all this, the table values do sometimes seem a bit light compared to reality...
 
Re: Box Fill

A square cornered 2-1/4" deep 4-square I've got sitting here is dead on 4" outside dimension on one side and maybe 1/32" shorter on the other side. Depth is about 1/32" shallower than 2-1/4 on the exterior. Materiel thickness on this one isn't too bad - close to an honest 1/16" (I haven't had any trouble with stripping out gnd screws on this these like I've had on some of the single gang nail ons that seem flimsier)

When I fled NY a couple of days ago right ahead of the deep freeze it was 20 degrees and didn't go over 30 till I hit Jacksonville. Its pretty cool tonight in WPB, maybe in the low 70's this afternoon.

[ January 09, 2004, 11:26 PM: Message edited by: tonyi ]
 
Re: Box Fill

The real cubes would be of course the inside.I was told once they plug the holes and fill the box with liquid to determine real cubes.No idea if that is bull or not
 
Re: Box Fill

I always assumed the listed area was lower than actual area because they included a percentage for terminations. Wires dont attatch to each other without some device, be it a SAK, Klippon or other device. (Sorry for the english terms, dunno what you equate SAKs to over here)

That was always my personal take on why box area's were smaller than the apparent size /shrug.
 
Re: Box Fill

The ones that don't count are the ones that start and end in the box (pigtails, gnd wires from box to devices and such)
 
Re: Box Fill

Jim
We are all rusty on some subjects, but when replying here maybe you could take a quick look at the codebook to refresh yourself. You do not want to teach someone how to do something without knowing the correct answers - that is how misinformation gets perpetuated in this industry.

Heres a question: How many 12/2 NM cables can you install to a 3x2x2 metal box using an NM connector to fasten the NM to the box, there is a duplex receptacle being supplied by the 12/2?
How many 12/2 NM conductors can you install to a 3x2x2 metal box using connectors, the conductors are spliced but there is no device.

Pierre
 
Re: Box Fill

Gonna take a shot at this:

For #1: Only 1 unless the 12/2 is w/ ground then you're over by 1.

For #2: 2 but again, if 12/2 w/ground then you're over by 1.
 
Re: Box Fill

I used to carry a reduced copy of table 314.16(A) in my wallet back when I was doing this regularly.

...I mostly do residential,so am very rust on things like this. Why? :confused: Don't you use boxes in residential work?

Anyway, you are allowed 4 #12's in a 3x2x2" box. You have 3 from the NM, 1 device and no clamp so that equals 4.

Without the device you could only add 1 #12 so for practical purposes one 12/2 NM is the limit with or without a device.

If you were using 12/2 AC you could use the box as a J box for two cables with a blank cover.

[ January 27, 2004, 05:17 PM: Message edited by: hbiss ]
 
Re: Box Fill

hbiss: I believe you have to count the two screws for the device so you'd be at 5, not 4. Don't have my book handy for a better explanation.

[ January 27, 2004, 06:33 PM: Message edited by: evandriel ]
 
Re: Box Fill

A device counts as two (2) conductors. A 3X2X2 box has a cubic volume of 10 as per table 314.16(A). 12 AWG has a volume of 2.25 cubic inches as per table 314.16(B).

12/2 NM cable has three (3) conductors, so the device and 3 conductors equals 5 conductors times 2.25 which equals ll.25 cubic inches. This would require a larger box - such as 3X2X21/2

for the splice example, the equipment ground conductor is counted once, so you have 5 conductors times 2.25 which equals 11.25 cubic inches. Too much for a 3X2X2 box.

Of course you could probably cram about 5 12/2 NM cables in there - at least I have seen that :D

Pierre
 
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