Another box calculation question

[syakoban]

I've read a number of posts on box calculations and seen opinions on other sites, yet it's still not clear to me. I deal with residential construction utilizing circuits for switch boxes going to fixtures, etc.

For example: a switch box on a 3-way circuit that feeds another branch

14-2 enters the box as the supply
14-3 exits as 3-way circuit
14-2 exits to feed the other branch
2 internal cable clamps utilized (top & btm)
1 3-way switch

I understand the code as:

14-2 3 conductors (w/ground)
14-3 4 conductors (w/ground)
14-2 3 conductors (w/ground)
2 clamps 2 conductors
1 switch 1 conductor

TOTAL 13 conductors or 26 cubic inches (2 cubic inches for each 14 gage conductor)

I've also seen interpretations as all grounds counting as 1 conductor total, yielding 11 conductors or 22 cubic inches. And opinions that the switch should count as 2.

Which is right? Am I missing something else?

Also, if only the supply was 12-2 would all conductors be calculated as 12 gage or would just the supply be calculated as 12 and the rest 14?

Thanks for the clarification!

[ December 29, 2004, 02:40 AM: Message edited by: syakoban ]

 

[physis]

Re: Another box calculation question

I think you already know enough to answer the question yourself. And if not, the process of figuring out how is more valuable than the answer.

You gotta hate that

 

[jimwalker]

Re: Another box calculation question

"TOTAL 13 conductors or 26 cubic inches" Get some sleep then try again.

 

[roger]

Re: Another box calculation question

See 314.16(B)(5) one EGC for your example.

Roger

 

[jimwalker]

Re: Another box calculation question

also all the grounds count as 1 so enter 2 cubes and device counts as 2 conductors

[ December 29, 2004, 06:07 AM: Message edited by: jimwalker ]

 

[bphgravity]

Re: Another box calculation question

What I like to do when performing box fill calculations is to list out the 5 required counts per 314.16(B) and then find the total. This way I know I don't miss anything, and I can ensure I apply the correct volume to each item.

So for your example,

1. Conductors = 7 x 2 = 14 in?

2. Clamps = 1 x 2 = 2 in?

3. Supports = N/A

4. Devices = 2 x 2 = 4 in?

5. EGC = 1 x 2 = 2 in?

Total = 22 in?

 

[bthielen]

Re: Another box calculation question

I think you already know enough to answer the question yourself. And if not, the process of figuring out how is more valuable than the answer.

I agree. It's not about code memorization. It's about code interpretation and understanding.

Bob

 

[syakoban]

Re: Another box calculation question

Thanks for the replies, but I don't "already know the answer" and am trying to understand the principle of the code.

What I'm gathering is that each device (switch, recepticle, etc.) counts as 2 conductors and that all grounds, no matter 1 or whatever, count as 1 conductor. Is that right?

Also, 2.0 is the multiplyer for 14 gage and 2.25 for 12, but what if there is both 12 and 14 in the box - do they use their respective multipliers or do they all use 2.25?

Thanks!

 

[roger]

Re: Another box calculation question

Do you have a code book? All of your questions are answered in article 314.16.

Each conductor will be caculated at it's "in?" per table 314.16(B)


Roger

 

[apauling]

Re: Another box calculation question

to each its own, not driven by largest wire

paul

 

[guesseral]

Re: Another box calculation question

Count the #14 as 2 count the #12 as 2.25, then if the #12 is present the grd allowance would be 2.25, if the #12 is connected to the device them the device would be 2.25x2. This as stated above is all spelled out in 314.16B follow steps 1/5 piece of cake!

Sorry forgot if the #12 is present the internal strap allowance would 2.25 also oops!

[ December 29, 2004, 12:38 PM: Message edited by: guesseral ]

 

[syakoban]

Re: Another box calculation question

Thanks all - I have a better understanding now.

I just looked at an actual example we're facing and although I beleive I understand the math, don't understand the practicality.

We are replacing a section of a room with outlets that are looped to receive power and then feed another, and so on, all around the room. It's a 20A circuit so it's wired with 12-2. Some feeds come in the bottom of boxes and exit the tops to feed the next, utilizing 2 clamps as opposed to 1 where both exit top. Calculating I come up with (2) 12-2 (2) clamps (1) outlet for 9 conductors*2.25 or 20.25 Cu. in. All the old boxes were only 2.5" deep. I spec'd. 3-1/2" (18 Cu. in.) Raco switch boxes.

According to code calculations, would we have to spec. a 4" box w/a mud ring just to put outlets in? I have never seen that. Is there some kind of exception for recepticles?

Thanks again!

 

[guesseral]

Re: Another box calculation question

8 volume allowance read 314.16A (2) closer,18cu. in. use the device box you have. But read the section closer and slower!!!

 

[bphgravity]

Re: Another box calculation question

Clamps only count once no matter how many clamps in the box. Are these non-metallic boxes with the push-in style clamps or metal boxes with actual internal cable clamps? I never count the clamps on plastic boxes.

4 x 2.25 = 9.00 for the conductors
1 x 2.25 = 2.25 for the clamps
2 x 2.25 = 4.50 for the device
1 x 2.25 = 2.25 for the egc's

Total = 18 in? or a standard single gang plastic box.

[ December 29, 2004, 05:40 PM: Message edited by: bphgravity ]

 

[hbiss]

Re: Another box calculation question

Clamps only count once no matter how many clamps in the box.

You sure about that?

-Hal

 

[bphgravity]

Re: Another box calculation question

Yes, very sure.

314.16(B)(2) "Where one or more internal cable clamps..."

 

[George Stolz]

Re: Another box calculation question

Originally posted by bphgravity:
I never count the clamps on plastic boxes.

I'm glad you said it before me. That's something that always seems to go unsaid.

What about fiberglass with plastic "K" clamps?

 

[kendog]

Re: Another box calculation question

If 12 was introduced to the box you would have to count the conductors as 2.25 cu in and the ground would then be counted as 2.25 cu in.

 

[George Stolz]

Re: Another box calculation question

Simma down now.

The 12's are counted as 2.25. The ground would be 2.25 if 12 was the biggest. What does that first part mean, Ken?

Pace yourself, Ken. Welcome to the forum.

 

[kendog]

Re: Another box calculation question

he asked about if 12 wire was added to the box if it would make a difference. I should have worded it a little better. I must be getting tired. LOL