Box Fill Calculations for sizes over #6 Awg?

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Mattula

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Table 314.16(B) lists sizes #18 - #6. I was wondering what the best way was to calculate for sizes above #6? I am splicing a 2/0 copper THHN feeder (3c +grnd) in a junction box. I also did not see any means of calculation for the bugs needed to make the mechanical connection.

I know that in table 5 (page 632) gives the square area for my wire size but how is the actual "free space" number derrived?

............Thanks Matt (LU.98 IBEW Philadelphia PA)
 
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Matt, let me see if I can help. :)

314.16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies. Boxes and conduit bodies shall be of sufficient size to provide free space for all enclosed conductors. In no case shall the volume of the box, as calculated in 314.16(A), be less than the fill calculation as calculated in 314.16(B). The minimum volume for conduit bodies shall be as calculated in 314.16(C). The provisions of this section shall not apply to terminal housings supplied with motors.

FPN: For volume requirements of motor terminal housings,
see 430.12.​

Boxes and conduit bodies enclosing conductors 4 AWG or larger shall also comply with the provisions of 314.28.​
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​
 
I read 314.28 A......So in my case:
2/0 (4 Conductor) MC cable being spliced in a metal box. My cable is running straight through the box. So I must multiply the diameter of my cable EIGHT times right. So do I multipy each individual wire's diameter (table 5) eight times .........then add them all together to find my boxes size in cubic inches?
 
Mattula said:
So do I multipy each individual wire's diameter (table 5) eight times .........then add them all together to find my boxes size in cubic inches?
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No , what is the trade size of the M/C.
1-1/2''?
2''?

If your not sure , what is the size of the connector?

Multiply that number bye 8 for your straight dimension.


ibew441dc
 
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Lets say its 2"......2"x 8=16. So 16 is my Volume Allowance for a single 2/0 copper conductor. Am I on the right path here?
 
There is no "box fill" calculation for #4 awg and larger. What you calculate is SOME of the dementions of your box. Cubic inch minimums are not involved here.
 
Alright John. So if using the calculation above 2" x 8=16".......16" would be the dimension of my box say a 16"/16" .......Right?
 
Mattula said:
Lets say its 2"......2"x 8=16. So 16 is my Volume Allowance for a single 2/0 copper conductor. Am I on the right path here?
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You are on the right track:)

Although,JohnJO906 is correct in his statement.

This was tricky for me early in my carrier, the way I remembered it was "straight times eight","Angle times six"


hope this helped

ibew441dc
 
Mattula said:
Alright John. So if using the calculation above 2" x 8=16".......16" would be the dimension of my box say a 16"/16" .......Right?
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Your almost there but , even though space is good you do not need that big of a box......
 
Matt, if you visit 2005 314.28(A) (1) or (2). Wire 4 or larger are treated by the size of conduit they are in and by the type of pull one makes at the box. i.e. straight, 90, or "U".

I hope this helps,

Richard
 
Mattula,


The wall that your 2" cable is entering in only has to be large enough to fit the connector. The 8times dimension only applies one direction, where the cable fitting enters and exits. You do not need to multiply Length,width,and height.

With a little practice you'll master this very important skill:)

ibew441dc
 
From this posting I have learned a few new things from everyones input. I know that when using the multiplier with trade size of an entering connector to size the box so pulling can be done with out damaging the cable.I am not pulling.......Im splicing. Splice takes up more room.....right. So I am try to find a definitive answer to my question so I can imperically calculate my boxes size. Spell it out for me here.
 
Mattula said:
I was wondering what the best way was to calculate for sizes above #6?
I am splicing a 2/0 copper THHN feeder (3c +grnd) in a junction box.
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First thing you would do is look at 314.16 ...at the end of the first section (before (A)) you'll see:
Boxes and conduit bodies enclosing conductors 4 AWG or larger shall also comply with the provisions of 314.28.
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314.28 tells us:
Boxes and conduit bodies used as pull or junction boxes shall comply with 314.28(A) through (D).
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314.28(A) tells us that the size we will use will be a MINIMUM value ~ you are always free to use larger.

Now we determine which direction the conduits enter/exit:
314.28(A)(1)
Straight Pulls. ...the length of the box shall not be less than eight times the metric designator (trade size) of the largest raceway.

314.28(A)(2)
Angle or U Pulls. Where splices or where angle or U pulls are made, the distance between each raceway entry inside the box and the opposite wall of the box shall not be less than six times the metric designator (trade size) of the largest raceway in a row. This distance shall be increased for additional entries by the amount of the sum of the diameters of all other raceway entries in the same row on the same wall of the box. Each row shall be calculated individually, and the single row that provides the maximum distance shall be used.

You have 3 #6 + an EGC in a conduit/raceway....
What size conduit/raceway are the wires in? Annex C has the proper sizing =?
What direction do the conduit/raceway enter/exit? 314.28(A)(1) or (B)(2) = ?
What size box do I need?

Therein lies the answer...Matt, post your answers - we'll double check'em for you


Mattula said:
I also did not see any means of calculation for the bugs needed to make the mechanical connection.
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You probably won't find anything in the NEC on that - other than 110.3(B).
My suggestion - [IILsco]
or you could just ask the counter-man for some #6 split-bolts...on the bolt will be the wire range...if memory serves me: #10-6 is one; and #6-2 another...READ the range to comply with 110.3(B) considering your application.
 
Mattula said:
I am not pulling.......Im splicing. Splice takes up more room.....right. So I am try to find a definitive answer to my question so I can imperically calculate my boxes size. Spell it out for me here.
Click to expand...

It's already done for you in the calculation on the box size in:
314.28(A)(2)
Angle or U Pulls. Where splices or where angle or U pulls are made
 
Mattula said:
Splice takes up more room.....right. So I am try to find a definitive answer to my question so I can imperically calculate my boxes size. Spell it out for me here.
Click to expand...

Mattula ,

I too can remember becoming frustrated.

314.28 Pull and Junction Boxes and Conduit Bodies

keywords : junction boxes

Last time I checked splices could be made in junction boxes. My point is that the NEC gives the minimum dimensions required.

If you feel you need more room get a bigger box.

The minimum you would need (taking lock-nuts into consideration) is 16''bye3''bye3'' .

Most likely a supplier would only have something like an 18''bye4''bye4'' if thats still not enough room go bigger , the important dimension, regardless of splice is the "straight time 8''

ibew441dc
 
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