Sizing large J-box containing distribution blocks

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sevlander

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We have a situation requiring junctioning of (6) sets (3) 350Kcmil parallel conductors to (3)sets of 600Kcmil parallel conductors in a single box. Terminal blocks are to used to connect the conductors within the box. All conduits will enter form the rear of the box opposite the removable cover. We are good with sizing the length and width.

My question is regarding how to size the the depth of the box from the conduit entry to the removable since different size conductors are in use. I believe I should be using table 312.6A to size this, but am not sure which column to use being we are mixing different size conductors. Can someone one give me some input on sizing this box?

Also, what calculation is needed to add the distribution style termination blocks we plan to use to splice the conductors.I am assuming the cubic inch dimensions of the blocks should just get added to the box size, but need confirmation.

Thanks!
 
We were not sure there even were any blocks suitable for this application. It was just an initial thought on cleaning up the splice box. It does not appear that there is any such animal available for our application. I would still like to know how to add them to a box calculation for possible future applications that could except them.

Even with another type of splicing device in place of the block, I need clarification on sizing the depth of box per my original question. Thanks
 
sevlander said:
...

Even with another type of splicing device in place of the block, I need clarification on sizing the depth of box per my original question. Thanks
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314.28 Exception requires not less than the distance of one wire per terminal using Table 312.6(A). That amounts to using the largest size wire to make the determination.

With 600kcmil conductors being the largest, the depth is required to be not less than 8". With the number of sets you have in your application, you'll likely need either a deeper box or one with substantially greater width and/or height than required.
 
Smart $ said:
314.28 Exception requires not less than the distance of one wire per terminal using Table 312.6(A). That amounts to using the largest size wire to make the determination.

With 600kcmil conductors being the largest, the depth is required to be not less than 8". With the number of sets you have in your application, you'll likely need either a deeper box or one with substantially greater width and/or height than required.
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I'm not sure I agree...I read the table as needing 12" minimum based on the fact that this is a triple parallel run of 600Kcmil at 1200amps. If I understand it properly, the 3 wires per terminal column is what we should be using for the parallel run of 3 conductors. thanks
 
sevlander said:
I'm not sure I agree...I read the table as needing 12" minimum based on the fact that this is a triple parallel run of 600Kcmil at 1200amps. If I understand it properly, the 3 wires per terminal column is what we should be using for the parallel run of 3 conductors. thanks
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Refer to the picture posted by Bob (petersonra). If you connected three conductors to one stud, you would have three conductors per terminal... but as pictured, it is only one conductor per terminal. That said, 314.28 Exception doesn't require you to use the three conductors per terminal column even if you have three conductors per terminal.
 
Smart $ said:
Refer to the picture posted by Bob (petersonra). If you connected three conductors to one stud, you would have three conductors per terminal... but as pictured*, it is only one conductor per terminal. That said, 314.28 Exception doesn't require you to use the three conductors per terminal column even if you have three conductors per terminal.
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*Except for the two conductors on one stud, lower left.
 
Smart $ said:
Refer to the picture posted by Bob (petersonra). If you connected three conductors to one stud, you would have three conductors per terminal... but as pictured, it is only one conductor per terminal. That said, 314.28 Exception doesn't require you to use the three conductors per terminal column even if you have three conductors per terminal.
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I'm still not sure that makes sense to me. What are the "wires per terminal" used for in table 312.6A? I would say if you had for example an 1200A breaker with lugs able to accept (3) 600Kcmil conductors you would need to go to the "3 wires per terminals" for sizing? I would think the same would apply to a terminal block.
 
sevlander said:
I'm still not sure that makes sense to me. What are the "wires per terminal" used for in table 312.6A? I would say if you had for example an 1200A breaker with lugs able to accept (3) 600Kcmil conductors you would need to go to the "3 wires per terminals" for sizing? I would think the same would apply to a terminal block.
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IIRC, since I'm not looking it up, the wires per terminal under that Table specify the minimum distance from the terminal to the enclosure wall opposing the wire-entry direction to the terminal. Essentially it is a wire bending space allowance... because the Code has no restriction on bending of non-cabled conductors other than this. However, note it is in an entirely different Article. Only the stipulations of the referring Article applies, and it states to use only the one-wire per terminal distance... but note this is a minimum distance requirement. You are not prohibited from having a greater distance.
 
Smart $ said:
IIRC, since I'm not looking it up, the wires per terminal under that Table specify the minimum distance from the terminal to the enclosure wall opposing the wire-entry direction to the terminal. Essentially it is a wire bending space allowance... because the Code has no restriction on bending of non-cabled conductors other than this. However, note it is in an entirely different Article. Only the stipulations of the referring Article applies, and it states to use only the one-wire per terminal distance... but note this is a minimum distance requirement. You are not prohibited from having a greater distance.
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OK, thanks, I think you finally got through my thick skull.Guess I have been doing this wrong for a while...at least my way I was over-sizing the minimum requirement, rather then under-sizing. Thanks!
 
sevlander said:
OK, thanks, I think you finally got through my thick skull.Guess I have been doing this wrong for a while...at least my way I was over-sizing the minimum requirement, rather then under-sizing. Thanks!
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Now that we got that part out of the way...

Table 312.6 will apply if you use power distribution blocks (or terminal buses) in the JB... but that typically involves the side-to-side dimensions, not depth [314.28(E)(3)].
 
Depth is rarely an issue, except for clearances to grounded surfaces.
Consider of a loadcenter, compare how much bending space is provided for the incoming terminations versus the depth which is often only 3.5".
 
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