Junction Box Design (NEC 314.71)

[posttoasties]

I'm trying to design a junction box and I am having difficulty interpreting NEC 314.71 for the sizing requirements. I have a straight pull through the junction box and it states that the length of the box needs to be 48 times the outside diameter, over sheath, of the largest shielded conductor or cable. I am running a single 3-conductor 5kV shielded cable into and out of the junction box. The O.D. of the cable is 2", the O.D. of the sheath over each conductor is 0.703”. So do I need to make the box 48*2=96” long or 48*0.703=34” long. Any insight into this would be greatly appreciated.

 

[davidbeach]

The largest outside diameter, so if your 3-conductor cable is 2" OD, your 96" dimension is correct. If you don't want your name used in vane by those who have to pull the cable in you won't make it the minimum dimension either.

 

[Cajun62234]

Junction box sizing clarification.
Here's my dilemma:
Extending 2 sets of 3#350kcil, 1#1G, 3½"C. via Junction Box (and compression fittings); straight through, no bends.

Handbook NEC 314.28(A)(1) states that 314.28(A)(2) applies if splicing in a straight pull; thus allowing 6x91mm (3½" = 91mm [trade size]) between entry and opposite wall. 6x91mm = 546mm ÷ 25.4 (mm/inches) = 21.496". This gives me the distance from conduit entry to opposite wall, but where are the requirements for volume allowances of the 6#350kcmil & 2#1 conductors; minimum width & depth of the junction box? Row-spacing of the 3½" conduits?? I know the answer is buried in the NEC somewhere, just can't find it...

 

[resqcapt19]

There are no volume requirements when you are above #6 in the NEC. I know that 314.16 says that for conductors #4 and larger you have to comply with both 314.28 and 314.16, but the code provides no guidance on how to calculate a volume for conductors larger than #6.
As far as the row spacing, there is no rule...just the practical one that requires you to have enough room to install the required locknuts and bushings.
Don

 

[Cajun62234]

Thanks...your reply is exactly what I finally concluded after beating the NEC to death to find an answer...

Thanks again.

 

[WAI1230]

Hello, As far as junction box design goes, is there any where in the NEC that states mounting height requirements? This question is for an outdoor junction box mounted on stainless steel unistrut.

Insight appreciated.

 

[dpc]

If it is strictly a junction box, there is no restriction on mounting height, although it does generally have to be accessible (but not "readily accessible".

 

[WAI1230]

Thanks dpc.