Article 314 pull box sizing question

[mshields]

I'm designing a vertical riser to run approximately 15 floors. This will consist of 3 inch EMT with pull boxes on each floor. So there will be a 3 inch on the bottom, a 3 inch comming out the top and in addition on each floor there will be 3/4 inch LB comming off top or bottom to take a single circuit over to the IDF room. Conductors in the 3 inch pipe will be no larger than #6. So 2 questions:

1. 314.28(A)(1) only applies if you are using 4 AWG or larger. What then applies when you've got smaller conductors.
2. I assume the above referenced section will give me an appropriately sized box (if perhaps over kill) as far as the length (or in this case height) is concerned, but what about the dimensions of the sides with the conduit comming into it. I assume an 8 by 8 would be ample, but what is the rule. I need to make these boxes as small as possible because of a shortage of space.

Thanks,

Mike

 

[jumper]

Will this work?

314.28(A)(3) Smaller Dimensions. Boxes or conduit bodies of dimensions
less than those required in 314.28(A)(1) and
(A)(2) shall be permitted for installations of combinations
of conductors that are less than the maximum conduit or
tubing fill (of conduits or tubing being used) permitted by
Table 1 of Chapter 9, provided the box or conduit body has
been listed for, and is permanently marked with, the maximum
number and maximum size of conductors permitted.

 

[Dennis Alwon]

I think as long as your fill is calculated there should not be an issue. Do you have de-rating figured in? I assume there will be many conductors in that 3" conduit. Would a few smaller conduits be better for that problem of de-rating?

 

[don_resqcapt19]

The only sizing rules that apply are for the volume of the conductors that will be in the box.
Do most of the conductors pass straight through? If some go all the way to the top of the building, there may be an issue with 300.19.

As far as the box size, as an installer I would want it at least 16 to 18" tall. A smaller box with a large number of conductors it harder to work with and there is more risk of conductor damage as you pull then in and out of the box.