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Best explantion I have been completey confused with these calculation .thanks I wish you could explain some other complicated parts of NEC .thanks

What size box would I need if I have (3) 4" conduits coming in the back of the box and (3) coming out of the top of the box. Each conduit has (4) 500 MCM that will be spliced in the box. Is 36"x36"x24" adequate?

No, it would need to be at least 32 inches deep or 32x32x32. The calc is 6*4=24, (6x the largest size conduit), then take the product of the largest conduit plus the sum of the remaining pipes, 24+4+4=32. Your height and width was ok, but your depth was inadequate because you entered the box from the back.

what size juction box do i need ?

2-4 inch on top

4-4 inch on side

with 4- 4/0 in each conduit

I am assuming you have a u-pull on the side with 4 pipes, but for purposes of our calculation, that won't matter. We use the side with 4 conduits in it. We take the largest conduit and multiply by 6, 6x4=24. Then add the sum of the remaining conduits to this product, 24+4+4+4=36. So your box would be 36x36 or larger.

I'm a bit confused. I have a similar situation:

(6)4", 2 rows of 3, entering at the bottom of the box and exiting out of the back. I come up with 6x4=24+3+3=32 also. So the box would need to be 32"H x 32"W and 32" Deep. Would you calculate the same?

what size of an enclosure is needed for a straight through 4" entering and 4" exiting with 4- 500 mcm"s going to terminal blocks?

A box has two 1 inch pipes with #4s doing an angle pull plus other 1 inch pipes with #10s and smaller pulling through it. Some of the smaller wies are spliced in the box. Do I just do the pipe calculations to get my box size or is there a box fill calculation involved here too?

I have 2 4in conduits and 2 2 1/2 entering the bottom of a pull box 2 4in go out the top a straight pull 4x8=32 one 21/2 goes left the other goes right do I take 21/2x6+4+4+21/2 to get my other side

I have two 6 inch wireways with removable covers entering the top and leaving the bottom of a junction box. Do I need to size the box based on 8 times rule?

Open bottom handholes...With conduit(s) coming up from bottom, do the two conduit feeding conductors through the handhole (one in, one out. both in the bottom (U pull)) require the 6 multiplier distance between the conduits rising into the box?

How would you size a junction box w/3-3" conduits on top, 8-2" on left side, 2-2" on right side, and 2-3" on bottom?

I have 15-4" conduits & 1-2" conduit entering the back and the same entering the bottom of the can. 82"x82"x?

The third dimension of the enclosure is never discussed when there are only two sides being used.

The above examples are very good and straight forward, however, all the given examples except the anonymous (04/24/13) do not talk about actual depth calculations. I think this is a very important item to be discussed - especially when it comes to sizing handholes and manholes.

Please show us some examples. I do some underground distribution LV and MV design projects and would find this very helpful.

Thank you!