Sizing Junction Box

[teamextreme]

Wondering if someone can double check me. I have to source a j-box for splicing wires for a DC battery system. I'll be installing an Ilsco power distribution block in it. There are 4 conduits entering the box, three 1.5" and one 2", each with 2x 4/0 1#4G. I don't know how the conduits will end up entering the box, so I want to take worst case scenario, which should be 8x the largest conduit size. So I came up with a 16"x16"x8" enclosure. Is there any scenario I'm missing where this would be too small?

 

[Terry D]

If Im reading this right, your putting the same size and number of conductors in each conduit. In that case, the 1.5 conduits would be to small. You can only put up to 2 4/0 in 1.5 conduit, by putting the # 4 in there, you would exceed your 40% fill capacity. All conduits need to be 2". To size it correctly, you need to know how the conduits are coming in the box. Plus you terminating everything to a distribution block. You have to be concerned with bending radius also. I would go with a much bigger box, possibly a 24x24 or larger. Remember the NEC is not a how to book, it is the bare minimum requirements for electrical installations. Make it easy on yourself, you have a lot going on in that box. Another option would be to use the Ilsco connectors instead of the block, and install a ground bar in there

https://www.eslsupply.com/ilsco-pbt...7797&CATARGETID=120164090000054918&CADEVICE=c

 

[teamextreme]

Oh, interesting. The conduit sizes are all speced by the engineer on the contract drawings. I didn't even bother to double check that. I'll have to bring that up as an issue. In terms of box size, my 16 inches should be ok for worst case scenario with 2", but I need to consider cable bend radius, correct? I'll have to look up NEC on that topic, never dealt with that before.

 

[Terry D]

Oh, interesting. The conduit sizes are all speced by the engineer on the contract drawings. I didn't even bother to double check that. I'll have to bring that up as an issue. In terms of box size, my 16 inches should be ok for worst case scenario with 2", but I need to consider cable bend radius, correct? I'll have to look up NEC on that topic, never dealt with that before.

I still am going to say that a 16 x16 is to small. what is the physical size of the block you are installing. Remember, that takes away from space for wiring inside the box. You really need to know how the conduits are going to be laid out. Or you need to lay them out to make terminating easy. I assume everything terminates in the box, nothing is feed thru. Look at the space above the main breaker in a 200 amp panel, and that's just for (3) 4/0

 

[pattenp]

http://www.nooutage.com/images/JBterminations.gif

 

[alfredeneuman]

2017NEC 314.28 (A)
(2) Angle or U Pulls, or Splices. Where splices or where angle or U pulls are made, the distance between each raceway entry inside the box or conduit body and the opposite wall of the box or conduit body 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.

 

[teamextreme]

Looking at 312.6B it looks like I need 7" of wire bending space. Since I don't know how the electricians will be routing conduit, worst case I need 7" on either side, plus the longest dimension of my PDB is 8.5". So that totals to 22.5".

As alfred pointed out, I need to also add the dimensions of the additional conduits, so that would be (8 x 2") + 2" + 2" (worst case if they increase to 2"). That would be 20" square. I think I'll go with a 24"x24" as suggested. That should be large enough to cover both wire bending space and conduit/pull dimensions. Whatcha think?

 

[alfredeneuman]

As alfred pointed out, I need to also add the dimensions of the additional conduits, so that would be (8 x 2") + 2" + 2" (worst case if they increase to 2"). That would be 20" square.

Read it again
(6 x 2") + 2" + 2" = 16"

What happened the 4th conduit? (three 1.5" and one 2")

 

[teamextreme]

6 was one mulitplier, but somewhere else in that NEC section was a requirement for 8x depending on the conduit orientation. So I'm going with worst-case. Sorry, I dropped a conduit. There are 2 of these, one has 3 conduits, the other has 4. I'm making them both the same, so accounting for worst-case of 4 conduits, and assuming they up-size to 2". So that math should be (8 x 2") + 2" + 2" + 2" = 22"

 

[alfredeneuman]

but somewhere else in that NEC section was a requirement for 8x depending on the conduit orientation.

314.28 (A)
(1) Straight Pulls. In straight pulls, the length of the box or conduit body shall not be less than eight times the metric designator (trade size) of the largest raceway.

8 times is the multiplier for straight through connections, but because of the splices does not apply here.

(EDIT: The correct measurement is 18")

 

[Terry D]

You guys are killing me going back and forth over a few inches. Put a 24 x 24 in and be done with it. Code states the minimum, there is nothing wrong with going a little bigger. You or whoever is installing will thank you. What doesn't make sense if you are putting the same size and same amount of conductors in all the conduits, why are they not the same size. Has anyone done a calculation to see if you could put (2) 4/0 and (1) #4 in a 1.5 conduit, if so, then why is one of them 2". Your putting some good size wire in there to be bending around to land on a distribution block. I don't think you mentioned if it is aluminum or copper. Copper is much harder to bend verses aluminum. What is the size of the bock, how are the conduits configured, for all you know, they could all be on the same side, all this needs to taken in consideration.

 

[teamextreme]

You guys are killing me going back and forth over a few inches. Put a 24 x 24 in and be done with it. Code states the minimum, there is nothing wrong with going a little bigger. You or whoever is installing will thank you. What doesn't make sense if you are putting the same size and same amount of conductors in all the conduits, why are they not the same size. Has anyone done a calculation to see if you could put (2) 4/0 and (1) #4 in a 1.5 conduit, if so, then why is one of them 2". Your putting some good size wire in there to be bending around to land on a distribution block. I don't think you mentioned if it is aluminum or copper. Copper is much harder to bend verses aluminum. What is the size of the bock, how are the conduits configured, for all you know, they could all be on the same side, all this needs to taken in consideration.

Agreed on all counts. I'm going with the 24" x 24". Engineer determined conduit sizes so I'm not sure why one is 2" and the others are 1.5", but I suspect it might be a longer run. I double checked and you're right though, 1.5" isn't big enough. I'm going to bring that to everyone's attention. PDB is 8.5" x 5.5". Wires are cu. Again, I don't know what the conduit config will be, that is why I want to consider worst-case scenario (including upsizing conduits to 2", as I'm sure this will likely be needed).

 

[alfredeneuman]

I agree fully.
There's nothing that says you can't go over the minimum size.
I was just trying to give examples of how, by Code, large boxes are sized.
There are others that may refer to the article later on. After all the thread is titled "Sizing Junction Box".

 

[Terry D]

I agree fully.
There's nothing that says you can't go over the minimum size.
I was just trying to give examples of how, by Code, large boxes are sized.
There are others that may refer to the article later on. After all the thread is titled "Sizing Junction Box".

 

[alfredeneuman]

Terry, you were the one that caused the initial confusion on post #2..... and you say we're killing you?

 

[Terry D]

Terry, you were the one that caused the initial confusion on post #2..... and you say we're killing you?

What was the confusion, the OP understood it. So you agree with me and now you say I confused you.

 

[alfredeneuman]

If Im reading this right, your putting the same size and number of conductors in each conduit. In that case, the 1.5 conduits would be to small. You can only put up to 2 4/0 in 1.5 conduit, by putting the # 4 in there, you would exceed your 40% fill capacity. All conduits need to be 2".

This complete paragraph.

"Engineer determined conduit sizes".
He must have calculated the fill and figured out the 2 4/0 and the #4 was right.
The charts just show how many conductors of the same size will fit, and doesn't take into account anything else.

I agree with you only that I was giving the minimum size and it could be larger

 

[Terry D]

This complete paragraph.

"Engineer determined conduit sizes".
He must have calculated the fill and figured out the 2 4/0 and the #4 was right.
The charts just show how many conductors of the same size will fit, and doesn't take into account anything else.

I agree with you only that I was giving the minimum size and it could be larger

It was not mentioned until the 3rd post that a engineer calculated this out. What caught my attention was that there was different sized conduits with the same conductors. That's all. And I know how the charts work. And after doing my own calculation, they will fit in a 1.5 conduit. Sorry for the confusion. But as you know, a accurate calculation could not be made for even the minimum size of J-Box with out knowing where the conduits are placed on the box.