Box for connecting SER to MHF

75gmck25 · Oct 1, 2017

This is for installation of a QO sub-panel in my garage, breakered at 60 amps in the main panel.

Inside the house: I have 2-2-2-4 SER running from my main panel across the house to my exterior basement wall and ending inside an unfinished storage closet.

Outside the house: I then have 2-2-2-4 MHF buried in conduit, running from my detached garage over to the point where I will drill through the basement wall. The MHF cable is rated for USE-2 RHH RHW.

My plan was to have the MHF come through the wall in 2" conduit, and mount an LB box on the inside of the wall. I would then come down the wall with the SER and connect the SER to the MHF inside the LB box. Then I looked at how small the LB volume is, considering the size of the wiring.

Question: Can the 2" LB serve as a junction box for the 2-2-2-4? Do I need more space to connect the SER to MHF, or is another type of box needed to meet code requirements?

Thanks,

Bruce

ishiboo · Oct 1, 2017

A LB can serve as a junction box if it has the cubic inch capacity stamped on it. I wouldn't want to splice MHF in one though for sure. I would come into a large plastic box for that. I think I spliced my 2-2-2-4 SER in the basement in a 8x8x6 or something close.

75gmck25 · Oct 1, 2017

I have concluded that it definitely makes more sense to use a much larger box than the 2" LB. The junction box will be mounted on an unfinished basement wall under the stairs, so box size doesn't really matter for aesthetics.

Bruce

pattenp · Oct 1, 2017

A 2" LB conduit body is to small to splice 2-2-2-4. Basically if you have 2" conduit entering a junction box the box needs to be 6 times the conduit size which will be a 12" X 12" box.

75gmck25 · Oct 1, 2017

The 2" conduit is probably a little bigger than I really needed, but I used that size to make the MHF easier to pull.

Do you really think I need a 12" X 12" box just to connect four wires (2-2-2-4)?

I figured an 8" X 8" box should be big enough - but maybe not. One advantage of the 12" X 12" box is that its also 6" deep (instead of 4"), so it has a lot more capacity than the smaller box.

Bruce

wyliesdiesels · Oct 1, 2017

75gmck25 said:
The 2" conduit is probably a little bigger than I really needed, but I used that size to make the MHF easier to pull.

Do you really think I need a 12" X 12" box just to connect four wires (2-2-2-4)?

I figured an 8" X 8" box should be big enough - but maybe not. One advantage of the 12" X 12" box is that its also 6" deep (instead of 4"), so it has a lot more capacity than the smaller box.

Bruce

What are you gonna use to splice the wires?

If insulated tap connectors, they take a lot of room and you will be glad you have that extra 2" of depth when having to cram it all back in the box.

ard · Oct 1, 2017

pattenp said:
A 2" LB conduit body is to small to splice 2-2-2-4. Basically if you have 2" conduit entering a junction box the box needs to be 6 times the conduit size which will be a 12" X 12" box.

Isn't it 12" only in one dimension?

pattenp · Oct 1, 2017

ard said:
Isn't it 12" only in one dimension?

Well since a box has three dimensions, the 12" is two of them. Typically 12" X 12" X 6" is the box you'll find.

pattenp · Oct 1, 2017

75gmck25 said:
Do you really think I need a 12" X 12" box just to connect four wires (2-2-2-4)?

I figured an 8" X 8" box should be big enough - but maybe not. One advantage of the 12" X 12" box is that its also 6" deep (instead of 4"), so it has a lot more capacity than the smaller box.

Bruce

A 8X8X4 box is actually large enough for the 2-2-2-4 splicing but the controlling factor is the size of conduit feeding the box. If you had used 1.25" conduit the 8X8X4 would be okay, but the 2" conduit could hold up to 5 or 6 #2/0 wires and the 8X8X4 box is too small for #2/0. The code makes you size the box comparatively to the conduit size so you aren't cramming too much wire in a junction box relative to the capacity of the conduit. I hope that makes sense.

Edit: This is for junction boxes containing #4 and larger conductors. See NEC 314.28

ard · Oct 1, 2017

pattenp said:
Well since a box has three dimensions, the 12" is two of them. Typically 12" X 12" X 6" is the box you'll find.

I wasnt being a smart ***. I thought the 12" was only one of the dimensions. Didnt know you need 12" on both.

:beer:

terabitdan · Oct 1, 2017

I'm curious, anything wrong with the OP using a bushing to enter the box as 1 1/4"? Say coming out the back of the outside LB wth 1 1/4" conduit into the inside junction box.

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75gmck25 · Oct 2, 2017

I considered the same question about downsizing the conduit size, since I really only needed 2" conduit to make the pull easier. However, I decided I will pick up a 12x12x6 box today and see how well it fits. The junction box is high on the wall in the back of a basement closet under the stairs, so its not like it will really get in the way.

I had glanced at NEC 314.28 a couple of times, but never realized that for large gauge wires the conduit size used is as significant as the number of wires in the box. I am more used to standard house wiring, where the number of wire connections and wires is the main factor for calculating box size.

Bruce

pattenp · Oct 2, 2017

I've used both 8" & 12" boxes for the splices on 2-2-2-4 and needless to say the 12" is a lot easier than a 8". The 8" box is okay but if you used 2" conduit I wouldn't reduce the conduit to 1.25" just for the purpose to get by with the 8" box if space is not an issue for the 12". The only thing is the price of a 12" box.

75gmck25 · Oct 2, 2017

I picked up the 12"x12"x6" box today and it was $35. Not much more than the smaller box.

I also picked up a 2" conduit box adapter, and the info says it is used to attach PVC conduit to the box. However, its so simple (and cheap) that I want to make sure I have the right fitting.

The 2" conduit will be coming through the brick wall into the basement, and I plan to mount the junction box over the hole in the wall. If I mount the conduit adapter and conduit onto the back of the box, I can just stick the conduit through the wall, and then make the final connection of conduit outside. Does this make sense, or am I missing something?

Thanks,

Bruce

ard · Oct 2, 2017

Sounds fine.

You have a few mm of 'adjustment' in terms of wet PCV glue, to get everything 'precise'- if there is an LB on the outside, then this will be flush against the wall- and on the inside the back of the box will 'just; sit on the shoulder of that 2" fitting. I'd mount inside first, locknut to the box- then finish up outside.

Jon_E · Oct 11, 2017

I spliced 2-2-2-4 SER to 2-2-2-4 MHF in a j-box mounted to the foundation of my house. I used 2" conduit for the MHF and was informed by the electrician assisting me that I needed a 12" x 12" box. It's a little out of place but not bad, but it is definitely the right size.

Box for connecting SER to MHF

75gmck25

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