Conduit Fill and Derating

jamiedolan

Yep, I am running isolated neutral bar, clip is gone. Everything appears to be working just fine. I'll double check voltage reading tommrow, and look at the neutrals with a clamp meter to make sure all is well, which I am sure it is. I'll have the last of the old circuits I am hooking up done in the am, then I need to start running EMT so I can re-wire several circuits.

I am really happy with how the MWBC worked out. It allowed me to junction everything right up there in the joists, hidden nice and neat in those 2 boxes that connect to the panel with EMT. Makes the panel look really neat and organize with just THHN in it.

Thanks again
Jamie

InPhase277

Jamie, I count 10 conductors entering that pipe. Since these are MWBCs to 2-wire NM, that means you have 10 + 12 = 22 conductors in that box (counting only one ground). The box looks like a 4 x 4 x 2-1/8". I do believe you have exceeded the volume limit for the box. You might look into an extension ring.

joey b

Although it may not be as cost efficient, could one run a shared neutral from 2 MWBCs that were located on the same leg, assuming that amperage allowance was not violated? for example, run 2 20 amp circuits (from the same supplying leg) with 12 guage wire for the hot and 8 gauge for the shared neutral? Seems this would also eliminate the possibility of the broken neutral situation.

Gigs

You'd wind up using more copper. 12AWG is (6529.95cmils), and 8AWG (16509.65cmils).

So in addition to creating a hazardously confusing wiring situation, you wouldn't save anything.

InPhase277

What he said. But yes, you could. You could put every neutral on one big wire as long as it had the ampacity to carry the current.

jamiedolan

I'll pick up an extension for the box. I don't see extensions listed on the chat 314.16(A). How do I know how much capicity the extentions adds and if it is adequate for the number of wires I have?
Thanks
Jamie

Gigs

Do the general volumetric calculation.

jamiedolan

Ok I see it now. I will just have to figure out what volume the extension adds, hopefully it is labeled on the box extension.
Thanks
Jamie

jamiedolan

12 gage is listed as needing 2.25 cu in X 22 = 49.5 cu in. That box is 30.3 cu in. So does that mean I need to find a box extension that adds almost 20 cu in to that box?

It didn't even seem that tight in the box at all, 50 cu in seems like a huge box for these wires. I guess I am getting better at making nice neat connections that take up less space.

Jamie

Stubbie

Jamie

Here is a link to get your volumes on extension rings and boxes.

http://www.tnb.com/contractor/docs/sc_indoorboxes.pdf

I am little confused at what you ended up with... for some reason I was thinking we were talking 2 14/2 cables connected to a 14/3 multiwire and 2 12/2 cables connected to a 12/3 multiwire. Under those considerations you would have 32 cu. in. of box fill.... 7 14's and 7 12's plus one 12 ground.

A 4x 4x 2 1/8 square has 30.3 cu in so you would have been over even with that.
Right now you have 46 cu in of box fill. So you will need an extension ring of at least 16 cu. on those 4x4's.

Here is another link to help you in the future


http://www.hubbellcatalog.com/raco/R...?FAM=RacoBoxes

Remember on box fill you have different allocations depending on the wire size 14 = 2 counts per wire and 12 = 2.25 counts per wire and one count for the grounds equal to the largest conductor in your situation.



I apologize for not getting this calculated for you as to the size boxes you needed but I was thinking somewhere in all this it had been mentioned but I guess i as wrong. I'll look back in the thread and see where I missed what you ended up with.

Stubbie

I believe what your saying is you used all 12 awg so you would be 49.5 cu in.

Are all the black cables 14's 12's or mixed? I'm worried you connected the 12's to 14 awg cables and put them on a 20 amp breaker.

jamiedolan

12/20's are on yellow THHN 12 gage

14/20's are on blue THHN 14 gage.

They are not all 12 gage, there is a mix, I just used that as a rough number since there is only .25 difference between the 2 wires.

I think what happened is that I was only counting the wire once when it came into the box and went out, I wasn't counting that as 2. So I was figuring out my box fill with half of the number of wires I should have been figuring it out with.

They are 12-2 and 14-2 cables that come into the box. Then a hot from each black (that comes into the junction on NM) runs back into the panel. Then 2 hots that share the same breaker have there neutrals tied together and a pig tail run back into the panel.

Jamie

jamiedolan

I just got some double gang mud rings, 15 cu in. So that gives me 45.3 cu in.

I do have a mix with some 12 and some 14 in all the boxes. Actually the box that is the most full is 6-14gage and 7-12gage plus 6 ground wires are bonded in that box from the NM, but they are 16gage ground wires in the NM for some reason.

I come up with a number of 34.77 for the amount of required space. So unless I am figuring out something incorrectly, with the mud ring, I will have more than enough room to satisify the box fill requirments. (althought physically there was plenty of room to work in the 4"x4" box.). I clamped in the wires in good locations (top feed as much as possiable).

Jamie

jamiedolan

I just posted updated photos of my electrical projects, and the panel in the project catagory.

Project Updates

Jamie