I need to bring a bunch of NM-B cables into a junction box and transition all of them to THHN inside EMT. I have a few questions as I've never done anything on this scale before.

Here's what I have:

2 6/2 AL
1 10/3 CU
1 10/2 CU
1 12/3 CU
2 12/2 CU
5 14/2 CU

I'll convert the 6/2 aluminum to copper in the junction box, so those become #8 with #10 ground.

What I came up with is:

4 #8
9 #10
10 #12
15 #14

Based upon entering those numbers into a conduit fill excel spreadsheet I downloaded, it says I need 1-1/2" EMT. Is there more to this than I have presented? The calculator did a 40% fill. What about derating?

 

Will all your wires above be THHN?

 

Yes. They will all be THHN.

 

Please tell us what it is your doing. Up front I can tell that derating is going to make this an ugly design.... Even if we can consider neutrals in the 3 conductor cables.

Only one ground is needed or you can use the emt. But multiple runs of conduit are likely to make this work.

For example all these in one pipe your #8, if its thwn-2 or thhn, will derate to a useable 25 amps. The 14's will be below 15 amps making them unusable and you get the idea I think.

 

If you going that route with the design you will run into couple issue which you may not be aware but let me run this down real quick.,,

You will need pretty " big " junction box something like 8X8X4 will work minuim size for this numbers of conductors to in there.

The other issue you will run into is numbers of conductors in conduit and that will really affect the ampcapity rating you may run into pretty serious derating there.

It will be easier to run multi conduits than single large conduit But there is a loophole if under 23" inches the the derating will not be effect at all if super short run that is the only time it will have loophole { some inspectors may frown on this }

For majorty of conduits I try to stay at 9 CCC { Current Carry Conductors } or less that do included netrual if there then useally no derating affect there at all so something you have to think about it.

For myself It will be eaiser to run two or three smaller conduits due easier to bend and easier to find parts what you need.

Merci,Marc

 

Let me give you some background on what I am proposing here...

The main panel in my house is located on the southeast corner of the garage. All of the NM cables go from the panel into a wooden chase that runs the length of the garage so that they can enter the house at the northeast corner of the garage and branch out from there into the house. The face of the chase was covered in drywall. One day I removed some of the drywall so I could inspect the wiring. I was curious as to how it was done. What I discovered is that rats had run up and down the chase and chewed on pretty much all of the NM cables, every few feet. Some of the cables were down to the bare copper.

As a temporary fix, I wrapped all of the exposed portions in electrical tape. I don't believe this is a permanent solution and I want to correct it. I realize that replacing all of the wiring inside conduit stills leaves all of the NM inside the attic susceptible to rats, but I've been in the attic and as far as I can tell none of it has been messed with.

Multiple runs of conduit may be an option. How many would I need and of what size to avoid derating?

Can someone explain derating?

 

Multiple runs is a good idea. You could also use metal cable (MC) or armored cable (AC) which both have metal spiral sheaths.

When wires are bundled closely together they build up heat. Because of this, the ampacity (maximum amps) of the wire must be reduced based on the number of nearby wires. You only count current carrying conductors (hots and neutrals), not grounds. It's a lot of info to retype here, so I will refer you to a thread from a few weeks ago which covers almost exactly the same thing you are doing:

http://forum.doityourself.com/electr...l-upgrade.html

Please read through that one and post back if you have any more questions.

 

Calculating your wire count you will need at least a 1 1/2" EMT pipe.

If you run all the wires in one pipe you will have to derate them at 40%. IE your #12 THHN wire would only be allowed to carry 12 amps. I suggest if your are going to run pipe, only fill them with a max of 9 wires. You will not have to worry about derateing them.

 

So I am going to run multiple conduits for this... And apparently 9 is the magic number here to avoid derating. 9 wires total or 9 current carrying conductors?

How is a MWBC handled? 2 hots + 1 neutral, does the neutral still count as a current carrying conductor?

 

Just to be clear, you still have to derate but it is really a non factor because if you do the math it all comes out fine for small conductors. (IE: #12THHN is rated at 30 amps. 30 x70%=21 amps. small conductor rule 240.4(D) allows only 20 amps on #12)

A neutral is almost always a current carrying conductor and will be needed to be counted. The only thing that is not is the ground, if present.

 

For conduit fill (40% of cross-sectional area), you use total number of wires including grounds. For derating, you use only the total number of current carrying conductors.

The three wires of a MWBC count as two CCC because the neutral carries the difference in current between the two hots. In a regular circuit, both the hot and neutral count as CCC. Grounds never count as CCC.

 

Ok, that makes sense.

Does it matter what is being supplied? My initial reference to a MWBC was for a 20 amp @ 120 volts circuit providing power to receptacles and equipment. The other MWBC I have is a 30 amp @ 240 volts supplying a single piece of equipment. Are these both considered the same thing as far as number of current carrying conductors?

 

No, it doesn't matter what is being supplied. Yes, they are considered the same thing. The neutral still carries only the difference in current between the two hots.

Good question!

 

Just for your information, not that it applies here: There are rules in three phase Wye systems where the neutral of a MWBC must be counted as a current carrying conductor. See 310.15(B)(4)(2005) For more info.

 

Good point, Tolyn. I doubt there are going to be too many DIYers working on 3-phase systems of any type, but there are bound to be some, somewhere...

 

I agree. But, I always figure the more info the better. That is why I did the bold type.