NM cable adjustment factor

[omega83]

Hey guys
When you do pipe fill with nm, you count nm cable as one wire. What do you do when you do the adjustment factor on T 310.15(b)(3)(a) on 2011 nec? Do you still consider the nm as 1 wire and 1 current carrying conductor or do you have to count all current carrying conductor in the cable? can you guys help me with this

 

[George Stolz]

You count all the current carrying conductors in the cable.

 

[omega83]

So for example,

you have 6 current carrying conductors and 1-12/2 nm wire( assuming both wires are current carrying) in a 3/4in emt pipe, you would have a total of 8 current carrying conductors? I thought that when you do a pipe fill on nm, it counts as one conductor and I assume that it would be the same when you did the adjustment factor.

 

[Ponchik]

So for example,

you have 6 current carrying conductors and 1-12/2 nm wire( assuming both wires are current carrying) in a 3/4in emt pipe, you would have a total of 8 current carrying conductors? I thought that when you do a pipe fill on nm, it counts as one conductor and I assume that it would be the same when you did the adjustment factor.

You are talking about two different things.
One conductor or cable for conduit fill. (chapter 9)
Then you have conductor derating. 310.15(b)(2)(a)

Also when you have two conductor circuit both are always current carrying.

 

[infinity]

You are talking about two different things.
One conductor or cable for conduit fill. (chapter 9)
Then you have conductor derating. 310.15(b)(2)(a)

Also when you have two conductor circuit both are always current carrying.

I agree with Edward, you would count the one flat NM cable for conduit fill based on it's width as if it were round and as one conductor. Derating is a different issue based on CCC's where in the cable or not.

 

[omega83]

You are talking about two different things.
One conductor or cable for conduit fill. (chapter 9)
Then you have conductor derating. 310.15(b)(2)(a)

Also when you have two conductor circuit both are always current carrying.

I understand that when you did pipe fill for nm, it is consider as one conductor after you find the area of it. I just was not sure what to do when you do the adjustment factor. If you still consider the nm as one conductor and count the whole cable as one current carrying conductor. I guess from what you guys are saying is that when you do pipe fill for nm, it is consider one conductor after you find area of it. Then if you are going to do adjustment factor, you have to count all the wires that in the nm as current carrying. you said " Also when you have two conductor circuit both are always current carrying". What if that nm cable is just a 120v circuit supplying a receptacle? 310.15(b)(5)(a) says that you don't have to count the neutral if all its doing is carrying the unbalance current. I understand if its a 240v then they both would be count right?

 

[Ponchik]

if you have a two conductor circuit, the current that goes out on black must come back on the Neutral. If it doesn't then you have a problem. So both conductors of a 2 wire circuit are current carrying.

On a MWBC (multi wire branch circuit) it is a different story. Only the difference of the two hot legs come back on the Neutral. However, if only one of the legs of a MWBC has current then the same current comes back on the Neutral.

 

[omega83]

if you have a two conductor circuit, the current that goes out on black must come back on the Neutral. If it doesn't then you have a problem. So both conductors of a 2 wire circuit are current carrying.

On a MWBC (multi wire branch circuit) it is a different story. Only the difference of the two hot legs come back on the Neutral. However, if only one of the legs of a MWBC has current then the same current comes back on the Neutral.

yes I understand what your saying but 310.15(b)(5)(a) says that you don't have to count the neutral if all its doing is carrying the unbalance current right? if not, what does that article means?

 

[david luchini]

yes I understand what your saying but 310.15(b)(5)(a) says that you don't have to count the neutral if all its doing is carrying the unbalance current right? if not, what does that article means?

Yes, the rule says that for a multiwire branch circuit, the neutral thatn only carries the unbalanced current need not be counted as a current carrying conductor.

That being said, your 12/2 nm is not part of a multiwire branch circuit. Both conductors are ccc's. Note that it says carries the unbalanced current from other conductors (more than one) in the same circuit. In your example, you have the neutral carrying the current from only one other conductor in the same circuit.

 

[omega83]

Yes, the rule says that for a multiwire branch circuit, the neutral thatn only carries the unbalanced current need not be counted as a current carrying conductor.

That being said, your 12/2 nm is not part of a multiwire branch circuit. Both conductors are ccc's. Note that it says carries the unbalanced current from other conductors (more than one) in the same circuit. In your example, you have the neutral carrying the current from only one other conductor in the same circuit.

I was looking up other thread in came across this one. if you look at this link, its seems to me that they say if its single phase you dont need to count neutral. When it a multiwire, if you have all phases with a neutral, you also don't count it.

http://forums.mikeholt.com/showthread.php?t=138921

 

[david luchini]

I was looking up other thread in came across this one. if you look at this link, its seems to me that they say if its single phase you dont need to count neutral. When it a multiwire, if you have all phases with a neutral, you also don't count it.

http://forums.mikeholt.com/showthread.php?t=138921

Yes, 120/240V single phase with two phase conductors and one neutral, the neutral wouldn't need to be counted as a current carrying conductor.

In a 3phase, 4wire system with three phase conductors and one neutral, the neutral wouldn't need to be counted as a current carrying conductor.

But,

In a 3phase, 4wire system with two phase conductors and one neutral, the neutral would need to be counted as a current carrying conductor.

And in a 120V 2 wire circuit, phase&neutral, or in a 240V 2 wire circuit, phase and phase, the both conductors would need to be counted as a current carrying conductors.

 

[omega83]

Thanks for your help, I see it now.