Feeder/Branch wire adjustment factor

[tarak]

I got query with adjustment factor when conductors are more than 3 including neutral. Could someone explain?

Do we derate cable for more than 3 conductors in conduit if 4th conductor is neutral following Table 310.15(B)(3)(a)? Below is an example of one situation where I have used adjustment factor “0.8” as feeder has more than 3 conductors including neutral.


1. Size the conductors and conduit for a feeder to a 208/120V 3-phase lighting and receptacle panel board with a 25,000 VA load.

Could someone help whether we consider derating as per Table 310.15(B)(3)(a) and treat neutral as current carrying conductor?

 

[Dennis Alwon]

Here is something written by Trevor at Mike HolMike HolMike Holt
Here's some examples of when to count the neutral as a CCC:
Here is something written by one of our members that may help.... BTW, welcome to the forum

208Y/120 volt system-different circuit types:

A)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C)- 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B)- In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C)- In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with one exception, *if the current is more than 50% nonlinear then the neutral would count as a CCC.

120/240 volt system-different circuit types:

D)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's

Notes:
D)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E)- In this circuit the neutral will only carry the imbalance between the two ungrounded condcutors so the neutral is not counted as a CCC.

 

[tarak]

I understood "A" and "C". but when we see "B", from 310.15(B)(5)(b), I understood that this way "If we have a 3-phase system with 3 phase conductors and a neutral (wye system) and you are only powering something with 2 phase conductors and a neutral, the neutral counts in this case correct?
So, do we have derate the input feeder (considering neutral) to that panel serving these loads?

 

[paulengr]

I understood "A" and "C". but when we see "B", from 310.15(B)(5)(b), I understood that this way "If we have a 3-phase system with 3 phase conductors and a neutral (wye system) and you are only powering something with 2 phase conductors and a neutral, the neutral counts in this case correct?
So, do we have derate the input feeder (considering neutral) to that panel serving these loads?

The B example is a bit crazy since it is rare. Think if 240/139 for a moment. If you have say 10 A on one hot and 12 A on the other you get 2 A on the neutral. So it’s mostly not a CCC. With the same loads on 208/120 though the two hots are offset by 120 degrees, not 180. So with the same 10 and 12 A you will get much more than 2 A. It’s not 22 A but it will not be insignificant.

The B example counts as 3 current carrying conductors so by itself no derating required. Just as in the normal 208/120 4 wire wye example C. This is in keeping with no derating up to 3. But if you ran a single additional current carrying conductor with it, then you derate.

 

[tarak]

I elaborated the same in this picture (just for understanding). So, we don't have to derate the input feeder (3ph, 4W) to that panel though it has some 1-ph (3wire) loads on it. correct! Because, it seems like you understood I am talking about the branch circuit (1ph, 3w load) within the panel. But I am talking about the input side of the panel which is (3ph, 4W). Thanks for helping me out.

 

[infinity]

I understood "A" and "C". but when we see "B", from 310.15(B)(5)(b), I understood that this way "If we have a 3-phase system with 3 phase conductors and a neutral (wye system) and you are only powering something with 2 phase conductors and a neutral, the neutral counts in this case correct?
So, do we have derate the input feeder (considering neutral) to that panel serving these loads?

Yes and No. The branch circuit neutral counts as a CCC the feeder neutral does not.

 

[sameguy]

No on the feeder.
It is assumed the panel load will be balanced, A/B, A/C, B/C, by the Engineer;
I don't have code book to look up 310.

 

[infinity]

The B example is a bit crazy since it is rare. Think if 240/139 for a moment. If you have say 10 A on one hot and 12 A on the other you get 2 A on the neutral. So it’s mostly not a CCC. With the same loads on 208/120 though the two hots are offset by 120 degrees, not 180. So with the same 10 and 12 A you will get much more than 2 A. It’s not 22 A but it will not be insignificant.

The B example counts as 3 current carrying conductors so by itself no derating required. Just as in the normal 208/120 4 wire wye example C. This is in keeping with no derating up to 3. But if you ran a single additional current carrying conductor with it, then you derate.

In your first example I think that you meant 240/120 not 240/139. 240/139 would be a WYE system just like 208Y/120.
In your last example (3 hots and 1 neutral) derating would only be required of the load were more than 50% non-linear.