Derating for conduit fill in a 120/240V system

[Pitt123]

In regards to de-rating cable per the adjustment factors for conduit fill in table 310.15(B)(3)(a) does the neutral in a 120/240V system count as a current carrying conductor? Considering that it can carry neutral current I would think it would be considered as one?

Also according to informational note 3 under this table you do not have to de-rate cable for conduit fill in a conduit that is under 24" (2ft) is that correct?

What about in a wire trough that may exceed 2 feet in length? Is there any derating needed for a wire trough? Is there anything in the NEC that dictates size of such a trough?

 

[charlie b]

The neutral conductor in a 120/240V single phase system will carry current, but only when the load on the phase conductors is not balanced. 310.15(B)(5)(a) (NEC 2011) tells us that the neutral does not count in that circumstance.

By the way, I would not use the phrase "derating for conduit fill." It is not quite the right description. To me, at least, "conduit fill" has to do with cross-sectional area (i.e., the 40% limit). In your question, what you are derating for is more than three CCCs in a raceway.

 

[Pitt123]

The neutral conductor in a 120/240V single phase system will carry current, but only when the load on the phase conductors is not balanced. 310.15(B)(5)(a) (NEC 2011) tells us that the neutral does not count in that circumstance.

O.k. so if I have (4) of these 240/120V circuits with each circuit having 2 hots and a neutral then I will have 12 conductors however for the sake ot 310.15(B)(5)(a) I only need to consider 8 conductors as current carrying and therefore only derate by 70%?

By the way, I would not use the phrase "derating for conduit fill." It is not quite the right description. To me, at least, "conduit fill" has to do with cross-sectional area (i.e., the 40% limit). In your question, what you are derating for is more than three CCCs in a raceway.

Good point!

 

[Dennis Alwon]

Remember Charlie stated this is true in your case for a single phase system. It would need to have 3 ungrounded conductors and one neutral for a 3 phases system for the neutral not to be a CCC.

 

[charlie b]

O.k. so if I have (4) of these 240/120V circuits with each circuit having 2 hots and a neutral then I will have 12 conductors however for the sake ot 310.15(B)(5)(a) I only need to consider 8 conductors as current carrying and therefore only derate by 70%?

Correct.