Neutrals- when counted

[websparky]

2002 NEC
310.15 (B)(4) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).

The above caught my attention today on a job site when I was considering conduit fill / derating.

The below I am used to and the above almost would include all neutrals after more than 3 current carrying conductors.

Am I reading this right?

(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.

 

[ryan_618]

Re: Neutrals- when counted

Dave, I'm not sure how you are reading it, but I read it as saying that nuetrals form non-linear/harmonic loads must be counted, but typical nuetrals need not be counted.

 

[websparky]

Re: Neutrals- when counted

Hi Ryan,

The (c) one I have no problem with. It's the (A) one that has caused me to re-think the scenario.

It seems to say that practically all neutrals are to be counted as CCC's after 3 or more are in the same raceway.

"carries only the unbalanced current from other conductors of the same circuit"

Is this one of the mis-worded terminologies?
"other conductors of the same circuit"

This seems to imply that it does not include a typical 3phase 4 wire circuit as mentioned in (C), which would seem to make it include almost all other neutrals.

Maybe it's just me??

 

[crash]

Re: Neutrals- when counted

I do not know too much about harmonics, so please correct me where I'm wrong.

My understanding is that the 60 HZ "phasor sum" of the currents on "hot" conductors must equal the 60 HZ current on the Neutral (unbalenced current). Therefore the algebraic sum of the currents in an unballenced wye connected (4 wire) load cannot eqceed the algebraic sum of the currents in a ballenced delta connected (3 wire) load of the same size. Since derating is based on heat, and heat is directly proportional to the algebraic sum of the currents, there is no need to consider the neutral when derrating conductors. However, regulated power supplies in electronic equipment cycle on and off ( in order to maintain the correct D.C. voltage within the equipment) faster than 60 HZ, and cause currents to flow at diferent frequencies, possibly 120HZ, 240HZ, 480HZ etc. This could mean that the neutral current could exceed the unbalenced current and therefore the neutral must be considered when derrating the conductors, in these circomstances

 

[nvcape]

Re: Neutrals- when counted

What happens to the neutral when run with 2 hots, for an example, lighting?

 

[Guest]

Re: Neutrals- when counted

Originally posted by nvcape:
What happens to the neutral when run with 2 hots, for an example, lighting?

Are you talking about using a super neutral or are you talking about putting the hots out of phase?

[ October 16, 2003, 11:52 PM: Message edited by: awwt ]

 

[Ed MacLaren]

Re: Neutrals- when counted

What happens to the neutral when run with 2 hots, for an example, lighting?

If you are referring to a single phase, three-wire circuit supplied from a three phase, four-wire system, the neutral must be counted.

Ed

[ October 16, 2003, 11:21 PM: Message edited by: Ed MacLaren ]

 

[crash]

Re: Neutrals- when counted

NVCAPE: The same formula would still apply, for example;
3-phase
10A @ 0 degrees + 10A @ 120 degrees = 10A @ 60 degrees ("neutral")
you would have three current carrying conductors

1-phase
10A @ 0 degrees + 10A @ 180 degrees = 0A(neutral)
you would have 2 current carrying conductors.

However the same problem could happen with hamonics and you could end up with a current on the neutral that is higher than anticipated.