Nuetral

[new_ee]

In a balanced 3 phase 4 wire wye system is there current on the neutral? I would think that the currents would cancel out.

How do you decide whether the neutral needs to be considered a "current carrying conductor" for ampacity puproses (when running with (3) phase conductors in a 4 wire system)?

What amount of nonlinear (computer equipment/inductive lighting) loads will cause you to consider the neutral a current carrying conductor?

 

[steve66]

Re: Nuetral

I would think that the currents would cancel out.

They do.

How do you decide whether the neutral needs to be considered a "current carrying conductor"

If there are non-linear loads, or single phase loads. Notice that per the NEC, if the neutral only carries the unbalanced current of a multiwire circuit, it is still non-current carrying. (Don't remember the article off-hand).

What amount of nonlinear (computer equipment/inductive lighting) loads will cause you to consider the neutral a current carrying conductor?

I think if 50% or more of the load is non-linear, the neutral is then considered a current carrying conductor.

Don't know where this number came from, but I have seen it thrown out in other posts. Believe it or not, I haven't seen anyone take issue 50% being the right cutoff number. Do you think we have found something everyone will agree on

[ February 07, 2006, 01:52 PM: Message edited by: steve66 ]

 

[dave_asdf]

Re: Nuetral



[ February 07, 2006, 01:58 PM: Message edited by: dave_asdf ]

 

[charlie b]

Re: Nuetral

Originally posted by steve66: . . . if the neutral only carries the unbalanced current of a multiwire circuit, it is still non-current carrying. (Don't remember the article off-hand).

310.15(B)(4)

I once did a simple proof that the amount of heating from four wires (A, B, C, and N), presuming the load is not balanced and that the neutral has current, will be no more than the heating that would occur with balanced load (i.e., no neutral current). That proof did not take harmonics into account, so it applied only to linear loads.

Originally posted by steve66:. . . I haven't seen anyone take issue 50% being the right cutoff number. Do you think we have found something everyone will agree on

 

[roger]

Re: Nuetral

Steve, the 50% figure comes from 310.15(B)(4)(c), only the term "major portion" is used instead of 50%

(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.

Roger

[ February 07, 2006, 02:29 PM: Message edited by: roger ]

 

[roger]

Re: Nuetral

Sorry Charlie b, I was typing when you posted I guess.

Roger

 

[new_ee]

Re: Nuetral

Thanks for all the help. I was wondering what the "major portion" was. I was going to guess 50% but, you know, I like to get a consenus.