Grounded vs. Nuetral

[ryan_618]

It was posted at this forum (by a vey knowledged individual) that in a 208y/120 system that "the grounded conductor is always a current carrying conductor". Can that individual please teach an ill-informed inspector like me please?

If this is the case, shouldn't every multi-wire branch circuit containing three phases + grounded be derated as more than three current carrying conductors?

 

[iwire]

Re: Grounded vs. Nuetral

Hi Ryan, It is my understanding that when you only use 2 phase conductors with one neutral from a 208Y/120 system that they will all be current carrying conductors, because the two phases in use are not 180 degrees apart.

They would be 120 and 240 degrees apart alternately so they do not balance out.

Use all 3 phase conductors and all is well.

 

[iwire]

Re: Grounded vs. Nuetral

Here is a reference for you.

310.15(B)(4)(b) In a 3-wire circuit consisting of two phase wires and the neutral of a 4-wire, 3-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(B)(2)(a).

 

[roger]

Re: Grounded vs. Nuetral

Ryan, it is very rare that all phases would be equal at their respective times making the grounded conductor neutral in the zero sense.

But, the unbalanced current on this conductor would only add heat in proportion to it's loading and could never add say another total 20 amps to (3) 20 amp phases, and as you know, would come closer to zero as the other phases increased towards 20 amps. (LINEAR loads no HARMONICS in this conversation please )

The most the grounded conductor could see would be 20 amps if only one phase were loaded and the other two were off.

Wye neutral load calculation

SQ root of

I^2A +I^2B +I^2C -(IAxIB)-(IBxIC)-(ICxIA)


Roger

[ October 01, 2003, 07:48 PM: Message edited by: roger ]

 

[pierre]

Re: Grounded vs. Nuetral

Ryan

A neutral conductor is considered a current carrying conductor.
310.15(B)(4) permits a neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted in the ampacity adjustment.

Pierre

 

[ryan_618]

Re: Grounded vs. Nuetral

Four very good replies in twenty minutes!!! Can't beat that. Thanks guys.

 

[karl riley]

Re: Grounded vs. Nuetral

And I didn't have to say a word. Great!

Karl

 

[roger]

Re: Grounded vs. Nuetral

But we're glad you're back.

Roger

 

[don_resqcapt19]

Re: Grounded vs. Nuetral

Ryan,
I misstated my answer in the other thread and have clarified it in that thread. Bob's first post in this thread said what I should have said in the other thead.
Don

 

[ryan_618]

Re: Grounded vs. Nuetral

Thanks Don.