Sizing a neutral (grounded)

[iwire]

Re: Sizing a neutral (grounded)

Originally posted by fleetfeet:
My point is that when and if we see this practice, is it entirely wrong (ie. hazardous, code violation, inefficient, etc.). Or, for the sake of theory, is is "workable".

If the installer is careful I believe it can be done safely and within the rules of the NEC.

Here is a example of it right from the 2002 NEC Handbook.

A 120/240-volt, single-phase, 3-wire system (branch circuits rated at 20 amperes; maximum unbalanced current of 80 amperes).

That aside I can not think of any applications of this that would save me labor and materials.

In the example above they show a 4 AWG neutral now I need to use something other than wire nuts to terminate the neutral. What ever that is will cost more and take longer to install.

Also pulling one large conductor with many small ones may result in damage to the smaller conductors.

 

[pogo]

Re: Sizing a neutral (grounded)

I'm curious, what exactly are you feeding? Is it a three phase load or three individual circuits? I have read the responses, but I think some peoples boats are sinking. Ha!

 

[iwire]

Re: Sizing a neutral (grounded)

Originally posted by pogo:
I'm curious, what exactly are you feeding? Is it a three phase load or three individual circuits? I have read the responses, but I think some peoples boats are sinking. Ha!

As far as I know we are feeding 9 individual 20 amp circuits from a 3 phase system, could be 120 or 277 really does not matter.

If you think peoples boats are sinking step and speak your mind.

 

[LarryFine]

Re: Sizing a neutral (grounded)

Iwire, I'm trying to imagine running a single 4-0 aluminum conductor throughout a house to land all the neutrals on.

"What a ******' nightmare!" - My Cousin Vinny

Edited by Ryan_618. Larry: Thanks for censoring yourself, but I would like to leave a little more to the imagination than just one * in a seven letter word!

[ September 04, 2005, 10:27 PM: Message edited by: ryan_618 ]

 

[curt swartz]

Re: Sizing a neutral (grounded)

Bob, why are you counting the grounded conductor(s) in your first example but not in the last example?

 

[iwire]

Re: Sizing a neutral (grounded)

Originally posted by curt swartz:
Bob, why are you counting the grounded conductor(s) in your first example but not in the last example?

Grounded conductors may or may not be current carrying conductors.

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).

(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).

(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.

No one said that these branch circuits feed non-linear loads. As far as I know they may feed heaters which means the neutral would not need to be counted for derating purposes.

 

[curt swartz]

Re: Sizing a neutral (grounded)

Bob, I agree I just wasn?t sure why you counted it in your first example.

 

[iwire]

Re: Sizing a neutral (grounded)

Hi Curt, I see what you are asking now.

In my first example we have an unusual installation, connected to that one neutral are the following

3 - Phase A conductors

3 - Phase B conductors

3 - Phase C conductors.

IMO this is either 9 circuits or maybe 3 circuits if we call it 3 multiwire branch circuits.

That being the case I believe the neutral must be counted as a current carrying conductor.

310.15(B)(4)(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).

In reality I do not see the neutral here as any different than the neutral of a panel feeder which I normally would not count.

But I do not see that we can call this unusual arraignment "the same circuit".

I am certainly open to other views, this is a bit of an odd wiring scheme and I do not see that anything in the NEC addressees this directly as far as derating.

In the picture I posted from the handbook it looks like derating was ignored. :roll:

It shows a 4 AWG for a possible 80 amp load run with 8 other current carrying conductors. We have to assume that the conductors are run together.

That would mean that the 4 AWG XHHW is only good for 65 amps. IMO that picture should show a 2 AWG XHHW.