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#67684 - 07/12/06 08:53 AM Neutal Puzzle  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Given: 120/208V three phase wye service. A 30A 'full boat' (three hots and a neutral) in a conduit. The loads are a 120V 24A resistive load and a 208V 24A resistive load.

For purposes of 310.15(B)(4) do we count the 'neutral' as a current carrying conductor?

Should this count as 4 current carrying conductors or 3?


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#67685 - 07/12/06 09:03 AM Re: Neutal Puzzle  
Roger  Offline
Joined: May 2002
Posts: 1,716
Jon, it would count as 4. BTW, this really wouldn't be a MWBC. Why the question?


#67686 - 07/12/06 03:00 PM Re: Neutal Puzzle  
wa2ise  Offline
Joined: Nov 2002
Posts: 784
Oradell NJ USA
The 208V load on say phase A and B, and the 120V load on phase C to the neutral? Then each and every wire sees 24 amps. None of the current on phase C finds its way to phase A or B.

#67687 - 07/12/06 04:20 PM Re: Neutal Puzzle  
n1ist  Offline
Joined: Feb 2002
Posts: 183
Malden MA
Regardless of which phase is feeding the 120V circuit, all of its current would return on the neutral, so it would be a current-carrying conductor.

#67688 - 07/12/06 04:49 PM Re: Neutal Puzzle  
jdevlin  Offline
Joined: Aug 2002
Posts: 402
welland ontario canada
What if you only turn on the 120 volt load? Then the neutral is current carrying.

#67689 - 07/12/06 06:16 PM Re: Neutal Puzzle  
Roger  Offline
Joined: May 2002
Posts: 1,716
jdevlin, all the current for the 120 v circuit will be returning on the grounded conductor (it is not a neutral in this case) even if the 208 circuit is on at the same time.

This would be the same case if the two circuits (the 120 and 208) were fed from a common breaker or on individual breakers tied together with a handle tie.

Unless all three phase loads are common to the grounded conductor (which is a neutral in this case) it is a current carrying conductor.


[This message has been edited by Roger (edited 07-12-2006).]

#67690 - 07/13/06 07:45 AM Re: Neutal Puzzle  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma

I absolutely agree that there are 4 current carrying conductors, and that for purposes of derating they should be treated as 4 current carrying conductors. They physics is pretty clear.

I brought this up because I was pondering the _actual wording_ of 310.15(B)(4), and trying imagine situations where the wording 'falls down'. After pondering 'idealized extreme' cases where the wording fails to match the intent, then I can ask the question: are there real world situations where this would matter?

I have three ungrounded conductors each carrying 24A. I have a grounded conductor acting as a return for the _net_ current being supplied by the ungrounded conductors. I believe that there is a strong argument that under the _wording_ of 310.1(B)(4) this conductor is not counted, even though I totally agree that it should be counted.

While my example above is not a true MWBC, since the single phase load is entirely separate from the line to line load, it is the extreme case of a MWBC that happens to serve line to line loads as well as line to neutral loads. MWBCs are permitted to service line-line loads. A circuit that has 24A from phase A to neutral, 1A from each of B and C to neutral, and 23A from B to C _would_ be a MWBC, but would have very similar physics as the extreme example. All 4 conductors would be loaded in excess of 20A.

What about the following changed example:
Given: 120/208V three phase wye service. A 30A 'full boat' (three hots and a neutral) in a conduit. All loads are 120V between hot and neutral. The loads are phase A: 120V 24A resistive, phase B: 120V 24A 30 degree leading power factor, phase C: 120V 24A 30 degree lagging power factor. No harmonics at all, just power factors caused by inductive and capacitive components. (To be clear, I am trying to set up a MWBC with return on the neutral where the actual current flow is the same as my example above.)

The real world situation that might apply is the neutral loading on a feeder to a panel that serves both line to line and line to neutral loads.


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