The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Topic Options
Rate This Topic
#67684 - 07/12/06 05:53 AM Neutal Puzzle
winnie Offline
Member
Registered: 09/15/03
Posts: 652
Loc: 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?

-Jon
Top
Work Gear for Electricians and the Trades
#67685 - 07/12/06 06:03 AM Re: Neutal Puzzle
Roger Offline
Member
Registered: 05/18/02
Posts: 1779
Loc: N.C.
Jon, it would count as 4. BTW, this really wouldn't be a MWBC. Why the question?

Roger
Top
#67686 - 07/12/06 12:00 PM Re: Neutal Puzzle
wa2ise Offline
Member
Registered: 11/29/02
Posts: 773
Loc: 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.
Top
#67687 - 07/12/06 01:20 PM Re: Neutal Puzzle
n1ist Offline
Member
Registered: 02/13/02
Posts: 176
Loc: 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.
/mike
Top
#67688 - 07/12/06 01:49 PM Re: Neutal Puzzle
jdevlin Offline
Member
Registered: 08/07/02
Posts: 402
Loc: welland ontario canada
What if you only turn on the 120 volt load? Then the neutral is current carrying.
Top
#67689 - 07/12/06 03:16 PM Re: Neutal Puzzle
Roger Offline
Member
Registered: 05/18/02
Posts: 1779
Loc: N.C.
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.

Roger

[This message has been edited by Roger (edited 07-12-2006).]
Top
#67690 - 07/13/06 04:45 AM Re: Neutal Puzzle
winnie Offline
Member
Registered: 09/15/03
Posts: 652
Loc: boston, ma
Roger,

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.

-Jon
Top

Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box

Recent Posts
Looking for older post
by sparkyinak
Today at 12:27 AM
License exam
by TRUE POWER
Yesterday at 11:33 PM
Members: Non-Members: Did you know?
by Admin
Yesterday at 02:34 PM
Safety at heights?
by HotLine1
01/21/17 08:51 PM
International Wire Colour Codes
by Texas_Ranger
01/21/17 03:29 PM

Who's Online
1 registered (Ruben Rocha), 142 Guests and 11 Spiders online.
Key: Admin, Global Mod, Mod
New in the Gallery:
Desk-mounted "power-board"
Top Posters
gfretwell 9066
Trumpy 8560
pauluk 7693
HotLine1 6833
sparky 5545
Member Spotlight
Tom
Tom
Member Since: 01/01/01
Posts: 1069

ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals