Sharing a neutral

[roger]

Pretty much. :grin:

Excellent!!! :grin:

Roger

 

[sameguy]

Summing up than we can have as many mwbc on (1) properly sized grounded conductor, so long as we have means of disconnect for each mwbc set.
Yes?
No?
I say yes.

 

[iwire]

To me this would have been better wording ....


200.4 Neutral Conductors. Neutral conductors shall not be permitted to be used for more than one set of ungrounded conductors unless specifically permitted elsewhere in this Code. [ROP 5-49]

Of course we would need a definition of 'set'.

 

[roger]

I say yes.

And I agree.

Roger

 

[sameguy]

Right now. slow typer.

 

[roger]

To me this would have been better wording ....


200.4 Neutral Conductors. Neutral conductors shall not be permitted to be used for more than one set of ungrounded conductors unless specifically permitted elsewhere in this Code. [ROP 5-49]

Of course we would need a definition of 'set'.

But, when we are dealing with ungrounded conductors of the same phase or leg, a "Neutral Conductor" doesn't exist.

Roger

 

[iwire]

But, when we are dealing with ungrounded conductors of the same phase or leg, a "Neutral Conductor" doesn't exist.

I hear you but didn't they already change that?

 

[roger]

I hear you but didn't they already change that?

Yeah, they did but, for those who are under code cycles earlier than 08, have a party. :grin:

Roger

 

[don_resqcapt19]

_______________________________________________________________
5-33 Log #1987 NEC-P05 Final Action: Accept in Principle
(200.8 (New) )
_______________________________________________________________
Submitter: Donald A. Ganiere, Ottawa, IL
Comment on Proposal No: 5-49
Recommendation: The proposal should be accepted as submitted.
Substantiation: There are cases where multiple ungrounded conductors having
no potential between them are being installed with a single oversized grounded
conductor. (example: Two #12s on 20 amp breakers both installed on A phase
with a #8 grounded conductor. There is no code rule that prohibits this practice
in the 2008 code. The two specific provisions that use the currently undefined
term of “common neutral, only apply to multi-wire type circuits. If this
proposal is accepted proposal 5-7 must also be accepted.
Panel Meeting Action: Accept in Principle
Revise the ROP draft text to read as follows:
200.4 Neutral Conductors. Neutral conductors shall not be used for more
than one branch circuit, multiwire branch circuit, or for more than one set of
ungrounded feeder conductors unless specifically permitted elsewhere in this
Code.
Panel Statement: The revised text clarifies that the restriction on use of the
neutral in more than one branch circuit is prescriptive and not permissive. The
panel concludes these changes resolve the issues raised by the submitter.
Number Eligible to Vote: 16
Ballot Results: Affirmative: 16

 

[John120/240]

I'm confused

I'm confused

REF post #15 where "mxslick" has 5 conductors A B C A C with one
neutral # 8. How would you simatenously disconnect 5 hots? Wouldn't
the two A phase & two C phase hots be additive on the neutral?
What am I missing ? Thank You John

 

[mxslick]

REF post #15 where "mxslick" has 5 conductors A B C A C with one
neutral # 8. How would you simatenously disconnect 5 hots? Wouldn't
the two A phase & two C phase hots be additive on the neutral?
What am I missing ? Thank You John

John, that situation predated the current Code handle tie requirement, (but since it was in a "dwelling unit", ie mansion, it was still in violation as it was not possible to disconnect all 5 circuits at the same time.) IIRC the first three breakers were next to each other, but the other two were in totally different spaces in the panel. Like I said a while back (post #23) the A/V contractor of the original install was a low-baller and had a hack EC that he was in love with using. Both the panel's wiring and the audio rack's wiring was a huge mess.

And yes, the two A and C phases would be additive on the neutral (A1 and A2, C1 and C2) that is why it was pointed out that the minimum neutral size had to be #8, to carry the potentially full load of 40 amps from either set of matched phases.

 

[John120/240]

Thank You for your insightful explanation. John

 

[e57]

All MWBC's are single circuits by their very nature, the NEC allows them to be considered "multiple circuits" which they are not.

Roger

Not looking to get into the battle this simple question (the OP) caused to start back up - but everything on the transformer or SDS feeding it is one circuit...

Does the OCP divide circuits?

 

[iwire]

Does the OCP divide circuits?

To the NEC yes.

Consider how tell which are service entrance conductors, feeders and branch circuits.

 

[busman]

It looks like this change will outlaw those pre-wired generator panels. They have a whole load of hots and just one neutral for them all. Thinking about the Reliance and Generac that come with a pre-wired greenfield whip.

Mark

 

[LarryFine]

Maybe the wording should be something akin to:

"All line conductors sharing a neutral must have voltages between any of them."

Or maybe:

No two (or more) line conductors without voltage between (or among) them shall share a neutral."

 

[dfortman55]

You must use a common trip multi-pole breaker to share the neutral.
Single phase panel you can share 2 hots with a neutral on a two pole breaker or two single pole breakers with an approved handle tie.

Three phase panel you can share 3 hots with a neutral but must use a 3 pole breaker or 3 single pole breakers with approved handle ties.

Check out 2008 illustrated code changes.

 

[Bennie Kennedy]

Sharing neutrals can create problems if there are non linear loads. Lighting with electronic ballast,computers,monitors,printers,electronic dimmers and other electronic devices. Harmonics add in the neutral conductor and because there is no protective device(breaker or fuse) to protect the neutral a fire can result. The recommendation is not to share neutrals. And over size the neutral where there are non-linear loads.

 

[roger]

Sharing neutrals can create problems if there are non linear loads. Lighting with electronic ballast,computers,monitors,printers,electronic dimmers and other electronic devices. Harmonics add in the neutral conductor and because there is no protective device(breaker or fuse) to protect the neutral a fire can result. The recommendation is not to share neutrals. And over size the neutral where there are non-linear loads.

Welcome to the forum.


So, do tell, where have you personally had to fix additive harmonic problems due to the use of MWBC's on non-linear loads?


Roger

 

[Joey12]

Breakers

Breakers

Now the breakers are required to have a ty handle so all ungrounded conductors sharing the same neutral are opened at the same time. This eliminates the chance of any voltage/current on that neutral if someone is working on those circuits. Smart idea but is it completely practical?