common neutral branch circuits

[david]

I am starting this thread to avoid taking away from another threads on branch circuits.

110.8 Wiring Methods.
Only wiring methods recognized as suitable are included in this Code. The recognized methods of wiring shall be permitted to be installed in any type of building or occupancy, except as otherwise provided in this Code.

210.3 Rating.
Branch circuits recognized by this article shall be rated in accordance with the maximum permitted ampere rating or setting of the overcurrent device. The rating for other than individual branch circuits shall be 15, 20, 30, 40, and 50 amperes. Where conductors of higher ampacity are used for any reason, the ampere rating or setting of the specified overcurrent device shall determine the circuit rating.

210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire branch circuit shall be permitted to be considered as multiple circuits. All conductors shall originate from the same panelboard.


Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

ARTICLE 225 Outside Branch Circuits and Feeders
225.1 Scope.
This article covers requirements for outside branch circuits and feeders run on or between buildings, structures, or poles on the premises; and electric equipment and wiring for the supply of utilization equipment that is located on or attached to the outside of buildings, structures, or poles.

FPN:For additional information on wiring over 600 volts, see ANSI C2-1997, National Electrical Safety Code.

225.2 Other Articles.
Application of other articles, including additional requirements to specific cases of equipment and conductors, is shown in Table 225.2.

225.7 Lighting Equipment Installed Outdoors.
(A) General. For the supply of lighting equipment installed outdoors, the branch circuits shall comply with Article 210 and 225.7(B) through (D).

(B) Common Neutral. The ampacity of the neutral conductor shall not be less than the maximum net computed load current between the neutral and all ungrounded conductors connected to any one phase of the circuit.


What I believe is common neutral circuits and multi-wire circuits are different animals. You are given permission to use multi-wire circuits if they are designed as defined above.

A circuit utilizing a neutral that is common to one or more others circuits not meeting the definition of a Multi-wire circuit is limited by the code to specific applications

David

 

[don_resqcapt19]

Re: common neutral branch circuits

David,

A circuit utilizing a neutral that is common to one or more others circuits not meeting the definition of a Multi-wire circuit is limited by the code to specific applications

Please cite the code section that limits the use of common grounded conductor circuits. I see the sections that specifically permit the use of a common neutral in some cases, but I do not see a section that prohibits it in all other cases. Just because the code permits A and B, does not automatically prohibit C and D.
Don

 

[ron]

Re: common neutral branch circuits

I would think that a 4 wire feeder to a 3 phase panelboard is sharing that single neutral within the feeder. Hope its not illegal, as most of the county is circuited that way.

 

[don_resqcapt19]

Re: common neutral branch circuits

ron,
The type of circuit that we are talking about is 2 #12 ungrounded conductors, both connected to the 20 amp breakers on the same phase and a #8 grounded conductor that is common to both ungrounded conductors. Why anyone would want to intall this type of circuit, I don't know, but my point is that the code does not speciffically prohibit that circuit.
Don

 

[pierre]

Re: common neutral branch circuits

Don
I also do not see where the situation you mention would not permitted. But if this were permitted could it not lead to overloading the feeder neutral if enough of the circuits shared a common neutral? How would an installer be able to ensure that the panel is balanced?

Pierre

 

[iwire]

Re: common neutral branch circuits

Originally posted by pierre:
But if this were permitted could it not lead to overloading the feeder neutral if enough of the circuits shared a common neutral?

How could you overload the feeder neutral?

 

[roger]

Re: common neutral branch circuits

Pierre, the feeder neutral would be gaurded at its calculated size in most cases. It would be far fetched to imagine no leg to leg or phase to phase loads on the feeder.


Don uses the real term we would have to use if we are talking about one leg or phase, GROUNDED CONDUCTOR, which would be current carrying and not a NEUTRAL anyways.

If enough or all circuits "shared a common neutral" the feeder neutral would be (for sake of conversation) balanced.

Roger

[ August 26, 2003, 07:59 PM: Message edited by: roger ]

 

[rb]

Re: common neutral branch circuits

I came across a dimmer system that was installed in a house located in our area. The system was comprised of eight 20 amp relays and a common neutral bar within a cabinet.The installation instructions allowed for two different methods for wiring the system. Method "A" called for a No. 1/0 grounded conductor to the common neutral bar for the eight 20 amp relays in the cabinet.

Method "B" called for individual No. 12 grounded conductors to the common neutral bar for each branch circuit. The instructions cautioned that when using method "B" great care should be taken to insure that the grounded conductors are of the same length. It went as far as to provide calculations of the imbalance of current that would occur if the grounded conductors were different lengths. Parallel No. 12's for the grounded conductors are obviously a violation of NEC 310-4.

Fortunately, in this case, the contractor used method "A" and installed a No. 2 conductor to the common neutral bar. We concluded that if the equipment was installed in accordance with its listing and labeling there was no problem. So we instructed the contractor to install a No. 1/0 conductor as called for in the instructions.

As far as method "B" is concerned I called UL and the engineer that specializes in dimmers never returned my calls after repeatedly leaving messages on his answering machine. It appears that the National Electrical Code is not a high priority for UL.

Enough of that rant.

Technically the grounded conductor in method "A" is a branch circuit conductor. I can find no provision in the code that allows the grounded conductor of a branch circuit to be calculated based on the maximum unbalance of the load. 210-3 provides that branch circuit sizes are based on the overcurrent device protecting the conductors, in this case twenty amps. Can you have a hybrid 20 amp - 150 amp branch circuit? It appears that these types of installations are bluring the distinction between feeder conductors and branch circuit conductors. Show me in the code where you are permitted to size a grounded branch circuit conductor based on the maximum imbalance of the load.

The grounded branch circuit conductor traditionally was protected by the overcurrent device provided in series with the ungrounded conductor. It appears that this fundamental is getting lost. Perhaps the code should be amended.

 

[david]

Re: common neutral branch circuits

Sorry guys to many meetings last couple of days. I?m headed to another this morning. I have to get back to this. Hopefully later today.

 

[don_resqcapt19]

Re: common neutral branch circuits

rb,

Show me in the code where you are permitted to size a grounded branch circuit conductor based on the maximum imbalance of the load.

What rule covers the sizing of the grounded conductor in a traditional branch circuit?
Don

 

[rb]

Re: common neutral branch circuits

Don,

210-3 appears to specify branch circuit sizes. It does not delineate between ungrounded and grounded circuit conductors. 210-23 provides limits to the maximum loads that may be placed on a twenty amp branch circuit. I contend that the No. 1/0 grounded conductor referenced in method "A" of my original post is, by definition, a branch circuit conductor. So my answer is 210-23. (Sorry about the 1999 references, I left my 2002 at work).

Ron Bethea

 

[roger]

Re: common neutral branch circuits

Ron, could this be grasping for straws? :roll:

Roger

 

[don_resqcapt19]

Re: common neutral branch circuits

rb,
I don't see anything in the two sections that you cited that apply to the conductor size.
Don

 

[karl riley]

Re: common neutral branch circuits

Roger, all circuits do share a common neutral by the time you get to the SE cable, and yet they may not be balanced. More on one phase than the other.

Karl

 

[roger]

Re: common neutral branch circuits

Karl, we agree that the service, if grounded is a multi wire circuit.

The imbalance at this point would not be determined by "one" or "numerous" neutrals, it would be the same in either case.

My statement

If enough or all circuits "shared a common neutral" the feeder neutral would be (for sake of conversation) balanced.

meant balanced as much as the service would be regardless of branch circuit methods if neutrals (not current carrying grounded conductors loaded on one leg or phase) were used.


Roger

[ August 28, 2003, 12:37 PM: Message edited by: roger ]

 

[rb]

Re: common neutral branch circuits

Don and Roger,

Is the grounded conductor in method "A" a branch circuit conductor or a feeder conductor? If so is it a 150 amp branch circuit or a 20 amp branch circuit? Can it be both?

And yes, I am grasping at straws. Work with me here.

[ August 28, 2003, 06:50 PM: Message edited by: rb ]

 

[roger]

Re: common neutral branch circuits

Ron, first I must say I meant no disrespect,in fact it's quite the contrary, I have alot of respect for you.

I'm not clear on A or B realy. Is the dimmer fed from one leg? This would be the only reason to worry about the #12's and would mean they are as you said in parallel, and "current carrying grounded conductors" not "neutrals".

What size OCPD is feeding the cabinet? Is it fed with a 150 amp OCPD or multiple 20's, in either case the 1/0 grounded conductor would be within the NEC in any 150 amp maximum "neutral" or "grounded conductor" branch circuit/s configuration/s IMO.

Roger

 

[karl riley]

Re: common neutral branch circuits

Roger,
I was being picky.

Karl

 

[roger]

Re: common neutral branch circuits

Karl, so was I.

Roger

 

[rb]

Re: common neutral branch circuits

Rodger,

Method "B" utilized a common neutral bar for all eight No. 12 branch circuit grounded conductors. So in effect you are paralleling No. 12 conductors. The common neutral bar connects the individual branch circuit grounded conductors together on their load end. ( I know, I couldn't believe what I was reading either.) This is why the instructions caution about maintaining the same length for each grounded conductor when using method "B".

Method "A" recommended a single No. 1/0 grounded conductor to the common neutral bar.

I took no disrespect from your reply. This is a very interesting topic. I remember arguing this topic on the old forum. If you remember me from the old forum you know that I say a lot of things "tongue in cheek". I sure as heck don't mind being proven wrong. Now back to the straws.

We can't define this arrangement as a multiwire branch circuit because there is no voltage between some of the ungrounded conductors. We can't define these conductors as feeder conductors because the twenty amp circuit breaker protecting the ungrounded conductors is the final overcurrent device. Therefore these must be branch circuit conductors.

210-3 provides that branch circuits recognized by this article are rated in accordance with the maximum permitted ampere rating or setting of the overcurrent device. Based on that, these conductors are part of a twenty amp branch circuit. Presumably these sections apply to the grounded conductor as well as the ungrounded conductor. If not, why is there a need for a permissive rule for multiwire branch circuits? 210-23 provides that "In no case shall the load exceed the rating of the branch circuit." Having established that the grounded conductor is part of a twenty amp branch circuit, how can the load exceed the rating of the branch circuit?

That is why I ask the question "Is this a branch circuit or is it a feeder?"