common neutral branch circuits

[iwire]

Re: common neutral branch circuits

Dave, I do not see any violation of 240.4, I even brought this up in my post of August 29, 2003 13:23.

Three 20 amp breakers backing up a 60 amp conductor still meets the requirement of 240.4.

Bob

 

[roger]

Re: common neutral branch circuits

Dave, I agree with Bob. The problem with this thread is as a general concensus we don't use this method, and for that reason only, we want to find a flaw or code violation in useing it.

James, although I know what you are talking about, please tell where "shared neutral" is referenced in the NEC?


Roger

 

[websparky]

Re: common neutral branch circuits

Bob,

Oh! I agree. Sorry, I should have made it clear that I was referring to the 3 12AWG 20A ungrounded conductors having a common neutral of 1 12AWG neutral.

It is odd, when searching through the code, how many times the word "common" is used interchangeably in reference to what we would refer to a "shared" neutral and also a true "common" neutral.

Maybe some rewriting is in order?

 

[iwire]

Re: common neutral branch circuits

I got you Dave, this thread is all over the place and hard to keep track of, if the conductors where all 12 awg I agree with you.

I know that many are aware of this but I think it is worth noticing that the circuit we are taking about (multiple ungrounded conductors returning on one grounded conductor) has No Neutral

The grounded conductor is not a neutral (shared, common or otherwise) in this circuit.

Bob

 

[websparky]

Re: common neutral branch circuits

Well Bob....... Now you have lost me!
I must be doing too many things at once!

If the circuit has no neutral.... what wire is the common wire? Or, am I really lost?

 

[don_resqcapt19]

Re: common neutral branch circuits

Dave,
It can't be a neutral if all of the hots come from the same phase. It is a grounded conductor. You can only have neutrals on multiwire circuits.
Don

 

[iwire]

Re: common neutral branch circuits

Well I could be wrong but I believe for a grounded conductor to be a "neutral" it has to be in a circuit that the grounded conductor only carries the imbalance of two or three grounded conductors from different phases.

If we where to wire a circuit with three ungrounded conductors all from the same phase returning on one grounded conductor, that grounded conductor would not be a neutral as it carries all the current, not just the imbalance.

If I am off on this let me know.

Bob

 

[websparky]

Re: common neutral branch circuits

Don,

OK.......... So I'm getting confused by the terminology. I guess when the code says "common neutral" it is referring to the intentionally grounded conductor of a "multiwire" branch circuit. Right?

2002 NEC Handbook;
If two ungrounded conductors and a common neutral are used as a multiwire branch circuit supplied from a 208Y/120-volt, 3-phase, 4-wire system, the neutral carries the same current as the phase conductor with the highest current and, therefore, should be the same size.

This above is referred to as a "common" neutral. For me, I have always called it a "shared" neutral.

2002 NEC Handbook;
The neutral for a 2-phase, 3-wire or a 2-phase, 5-wire circuit must be sized to carry 140 percent of the ampere rating of the circuit as required by 220.22.

This is also referred to as a "common" neutral.

Then, there is the system that you are describing and you are calling it a "grounded" conductor?

Please help un-confuse me

[ August 29, 2003, 10:44 PM: Message edited by: websparky ]

 

[david]

Re: common neutral branch circuits

What we are discussing is a common neutral circuit, the reason it is a common neutral there is nothing sayings that all circuits have to be on the same phase. We were using the example of all three circuits on the same phase. This was an attempt not to blur this discussion even more. Say three 20- amp circuits two from phase A and one from phase B we all will agree it?s a common neutral. This still is not a multi wire circuit by definition.

[ August 30, 2003, 12:37 AM: Message edited by: david ]

 

[david]

Re: common neutral branch circuits

A multi-wire circuit is a common neutral circuit. Not all common neutral circuits are multi-wire circuits. A multi-wire circuit is defined in article 100 and has specific characteristics. It?s like the small appliance circuit in a kitchen. If you say the two or more required small appliance circuits you talking about circuits with specific characteristics. If you say a circuit feeding a small appliance your talking about a different animal.

 

[david]

Re: common neutral branch circuits

I don?t think I?m going to hit this one out of the park. But I felt I got a nice solid hit and made it to first base. Then I got thrown out for trying to steal second when I used this example
?A contractor utilizes this common neutral circuit by taking three 20 amp breakers fed with # 12 ungrounded conductors fed from the same phase and uses a # 12 common neutral. You are saying this circuit is recognized by article 210; I have no code reference that is specific to call this a violation. According to you I have to pass it.?

Of course the circuit conductors of a branch circuit have to be sized to carry the load.


When I asked what was going to happen to this circuit when the neutral was lost at it?s supply I was jumping ahead to when the circuit designed added a circuit from a second phase.

 

[websparky]

Re: common neutral branch circuits

David,

Bolt type added by me.

"Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them
,(these conductors are on different phases)
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."

Your example below is incorrect.

"What we are discussing is a common neutral circuit, the reason it is a common neutral there is nothing sayings that all circuits have to be on the same phase. We were using the example of all three circuits on the same phase."

[ August 30, 2003, 09:59 AM: Message edited by: websparky ]

 

[websparky]

Re: common neutral branch circuits

 

[roger]

Re: common neutral branch circuits

Dave, the "grounded conductors" in the "Delta diagrams" (C & D) are indeed "neutrals".

We have a center grounded winding no different than diagram (B)

Remove the windings between X1-X2 & X2-X3 and you have the same as (B), the other two windings would have no bearing on this.

A "corner grounded Delta" is a completely different animal.

Roger

[ August 30, 2003, 11:19 AM: Message edited by: roger ]

 

[david]

Re: common neutral branch circuits

Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable

(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).

websparky

Thank you for the drawling. I am aware of the technical distinction the guys are making about the terminology, grounded conductor and neutral. However we are discussing what the NEC considers a common neutral notice above in the NEC both the conductors in your drawling fall under the heading neutral. What you are calling a grounded conductor that is a neutral, above is called a neutral that carries only the unbalanced current. And what you labeled as a grounded conductor that is not a neutral above is called the neutral, a common conductor.

The only example of the branch circuit we are discussing in this thread that I can find in the NEC is found in 225.7 and this conductor termed common neutral is definitely caring current.

225.7 Lighting Equipment Installed Outdoors.
(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.

 

[david]

Re: common neutral branch circuits

This thread was started to address a common neutral branch circuit as described in the NEC. The terminology I was using was derived from the discription of a common neutral branch circuit in 225.7 and a common neutral branch circuit being defined as a multi-wire circuit in article 210.

Common neutral:

I contend for the point of this discussion that both of these circuits are common neutral circuits. I also contend that a multi-wire circuit has a specific definition. The common neutral branch circuit addressed in 225.7 is not a multi-wire circuit.

What I have said is that a common neutral branch circuit of the type described in 225.7 is not recognized by article 210. I said you are given special permission to use this type of common neutral branch circuit out side for lighting. I further have said this type of common neutral branch circuit would not be allowed inside a building. I contend that the only common neutral branch circuit regonized by article 210 is the multi-wire circuit.

National Electrical Code Handbook McGraw-Hills Twenty-Fourth Edition page 195.

? A common neutral may be used only with feeders. It may never be used with branch circuits. A single neutral of a multi-wire branch circuit is not a ?common neutral?. It is the neutral of a single circuit even though the circuit may consist of 3 or 4 wires. A feeder common neutral is used with more than one feeder.?

Handbook McGraw-Hill?s Twenty-Fourth Edition page 240.

225-7. Lighting equipment installed out doors. Part(B) permits a common neutral for outdoor branch circuits-something not permitted for indoor circuits(a neutral of a 3-phase, 4 wire circuit is not a common neutral).

http://ceenews.com/ar/electric_common_neutrals_gym/index.htm

Don said, prove it directly from the NEC not excepting the hand book comments. You can see that challenge has become difficult.

[ August 30, 2003, 11:54 AM: Message edited by: david ]

 

[joe tedesco]

Re: common neutral branch circuits

Using a common grounded conductor (neutral) inside of a building run with multiple branch circuits is not going to present a serious hazard, and it is not a terrible problem, but would require a bit more on behalf of a designer, for example, when sizing the raceway per Table 5, derating, balancing the loads properly, box sizing and fill, etc.

I have seen telephone company designs where they used a larger 2 AWG "common neutral" and there were no problems with that type of design and worked just fine.

PS: I provided the diagram that is missing the specific information attached to it, and sent I the article to david, because it too seems to support some of his comments.

Electrical Neutral - the point, in an electrical system, where the voltage is the instantaneous average of the line terminal voltages during normal operation; or the conductor, in a circuit consisting of three or more conductors, that, is so energized, that in the normal steady state, the voltages from every other conductor to the neutral conductor are definitely related and usually equal in amplitude; the current in the neutral
conductor is the vector sum of the currents in the related conductors; and when connected to ground at the point of a symmetrical system which is normally at zero voltage, it can be the same as the grounded conductor.

IAS IEEE INDUSTRY APPLICATIONS SOCIETY PLEASE REPLY TO:

L. Bruce McClung
Union Carbide Corporation
3200 Kanawha Turnpike
South Charleston, WV 25303 USA
Tel: (304) 747-4916
Fax: (304) 747-5774

IEEE INDUSTRY APPLICATIONS SOCIETY
June 25, 1999
IEEE-SA Standards Board Re: Electrical Neutral
At the March Standards Board Meeting, a resolution was passed requiring the IAS/PES to
develop a definition for "Electrical Neutral".

Messrs. Don Zipse, Lynn Saunders, and Bill Lewis, all IEEE-IAS members, have collaborated with Bruce McClung to develop the following definition:
Electrical Neutral - the point, in an electrical system, where the voltage is the instantaneous average of the line terminal voltages during normal operation; or the conductor, in a circuit consisting of three or more conductors, that, is so energized, that in the normal steady state, the voltages from every other conductor to the neutral conductor are definitely related and usually equal in amplitude; the current in
the neutral conductor is the vector sum of the
currents in the related conductors; and when
connected to ground at the point of a symmetrical
system which is normally at zero voltage, it can be the same as the grounded conductor. The attached series of sketches illustrates an Electrical Neutral.

Sincerely,
L. Bruce McClung
IAS Standards Department Chair
THE INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS, INC.

See also NFPA 70B Chapter 27

27.2.2.4.1 Generators operating in parallel should have the same winding pitch to minimize problems. Where generators operate in parallel with a "common neutral", third harmonic currents can circulate between the machines and cause overheating. High resistance grounding of these generators can adequately limit the harmonic current.

I searched the NFC and no definition of "shared neutral" I could find, except for here:

http://www.kennedyelectric.com/SureTest/Suretest-SharedNeutrals.htm

It is just another slang term used by the field, I use the term "balanced "and the NEC uses the term "equally proportioned" See 210.11(B)

 

[websparky]

Re: common neutral branch circuits

David,

Thanks for your patients and your explanation.

It does clear-up a lot of the terminology!

I agree with Don, that the NEC does not clearly prohibit this type of neutral. In that regard Don, maybe we all could work on some clearer language for future code cycles? Maybe this should be defined?

Thanks Joe for your help and expertise. As always, you seem to have the supporting information at hand!

Thanks to all!
Dave

 

[iwire]

Re: common neutral branch circuits

I would still say the name of this topic "common neutral branch circuits" is incorrect.

A circuit connected at the panel A,A,A,N has no neutral, it has 3 ungrounded and one grounded conductor.

The name of this topic should be "common ungrounded conductor branch circuits"

Minor difference but many items in the code are, GEC, EGC, etc.

Bob

 

[bennie]

Re: common neutral branch circuits

I have expressed my viewpoint on redefining the word "neutral" before.

The proposed definition will make the word a noun instead of an adjective. This will further confuse people, as it violates the rules of grammer.

The word, neutral, should never appear by itself. It is a modifying term for "conductor" which is a noun.

A conductor that starts out as a neutral conductor should retain the same identity from start to finish, even on a 2 wire circuit.