phase a, phase a , and neutral ?

[hillbilly1]

Actually you did:



There would only be a C in a 3 phase system.

What part of "A" phase + "B" phase OR "A" phase + "C" phase is three phase? It may be a three phase system, but when only two legs are used it is SINGLE PHASE.

 

[infinity]

What part of "A" phase + "B" phase OR "A" phase + "C" phase is three phase? It may be a three phase system, but when only two legs are used it is SINGLE PHASE.

If the system contains A, B & C phase it's a three phase system. Even if you had a MWBC using two of the three phases and sharing a neutral the neutral would carry current almost equal to that of the two phase conductors. It would not be a simple arithmetic problem as in a single phase system. For a single phase system A= 10 amps, B= 10 amps, N= 0 amps.

Take a look at 310.15(B)(4)(b).

BTW I appreciate the sarcasm.

 

[hillbilly1]

If the system contains A, B & C phase it's a three phase system. Even if you had a MWBC using two of the three phases and sharing a neutral the neutral would carry current almost equal to that of the two phase conductors. It would not be a simple arithmetic problem as in a single phase system. For a single phase system A= 10 amps, B= 10 amps, N= 0 amps.

Take a look at 310.15(B)(4)(b).

BTW I appreciate the sarcasm.

OK, then explain where the neutral current will go if it is a 208 0r 240 volt load using only "A" phase and "B" phase both at 10 amps with NO neutral. Hmmm, another single phase load. 310.15(B)(4)(b) is an interesting statement, It does not include three phase Delta's. I will have to do some research on this one, because it does not make sense. Maybe you can teach an old dog new tricks!

 

[charlie b]

OK, then explain where the neutral current will go if it is a 208 0r 240 volt load using only "A" phase and "B" phase both at 10 amps with NO neutral.

There is no neutral current to go anywhere. I don't understand the question. :-?

 

[infinity]

OK, then explain where the neutral current will go if it is a 208 0r 240 volt load using only "A" phase and "B" phase both at 10 amps with NO neutral. Hmmm, another single phase load. 310.15(B)(4)(b) is an interesting statement, It does not include three phase Delta's. I will have to do some research on this one, because it does not make sense. Maybe you can teach an old dog new tricks!

OK old dog here's a new trick. The link:

http://ecmweb.com/mag/electric_commercial_load_calculations/

 

[mivey]

Roll over Rover

Roll over Rover

OK old dog here's a new trick. The link:

http://ecmweb.com/mag/electric_commercial_load_calculations/

Not quite Rob. He stated there was no neutral. The method you are showing would have a neutral and line current of 11.547 amps instead of the 10 you would get for the line-line case proposed by hillbilly1.

Take a load of (2) 10.3923 ohm resistors in series and a 208 volt supply line-line and you will get 10 amps.

Tie the neutral to the junction of these two resistors and now you will get 11.547 amps in the line and neutral.

 

[weressl]

I have a reasonable understanding of electrical theory and am comfortable with the NEC........

But I can not pin down a rule that specifically restricts running circuits in this manner.(A,A,& Neutral or B,B,B & neutral, ect.)


Originally I thought I would find the answer around one of these sections and came up short 310.4 ,Article 100 def. multi-wire branch circuit,300.3(B).....

Anybody got an answer for me?

Thanks

The Code is not an engineering document. The reasons why you can't share a neutral of the same phase are outside of the Codes perview.

If that is not sufficient for you then you should ask back for your tuition for apprenticeship because you should have sufficient technical knowledge to know this.

 

[infinity]

Not quite Rob. He stated there was no neutral. The method you are showing would have a neutral and line current of 11.547 amps instead of the 10 you would get for the line-line case proposed by hillbilly1.

Take a load of (2) 10.3923 ohm resistors in series and a 208 volt supply line-line and you will get 10 amps.

Tie the neutral to the junction of these two resistors and now you will get 11.547 amps in the line and neutral.

Actually that is not the post I was responding to. If you look back towards the beginning of the thread the comment was this:

I can't think of a code section, but the electrical gods would be unhappy with you, using the same phase for both or all three hots can seriously overload the neutral, If you have 15 amps on one circuit on "A", added another "A" circuit with say, 15 amps to make it simple, would produce 30 amps on the neutral. But if you changed one of the circuits to "B" or "C" the neutral load would be zero.

My response was that to have an A, B, & C phase you would have a 3 phase system. By switching from the A-A & N to A-B & N or A-C & N, as suggested above, you would not have zero amps on the neutral.

 

[crossman]

If that is not sufficient for you then you should ask back for your tuition for apprenticeship because you should have sufficient technical knowledge to know this.

Poor form is as poor form does.

That is all I have to say about that.

 

[infinity]

The Code is not an engineering document. The reasons why you can't share a neutral of the same phase are outside of the Codes perview.

So then do you agree that it's is permitted to have an oversized neutral sharing circuits on the same phase?

 

[mivey]

Actually that is not the post I was responding to. If you look back towards the beginning of the thread the comment was this:




My response was that to have an A, B, & C phase you would have a 3 phase system. By switching from the A-A & N to A-B & N or A-C & N, as suggested above, you would not have zero amps on the neutral.

I agree with your assessment of the earlier question.

Silly me tied the dog reference question to the dog reference answer.

[add: not to mention it was the question you referenced in your answer.

 

[winnie]

The circuit described in the original posting is called a "common neutral circuit", and is explicitly permitted for _outside_ branch circuits. (Or at least was as of the 2005 code; I'm on the road and can't check the 2008 code.) This is where the handbook image that iwire posted came from.

It is neither explicitly permitted nor prohibited for other branch circuits.

If you rigidly follow the concept that anything that the code doesn't prohibit is permitted, then such a circuit is permitted.

However an extremely common interpretation of the code is that the explicit permission for outside circuits, combined with the _lack_ of explicit permission for other circuits, implies that such circuits are prohibited for other other circuits.

The only situation that I can see where there _might_ be some benefit is to use the full 310.16 ampacity of 10 and 12 ga conductors.

-Jon

 

[ibew441dc]

The Code is not an engineering document. The reasons why you can't share a neutral of the same phase are outside of the Codes perview.

If that is not sufficient for you then you should ask back for your tuition for apprenticeship because you should have sufficient technical knowledge to know this.

Sounds like you just have another opinion with nothing to back it up.....thanks for your ''staff engineering insight'' , sure added to the discussion. Sorry if my original post offended you, next time I'll make sure to run it by you first.

 

[infinity]

The circuit described in the original posting is called a "common neutral circuit", and is explicitly permitted for _outside_ branch circuits. (Or at least was as of the 2005 code; I'm on the road and can't check the 2008 code.) This is where the handbook image that iwire posted came from.

It is neither explicitly permitted nor prohibited for other branch circuits.

If you rigidly follow the concept that anything that the code doesn't prohibit is permitted, then such a circuit is permitted.

However an extremely common interpretation of the code is that the explicit permission for outside circuits, combined with the _lack_ of explicit permission for other circuits, implies that such circuits are prohibited for other other circuits.

The only situation that I can see where there _might_ be some benefit is to use the full 310.16 ampacity of 10 and 12 ga conductors.

-Jon

Yes that illustration is from Article 225 which says this:

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 electrical equipment and wiring for the supply of utilization equipment that is located on or attached to the outside of buildings, structures, or poles.

Nothing in the scope of that article about interior building wiring methods.

 

[iwire]

Nothing in the scope of that article about interior building wiring methods.

That is correct but I don't see that anything in 225 that actually says a common neutral shall be permitted, all I see is that when you do use a common neutral it shall be sized properly.

It is my opinion that there is nothing in the NEC that prohibits a common neutral.

 

[roger]

Sounds like you just have another opinion with nothing to back it up.....thanks for your ''staff engineering insight'' , sure added to the discussion. Sorry if my original post offended you, next time I'll make sure to run it by you first.

Don't worry about Laszlo, we don't.

If you have a question feel free to ask it. As shown by the respondents in this thread, most will be courteous in their replies.

Roger

 

[Pierre C Belarge]

The Code is not an engineering document. The reasons why you can't share a neutral of the same phase are outside of the Codes perview.

If that is not sufficient for you then you should ask back for your tuition for apprenticeship because you should have sufficient technical knowledge to know this.

I did not read through the all of the responses...if you already explained this, tell me at what post number you did.


Otherwise, in terms we can understand or a link, please explain to me/us why this is not permitted outside of the code language.

 

[weressl]

It's happened before. But the only ones I have seen were separate breakers, two of which could be put into the same space as a single, "normal size" breaker. Are you saying that there are "tandem breakers" that share a single overcurrent device, but that supply separate loads? I am not asking about two breaker handles in a single case, for that IMHO would comprise two breakers, and could not, therefore, supply a single "branch circuit."

Well, the OP idea is so outlandish - to be nice - that I wouldn't put it past that they would also terminate both circuits on the same receptacles or same circuit whatever.

 

[winnie]

Well, the OP idea is so outlandish - to be nice - that I wouldn't put it past that they would also terminate both circuits on the same receptacles or same circuit whatever.

Yet this 'outlandish' concept is an explicit part of the NEC, if only for _outside_ circuits. Similarly, feeders with a common neutral are explicitly described.

Clearly, at some point in the past, someone thought that these were a good enough idea that the NEC placed strictures on how to use these methods.

My gut tells me 'Unexpected and confusing design with no significant benefits.' Do you have a firmer reason for not using such circuits?

-Jon

 

[crossman]

Well, the OP idea is so outlandish - to be nice - that I wouldn't put it past that they would also terminate both circuits on the same receptacles or same circuit whatever.

And, do you have any concrete reasons to make such an outlandish assumption about the OPs knowledge and education?

Everyone on this thread understands the theory behind it. It is a valid point to ask if the code specifically prohibits it. There is nothing outlandish about it at all.