phase a, phase a , and neutral ?

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winnie said:
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
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OK, let me see if you can answer this question:

Let's use A1 and A2 for the two different circuit breakers.

Breaker A1 has a load of 12A.

Breaker A2 has a 17A load.

What would be the current in the common grounded conductor(neutral)?
 
weressl said:
What would be the current in the common grounded conductor(neutral)?
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29A.

My response question is:

What is the problem with 29A flowing on a suitably rated conductor?

Where the NEC specifically describes the use of these circuits, it requires that the neutral be sized to carry the maximum possible unbalance current.

For example, A1 could be a 20A breaker supplying a 12ga conductor, and A2 could be a 20A breaker supplying a 12ga conductor. If the shared neutral is a 8ga conductor, then the maximum unbalance current (40A) would not overload the neutral.

-Jon
 
winnie said:
29A.

My response question is:

What is the problem with 29A flowing on a suitably rated conductor?

Where the NEC specifically describes the use of these circuits, it requires that the neutral be sized to carry the maximum possible unbalance current.

For example, A1 could be a 20A breaker supplying a 12ga conductor, and A2 could be a 20A breaker supplying a 12ga conductor. If the shared neutral is a 8ga conductor, then the maximum unbalance current (40A) would not overload the neutral.

-Jon
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This is NOT a case of unballanced currents. Unbalance occurs between different phases, such as A and B, but not A and A.

I am not aware of NM or BX cables constructed with different size neutral. Nor am I aware of receptacles accepting larger than #10 conductors.

I do not think this question should have any merit in practical use. Hence my 'outlandish' qualifier. A similar question would be; can we use nuclear fission for welding. Sure you can, but do you really want to do it?
 
weressl said:
This is NOT a case of unballanced currents. Unbalance occurs between different phases, such as A and B, but not A and A.

I am not aware of NM or BX cables constructed with different size neutral. Nor am I aware of receptacles accepting larger than #10 conductors.

I do not think this question should have any merit in practical use. Hence my 'outlandish' qualifier. A similar question would be; can we use nuclear fission for welding. Sure you can, but do you really want to do it?
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Southwire has 12/1 and 12/2 and 12/3 12/4 MC with # 10 neutral and 12/3 with #8 or #10 neutral.

Also, you would not have to have receptacles that accept larger conductors.
 
Some folks have mentioned Art 225 and common neutrals.

Earlier I mentioned 215.4(A) which allows multiple feeders to have a common neutral. If we had 2 sets of 3 phase 208Y/120 feeders with a common neutral feeding 120 volt loads, it is not impossible that only the A phase loads could be on giving a situation exactly as asked by the OP.

Laszlo, again, I am quite certain that everyone understands that the current on the neutral would be the addition of whatever current was on the individual circuits. We all get that.

The question is, is it a specific code violation?
 
crossman said:
Laszlo, again, I am quite certain that everyone understands that the current on the neutral would be the addition of whatever current was on the individual circuits. We all get that.
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Oh no we don't, only Laszlo has that advanced knowledge. :grin:
 
iwire said:
Oh no we don't, only Laszlo has that advanced knowledge. :grin:
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That is why I refered to it as an 'outlandish' questions. I did not presented as something only I know, I responded to the question. Now if EVERYBODY understood and It is so fundamental then why was it questioned in the first place?

So your ad hominem goading is that is out of order and uncalled for. Not that it is the first time either. That is not how a 'moderator' should behave. Look up the word sometime.
 
weressl said:
That is why I refered to it as an 'outlandish' questions. I did not presented as something only I know, I responded to the question. Now if EVERYBODY understood and It is so fundamental then why was it questioned in the first place?
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Sorry, I have no idea the point you are trying to make.
 
weressl said:
This is NOT a case of unballanced currents. Unbalance occurs between different phases, such as A and B, but not A and A.
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I disagree.

_Balanced_ current can only occur when you have different phases sharing the same neutral. If the two circuits sharing the neutral are on the same phase, then the net current on the neutral is clearly unbalanced.

That said, as I recall the wording of the paragraph in section 215 that deals with such common neutral circuits, the description implied that multiple circuits from multiple phases were being used. For example 6 20A circuits A,A',B,B',C,C' all sharing a single 40A neutral.

Clearly in such a situation you have the balanced current which doesn't flow on the neutral, and the unbalanced current which does flow on the neutral.

Again, the only thing that I disagree with is the way that you characterized the circuit described as 'outlandish' and simply dismissed it out of hand. It can't be too outlandish if it is actually described as a method of installation in the NEC. I _agree_ with you that is almost certainly is not a good installation practice, however this is my gut understanding...but if someone came along with a good reason to want to install such a circuit, I have no good engineering reason that such should be prohibited, and was wondering if you had any good reason other than your own common gut understanding.

-Jon
 
winnie said:
I disagree.

_Balanced_ current can only occur when you have different phases sharing the same neutral. If the two circuits sharing the neutral are on the same phase, then the net current on the neutral is clearly unbalanced.

That said, as I recall the wording of the paragraph in section 215 that deals with such common neutral circuits, the description implied that multiple circuits from multiple phases were being used. For example 6 20A circuits A,A',B,B',C,C' all sharing a single 40A neutral.

Clearly in such a situation you have the balanced current which doesn't flow on the neutral, and the unbalanced current which does flow on the neutral.

Again, the only thing that I disagree with is the way that you characterized the circuit described as 'outlandish' and simply dismissed it out of hand. It can't be too outlandish if it is actually described as a method of installation in the NEC. I _agree_ with you that is almost certainly is not a good installation practice, however this is my gut understanding...but if someone came along with a good reason to want to install such a circuit, I have no good engineering reason that such should be prohibited, and was wondering if you had any good reason other than your own common gut understanding.

-Jon
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Unbalanced current refers to the out of phase portion of the current flow difference when there are a mixture of different phase to phase loads in a three phase system.

In a circuit where common grounded conductor used for the two or more circuitsof the same phase, the current flow will ALWAYS be the sum of currents flowing in each of th ungrounded conductors.
 
weressl said:
... I am not aware of NM or BX cables constructed with different size neutral. Nor am I aware of receptacles accepting larger than #10 conductors.

I do not think this question should have any merit in practical use. Hence my 'outlandish' qualifier.
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Not all of us use cable wiring methods so the cable comment does not apply. There are any number of practical applicaitons for this. The most common would be a panel replacement where the original panel is being used and a J-box and you install one correctly sized neutral along with all of the phase conductors between the new panel and the junction box. As far as at the receptacle you can always pigtail a smaller conductor at that point. It is unlikely that you would run the "common" neutral to a device. It would stop and splice to smaller conductors at junction point. The smaller neutrals would be sized based on the rating of the OCPD that protects the assoctiated phase conductor.
 
don_resqcapt19 said:
Not all of us use cable wiring methods so the cable comment does not apply. There are any number of practical applicaitons for this. The most common would be a panel replacement where the original panel is being used and a J-box and you install one correctly sized neutral along with all of the phase conductors between the new panel and the junction box. As far as at the receptacle you can always pigtail a smaller conductor at that point. It is unlikely that you would run the "common" neutral to a device. It would stop and splice to smaller conductors at junction point. The smaller neutrals would be sized based on the rating of the OCPD that protects the assoctiated phase conductor.
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In theory it is a nice and workable solution.
But what happens if your installer sized the common neutral to say 20 ea. 20A circuit breakers, yet he only needed to install 16 and all the rest of them are spare spaces. So subsequent needs add circuit breakers and all connect to the common neutral. But what happens when somebody comes along and installs the 21st. breaker, still using the common neautral?

Which scenario is more likely? The guy just adds a breaker or does he go and investigate the size of thet pesky common neutral?
 
weressl said:
In theory it is a nice and workable solution.
But what happens if your installer sized the common neutral to say 20 ea. 20A circuit breakers, yet he only needed to install 16 and all the rest of them are spare spaces. So subsequent needs add circuit breakers and all connect to the common neutral. But what happens when somebody comes along and installs the 21st. breaker, still using the common neautral?

Which scenario is more likely? The guy just adds a breaker or does he go and investigate the size of thet pesky common neutral?
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You can do the what happens to any installation. The code does not require anyone to provide for the future.
 
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