Phase to neutral wiring
- Thread starter stevej
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- Lockport, IL
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- Retired Electrical Engineer
Re: Phase to neutral wiring
It doesn't. A three-phase system that only serves three-phase loads (e.g., a motor control center) does not require a neutral. A single phase branch circuit to a 240 volt load does not require a neutral.
But I am not sure I understood your question in the way you had intended. What is it that you are trying to do, to confirm, to refute, or to understand?
It doesn't. A three-phase system that only serves three-phase loads (e.g., a motor control center) does not require a neutral. A single phase branch circuit to a 240 volt load does not require a neutral.
But I am not sure I understood your question in the way you had intended. What is it that you are trying to do, to confirm, to refute, or to understand?
electricman2
Senior Member
- Location
- North Carolina
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- Retired Electrical Contractor
Re: Phase to neutral wiring
For grounded AC systems, the grounded conductor (neutral) is required to be run with the phase conductors to the service disconnecting means per 250.24(C) but beyond that is not required
For grounded AC systems, the grounded conductor (neutral) is required to be run with the phase conductors to the service disconnecting means per 250.24(C) but beyond that is not required
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Phase to neutral wiring
I'm still not sure I understand you. I am guessing that you are not talking about the "service," the conductors coming from the utility, but rather are discussing a branch circuit. It might be a "multi-wire branch circuit," but I'm not sure about that either.
Are you saying that from the service panel (or from some other sub-panel downstream of the service), you have one conduit with five wires: 3 ungrounded ("hot") wires from phase A, 1 ungrounded ("hot") wire from phase C, and one grounded ("neutral") wire? If so, before I can look up the code that is being violated (and I think there is one), please tell me one more thing: Are these serving one single load, or are there four loads that are sharing the single neutral wire?
I'm still not sure I understand you. I am guessing that you are not talking about the "service," the conductors coming from the utility, but rather are discussing a branch circuit. It might be a "multi-wire branch circuit," but I'm not sure about that either.
Are you saying that from the service panel (or from some other sub-panel downstream of the service), you have one conduit with five wires: 3 ungrounded ("hot") wires from phase A, 1 ungrounded ("hot") wire from phase C, and one grounded ("neutral") wire? If so, before I can look up the code that is being violated (and I think there is one), please tell me one more thing: Are these serving one single load, or are there four loads that are sharing the single neutral wire?
Re: Phase to neutral wiring
I think they are each feeding different ckts, All 120v to N.
For example, one a 3 phase system, you have 3 "hots", 1 "n" and maybe a ground wire. On mult-wire branch ckts,you can't use the "n" for something with 2 "A","B" or "C" but you can use this "n" for 1 hot of different phase. I guess the same holds true for a 1 phase system. You can't run 2 blk wires to different repts with 1 "n" i.e, #1,2,3. This is something almost every EC knows from day 1. I just had never asked to prove it.
I KNOW something is vololated but I'm looking for proof.
I think they are each feeding different ckts, All 120v to N.
For example, one a 3 phase system, you have 3 "hots", 1 "n" and maybe a ground wire. On mult-wire branch ckts,you can't use the "n" for something with 2 "A","B" or "C" but you can use this "n" for 1 hot of different phase. I guess the same holds true for a 1 phase system. You can't run 2 blk wires to different repts with 1 "n" i.e, #1,2,3. This is something almost every EC knows from day 1. I just had never asked to prove it.
I KNOW something is vololated but I'm looking for proof.
Re: Phase to neutral wiring
Stevej, if your system is in fact a 120/240 delta, you would only be able to use A phase and B phase with the grounded conductor, C would be a high leg and would be around 208 volts measured to the grounded conductor.
Are you sure you don't have a 120y/208v system?
Now, what are the size of the ungrounded and grounded conductors?
Roger
Stevej, if your system is in fact a 120/240 delta, you would only be able to use A phase and B phase with the grounded conductor, C would be a high leg and would be around 208 volts measured to the grounded conductor.
Are you sure you don't have a 120y/208v system?
Now, what are the size of the ungrounded and grounded conductors?
Roger
Re: Phase to neutral wiring
For three phase wye applications, each phase is 120 degrees apart from each other and the total neutral load is balanced out. If you share the neutral between the same phase, then the neutral load will be additive, which in many cases will overload the neutral. For example, lets say you have three 20A breakers on different circuits - each phase A, sharing the neutral, and the loads for each are 7A,9A,14A. The neutral load will add up to 30A, thus overloading it. If the neutral is shared between opposite phases, the neutral will only carry the unbalanced portion. This can be explained better but I think this is the basic idea. I'm not sure of code references but there is an engineering explanation.
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David - St. Louis,MO
For three phase wye applications, each phase is 120 degrees apart from each other and the total neutral load is balanced out. If you share the neutral between the same phase, then the neutral load will be additive, which in many cases will overload the neutral. For example, lets say you have three 20A breakers on different circuits - each phase A, sharing the neutral, and the loads for each are 7A,9A,14A. The neutral load will add up to 30A, thus overloading it. If the neutral is shared between opposite phases, the neutral will only carry the unbalanced portion. This can be explained better but I think this is the basic idea. I'm not sure of code references but there is an engineering explanation.
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David - St. Louis,MO
Dnkldorf
Senior Member
Re: Phase to neutral wiring
Am I confused here?
When does nuetral current cancel in a wye system using multiwire circuits?
The phase angles are off by 120 degrees, aren't they.
I thought nuetral currents cancel on the nuetral only on split phase systems because the phase angles added up to 0.
What am I missing?
Dnk......
Am I confused here?
When does nuetral current cancel in a wye system using multiwire circuits?
The phase angles are off by 120 degrees, aren't they.
I thought nuetral currents cancel on the nuetral only on split phase systems because the phase angles added up to 0.
What am I missing?
Dnk......
Re: Phase to neutral wiring
dpenbert has the best answer. I think we are reading too much into the question.
Basicly, why do we have to use the "N" with a "A", "B", "C" with 3 phase wiring and a "N" with a "A" and "B" with a 1 phase wiring. Where does the NEC say do this?
We all know we have to do this, but does the code says we have to do this?
dpenbert has the best answer. I think we are reading too much into the question.
Basicly, why do we have to use the "N" with a "A", "B", "C" with 3 phase wiring and a "N" with a "A" and "B" with a 1 phase wiring. Where does the NEC say do this?
We all know we have to do this, but does the code says we have to do this?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: Phase to neutral wiring
OK. Now that I think I understand the question, I have to admit that (1) It's a good question, and (2) I don't know the answer. But let me restate the question, in what I hope will be a clearer way, and see if anyone else can cite the code article (if any) that would be violated.
</font>
However, the white wire will see 32 amps of continuous current, and that exceeds its rating. Obviously no good. But I'm drawing a blank: What article says you can't do that?
Now let me complicate matters a bit. Let's make the white wire a #8. Use pigtails at each load, to allow connection of the white wire. Now the white wire will not be loaded beyond its ampacity. Is there a code violation, and if so, please provide the article.
Stevej: Did I paraphrase your question correctly?
OK. Now that I think I understand the question, I have to admit that (1) It's a good question, and (2) I don't know the answer. But let me restate the question, in what I hope will be a clearer way, and see if anyone else can cite the code article (if any) that would be violated.
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Start with the main panel in a house. Let it be the only panel in the house. Let it be single phase, 120/240 volt.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Connect black #12 AWG THHN wires to the 20 amp breakers in slots 1, 2, 3, and 5.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Connect one white #12 AWG THHN to the neutral bar, and one bare #12 to the ground bar.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Run all six of these wires out the panel, through a single 3/4 inch conduit.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Connect each of the four black wires to a separate load. Bring the same white wire to each of the loads.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Let's say that all four loads run at the same time, all are continuous, and each draws 16 amps.</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Do you have a code violation, and if so, please provide the article.</font>
However, the white wire will see 32 amps of continuous current, and that exceeds its rating. Obviously no good. But I'm drawing a blank: What article says you can't do that?
Now let me complicate matters a bit. Let's make the white wire a #8. Use pigtails at each load, to allow connection of the white wire. Now the white wire will not be loaded beyond its ampacity. Is there a code violation, and if so, please provide the article.
Stevej: Did I paraphrase your question correctly?
jwelectric
Senior Member
- Location
- North Carolina
Re: Phase to neutral wiring
When you are able to answer this then and only then will we be able to explain what happens in a circuit that has no grounded (neutral) conductor such as a water heater or air conditioner.
Just what do you think that the grounded (neutral) conductor does?
When you are able to answer this then and only then will we be able to explain what happens in a circuit that has no grounded (neutral) conductor such as a water heater or air conditioner.
Dnkldorf
Senior Member
Re: Phase to neutral wiring
Jw, just when I thought I knew something I read this.
http://www.allaboutcircuits.com/vol_2/chpt_10/2.html
Care to enlighten me further.......
Dnk.......
Jw, just when I thought I knew something I read this.
http://www.allaboutcircuits.com/vol_2/chpt_10/2.html
Care to enlighten me further.......
Dnk.......
Re: Phase to neutral wiring
Charlie, in your example if you used a conductor with an ampacity of 32 amps or greater, it would be legal.
225.7 (B) specifically allows it.
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.
Roger
[ August 31, 2005, 06:30 PM: Message edited by: roger ]
Charlie, in your example if you used a conductor with an ampacity of 32 amps or greater, it would be legal.
225.7 (B) specifically allows it.
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.
Roger
[ August 31, 2005, 06:30 PM: Message edited by: roger ]
- Location
- Illinois
- Occupation
- retired electrician
Re: Phase to neutral wiring
Charlie,
The code is not really clear on your first example. The only section that I can think of to prohibit that installation is:
Don
Charlie,
The code is not really clear on your first example. The only section that I can think of to prohibit that installation is:
As far as the second example, I don't see anything in the code to prohibit its use. There are two specific sections that permit this use, but none that prohibit it.
Don
tx2step
Senior Member
- Location
- North Texas, DFW area
Re: Phase to neutral wiring
Where is the second section that permits this?
I agree that I don't know of any code that prohibits using a common neutral (such as Charlie described in his second example) for any application that I'm aware of. It looks to me like you are free to use a "common neutral" anywhere you'd like as long as you size it properly.
On the other hand, I can't think of any place where I'd want to use one. Can you think of any examples where one would be an advantage, preferable or desirable?
Don, I know that 225.7 (as quoted earlier by Roger) allows this for outdoor lighting.
Where is the second section that permits this?
I agree that I don't know of any code that prohibits using a common neutral (such as Charlie described in his second example) for any application that I'm aware of. It looks to me like you are free to use a "common neutral" anywhere you'd like as long as you size it properly.
On the other hand, I can't think of any place where I'd want to use one. Can you think of any examples where one would be an advantage, preferable or desirable?
Re: Phase to neutral wiring
In a wye distribution, each phase voltage and current is 120 degrees behind(lagging) the phase before it. If each phase carries equal amounts of current then you have a balanced system. In other words the equivalent currents will cancel each other out because they combine at the common neutral after return from the source. Of course, this is theoretical and not realistic to achieve a perfectly balanced system unless the loads are purely resistive (linear) and in equilibrium. To prove this, use this formula: square root((A^2+B^2+C^2)-(AB+BC+CA)) = Neutral Current. If all phases are equal you will get zero.
I understand this is a code related question, just wanted to try and clear up what I said for Dnkldorf. I am also curious if sharing neutrals between same phases could be a violation, because that would not be a very good practice IMO, and I know there are many people who are against sharing neutrals between different phases and equipment but that will continue because of economics. Just my 2 cents.
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David - St. Louis,MO
In a wye distribution, each phase voltage and current is 120 degrees behind(lagging) the phase before it. If each phase carries equal amounts of current then you have a balanced system. In other words the equivalent currents will cancel each other out because they combine at the common neutral after return from the source. Of course, this is theoretical and not realistic to achieve a perfectly balanced system unless the loads are purely resistive (linear) and in equilibrium. To prove this, use this formula: square root((A^2+B^2+C^2)-(AB+BC+CA)) = Neutral Current. If all phases are equal you will get zero.
I understand this is a code related question, just wanted to try and clear up what I said for Dnkldorf. I am also curious if sharing neutrals between same phases could be a violation, because that would not be a very good practice IMO, and I know there are many people who are against sharing neutrals between different phases and equipment but that will continue because of economics. Just my 2 cents.
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David - St. Louis,MO
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