neutral as current carrying conductor

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Re: neutral as current carrying conductor

Originally posted by bajan: . . . between Neutral and earth there is an extremely small potential
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True. From that fact we may infer that there is no current between the neutral conductor and planet Earth. However, there is a potential difference between the (let us call it) ?neutral side of the load device? and the (let us call it) ?neutral point of the panel.? This potential difference will cause a current to flow from the load to the panel (and from there, ultimately, back to the source) along the neutral conductor. Therefore, we count the neutral as a current-carrying conductor.

What is different about balanced, linear, three phase currents? What allows us not to count the neutral? The answer starts with the same discussion in my first paragraph. There will be a current leaving a load on Phase A, and it will travel along the neutral wire from the load to the point at which the ?shared neutral? splits off to form three separate neutrals (one associated with each phase). There will also be a current leaving a load on Phase B, and a third current leaving a load on Phase C. The three neutral currents will join at the split point, and from this point the net neutral current will flow back to the panel. But if you add the three currents (recall they are not in phase with each other), and if the load is balanced, and if there are no significant harmonics, then the sum of the three neutral currents will be zero. No neutral current will flow back to the panel. Instead,
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  • <font size="2" face="Verdana, Helvetica, sans-serif">The Phase A neutral current flows into the Phase B and Phase C loads</font>
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  • <font size="2" face="Verdana, Helvetica, sans-serif">The Phase B neutral current flows into the Phase A and Phase C loads, and</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">The Phase C neutral current flows into the Phase A and Phase B loads.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">Finally, if the loads are not balanced, the three neutral currents will not add to zero. Also, if there are significant harmonics, then the harmonic currents contained within the neutral currents will not cancel each other, but rather each will add on to the others, creating an even higher current. For these reasons, you must count the neutral if the load is designed to be unbalanced (but who does that, anyway?) or if there are significant harmonics.
 
Re: neutral as current carrying conductor

charlie, if you have 3 devices on the 3 phases, only one is being used, the other two devices are sitting idle, will a current pass through neutral coming off device in use that will not be balanced out by the other 2 phase neutrals which are not in use?
 
Re: neutral as current carrying conductor

Ronson:

The answer to your question is yes. If only one phase is loaded, the line current on the hot conductor will flow through the load and back through the neutral.

Steve
 
Re: neutral as current carrying conductor

i can verify that a neutral can carry a load from firsthand experience. my first year, i was told that a neutral would not shock me. we were wiring a new restaurant. one of the lights had stopped lighting up or was flickering. i went up to check on it. found one of neutrals loose in a 4 sq feeding this light. took off wire nut & proceeded to twist neutral back onto other neutrals barehanded. i had my chest &/or elbow up against grid ceiling & got lit up. was told that the neutral must have been looking for ground & found it through me! i was told that the voltage/amperage was probably greater than 110/15A because it was going through the neutral. it sure felt like it. why would it have felt greater?

[ July 02, 2004, 06:51 PM: Message edited by: ronson ]
 
Re: neutral as current carrying conductor

Why even bother pulling a neutral if its not going to be current carrying?

But in this question it would have to be considered current carrying , what if the recpts. on a and b had nothing plugged into them and the recpt. on c had a 1500 watt heater plugged into it?

Ronald

[ July 02, 2004, 11:41 PM: Message edited by: ronaldrc ]
 
Re: neutral as current carrying conductor

i think a big part of the problem i have had understanding what a neutral is, is that the word "neutral" is kind of a misnomer. when i think of the word "neutral", i think of something being "neutralized" or "zero". this may be true if the load is balanced and cancels all phases (3 or "single" which is another misnomer?). this should be true from the line side of service. on the load side, apparently the neutral side of any single phase of a system may carry current back to the panel where it "zeros" out or the neutral will "zero" out when it meets the neutrals of the other phases on its system. am i right? what is a good book i can get that would explain this in more detail?

[ July 02, 2004, 11:14 PM: Message edited by: ronson ]
 
Re: neutral as current carrying conductor

Originally posted by ronaldrc: Why even bother pulling a neutral if its not going to be current carrying?
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Because in the event of an imbalance, or in the event of an extreme imbalance more commonly called a ?single phase fault,? the existence of a neutral wire reduces the impact on the unaffected phases. It is a design choice, not a code issue.
What if the recpts. on a and b had nothing plugged into them and the recpt. on c had a 1500 watt heater plugged into it?
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It?s a good question. The answer requires us to remember the reason that we de-rate for having more than 3 current carrying conductors in a raceway. The reason is that the heat generated within any one wire causes an increase in the ?ambient? temperature seen by the other wires. If there is a risk of too much heat being generated, then we reduce the risk by reducing the current allowed to flow in any of the wires.

If you have a balanced three-phase feeder with no significant harmonics, then the current carried by the neutral is zero. If it is slightly out of balance, the neutral current will be too small to add to the heat generated within the raceway. Now to your question: If there is a load on only one phase, then you are right in saying that the neutral will ?carry current.? But that is different than our having to treat it as a ?current carrying conductor,? as defined in the NEC. Why the difference? Because as the neutral is carrying current (based on a Phase A load), the Phase B and Phase C conductors are carrying no current, and therefore creating no heat. In your scenario, there are only two conductors that are generating heat. If you then turned on a Phase B load, then there would be three conductors generating heat (Phase A, Phase B, and Neutral). Finally, if you turned on a Phase C load, there would still only be three conductors generating heat (Phase A, Phase B, and Phase C, with the Neutral being idle). In no case will there be four conductors carrying current and generating heat. That is why the neutral is treated as not being a current carrying conductor.

[ July 07, 2004, 10:44 AM: Message edited by: charlie b ]
 
Re: neutral as current carrying conductor

charlie, in a single phase system, would a neutral shared between two ckts, one on each phase, both ckts in use, carry no current?

[ July 07, 2004, 06:56 PM: Message edited by: ronson ]
 
Re: neutral as current carrying conductor

Originally posted by ronson:
charlie, in a single phase system, would a neutral shared between two ckts, one on each phase, both ckts in use, carry no current?
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Only if both legs were loaded exactly the same, see the illustration above, that is an example of this question.

Roger
 
Re: neutral as current carrying conductor

Charlie

It more and likely wouldn't become unbalanced unless you use the neutral as a current carrying conductor. :)

Anytime I think I'm going to be using 120 volts from my circuits I pull a neutral, wouldn't you? :confused:
 
Re: neutral as current carrying conductor

roger, does "loaded" mean how many amps a device (e.g. a hair dryer) is pulling? then could two different ckts using the same neutral have two different loads because of two different devices (e.g. a hair dryer pulling 10A on one & an electric razor pulling 5A on the other)? would this then result in the unbalanced load that would end up on the neutral?
 
Re: neutral as current carrying conductor

Good morning Charlie

As usual you put into words very well why we do not consider the neutral as a current carrying conductor as far as considering the heat it produces.

If I am reading tHirt911s' post wrong please excuse me.

But the way I am comprehending it is that tKirt911 does not understand that a neutral when used as a conductor pulls half of the load when used with a three wire circuit even if the other two legs are not in use.

Roger explains it very good with his drawings please study. tKirt911 :)

Please excuse me Ryan I got the name wrong the first time.

Ronald :)

[ July 09, 2004, 02:58 PM: Message edited by: ronaldrc ]
 
Re: neutral as current carrying conductor

Originally posted by ronaldrc:But the way I am comprehending it is that tkirk911 does not understand that a neutral when used as a conductor pulls half of the load when used with a three wire circuit even if the other two legs are not in use.
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I do not understand the way you worded this sentence. If there are three wires, and the two hot legs are ?not in use,? then there is no current at all, and the neutral will not be pulling half of any load at all. Can I ask you to reword your comment?

[ July 15, 2004, 10:18 AM: Message edited by: charlie b ]
 
Re: neutral as current carrying conductor

Sorry Ryan about using your name in place of tKirt911.

I know from reading your post you understand what a balanced circuit is.

But if one doesn't its no big deal we all have to learn. I know I wouldn't if it hadn't been explained to me in detail with a diagram.

Sorry Charlie I didn't relize I left Ryans name in the upper paragraph I changed it in the lower sentence about Rogers diagrams.

Ronald :)
 
Re: neutral as current carrying conductor

12/2 ?????? are you talking about romex,mc or what? and if so why is it going to be in conduit?
 
Re: neutral as current carrying conductor

I get so frustrated when I open an existing electrical panel and I see where some other electrician, maintainence person, or handyman has incorrectly run what they thought was a three wire circuit with nuetral. They have put both ungrounded feeds on the same phase or line. There is no nuetral in this configuration. The white wire is only a grounded conductor that very likely will be overloaded.

Unfortunately, this happens way too often in residential installs and some commerecial work. I'm sure that some accidents have occurred because of someone's incompetence.
 
Re: neutral as current carrying conductor

I guess I cant resist throwing in my two cents. Comming from a background in power generation I would say that if you dont have unbalanced or single leg loads you dont need a nuetral or even a ground of any kind for it to work properly. there are ungrounded systems in industrial applications that work safely, but I'm getting of track. when you have all three phases or a boat as its commenly reffered to the nuetral only carries the unballanced portion of the load ie: a=2000va b=1500va c=1500va the nuetral will have a 500va load on it. Non linear loads such as electronic ballasts,servers,and other electronic equipment that make more than 25% of the load will have a nuetral sized @ 200% of circuit capacity. I'm not sure if 25% is according to code. Eric
 
Re: neutral as current carrying conductor

Originally posted by srvrrmguy:. . . I would say that if you don?t have unbalanced or single leg loads you don?t need a neutral or even a ground of any kind for it to work properly.
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Quite true. I have designed distribution systems that only serve 3 phase loads, and I have not installed a neutral for those systems.

As to needing a ground (presuming you meant ?equipment grounding conductor?), no system and equipment ever needs a ground for it to work properly. The EGC is there for those times in which something chooses not to work properly. When something fails (i.e., faults), then the EGC will help terminate the event be enabling the breaker to trip. But the EGC never contributes to the normal operation of the system.
 
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