Derating and Neutral as Current Carrying Conductor

[cnl4]

I am looking for some input on my interpretation of 310.15(B)(4)(a).

As I understand it, this section is referring to a neutral conductor in a polyphase system that only carries unbalanced current from a "circuit" such as a 3 phase 4 wire Wye.

Examples would be a panel feeder, or a full boat (3 hot conductors of different phases, a common neutral, and hopefully an equipment grounding conductor:roll: ) serving convenience receptacles where the loads have been properly engineered.

The reason I am asking the question is that the company I work for has recently been embroiled in a debate with an engineering firm over what counts as a "current carrying conductor".

Briefly, we were forced by the Engineer and Construction Manager to abandon, and fill with concrete, all branch circuit conduits run in the underground and utilize 2" underground conduits that we believed were intended as future conduits for branch circuits (each panel has one 2" conduit that crosses an open area underground).

Their position is that in a 20 amp branch circuit, with a dedicated neutral, the neutral is not a CCC . This is allowing them to use a much lower derating factor (one of these conduits has 23 single phase circuits with dedicated neutrals) than we feel needs to be used. If they were using proper derating factors, the 2" conduits would not be sufficient and they would have to give us a change order to reroute this stuff overhead.

So, in a nutshell, what is eveyone's opinion on when it is necessary to count a neutral as a current carrying conductor?

 

[raider1]

If you have a 20 amp circuit with a dedicated neutral then the neutral will carry the same amount of current as the ungrounded conductor and as such should be counted as a current carrying conductor for derating purposes.

A multi-wire branch circuit that incorporates 3 ungrounded condcutors sharing a common grounded conductor in a 3 phase 4 wire system, would have a neutral that carrys only the unbalanced current from the 3 phase conductors. This neutral would not be counted as a current carrying conductor, unless the circuits have a major portion of the load consisting of nonlinear loads.

Hope this helps,

Chris

 

[charlie b]

I agree with Chris, and I think it is so obvious that I cannot fathom the engineer?s train of thoughts. 310.B(4)(a) is very clear. A neutral would not count if it only carries unbalanced current from other conductors (plural) of the same circuit. If by ?dedicated circuit,? you mean what I think you mean (one phase, one neutral, plus ground), then there are no other conductors (plural) to carry current and to have a left over (i.e., unbalanced) current for the neutral to carry. The neutral carries it all.

This was a serious, fundamental error on the part of the engineer.

 

[cnl4]

If you have a 20 amp circuit with a dedicated neutral then the neutral will carry the same amount of current as the ungrounded conductor and as such should be counted as a current carrying conductor for derating purposes.

A multi-wire branch circuit that incorporates 3 ungrounded condcutors sharing a common grounded conductor in a 3 phase 4 wire system, would have a neutral that carrys only the unbalanced current from the 3 phase conductors. This neutral would not be counted as a current carrying conductor, unless the circuits have a major portion of the load consisting of nonlinear loads.

Hope this helps,

Chris

It does help. For a minute there I thought I was losing my touch. Having an Engineer tell you that what you have known for years is wrong is disconcerting :wink: .

 

[roger]

Show the following simple illustration to the engineer.

Roger

 

[raider1]

Having an Engineer tell you that what you have known for years is wrong is disconcerting

I agree, engineers are just like any other profession there are great engineers and there are ones that need to go back to school, just like there are great electricians and ones that I wouldn't have wire my dog house.

Chris

 

[mikeames]

We have been around this horn before. If the engineers used the proper terms this would not be an issue. In a 120 volt circuit there is no technical "neutral" end of story. One phase conductor, one grounded CONDUCTOR and one EGC. So there is no neutral to derate.

 

[charlie b]

If the engineers used the proper terms this would not be an issue. In a 120 volt circuit there is no technical "neutral" end of story.

Nah, that?s just the beginning of the story. And it?s not so easy for us enjuinears to fingure out. :roll:

Be so kind as to look at the title of article 310.15(B)(4), and look also at the number of times the word ?newutral? appears in that article.

 

[cnl4]

Thanks for the input all.

To clarify, yes, by "dedicated neutral" ( a very common term, at least in my geographic area) I mean each single phase hot conductor originating at a single pole breaker is to have it's own white grounded conductor.

In a 120 volt circuit there is no technical "neutral" end of story.

Not quite true there mikeames. If you have circuits 1,3,5 in a 120/208,3 phase,4 wire system on single pole breakers feeding some sort of loads and you only provide one grounded circuit conductor for the 3 circuits you have 3 120v circuits and a "neutral" conductor. Same would hold true for circuits 1 and 3 in a "single" phase 3 wire system if they shared a common grounded conductor.

And for what it's worth mikeames, you have a long, uphill fight ahead of you if you are going to embark on a crusade to get sparkys to quit calling that white wire the neutral, regardless of what it truly is.

For posterities sake, does anyone know for a fact whether the term "neutral" has to do with currents of different phases canceling ("neutralizing" ) each other, or if it has to do with the grounded conductor being "neutral" in respect to the system ground?

Just thought I'd throw that in to stir the pot. It is entirely possible I made the part about "neutral in respect to ground" up. :grin: I would be interested to hear if anyone knows the true history of the term though.

 

[LarryFine]

I agree, engineers are just like any other profession there are great engineers and there are ones that need to go back to school, just like there are great electricians and ones that I wouldn't have wire my dog house.

Well, that was "ruff!"

 

[LarryFine]

Not quite true there mikeames. If you have circuits 1,3,5 in a 120/208,3 phase,4 wire system on single pole breakers feeding some sort of loads and you only provide one grounded circuit conductor for the 3 circuits you have 3 120v circuits and a "neutral" conductor. Same would hold true for circuits 1 and 3 in a "single" phase 3 wire system if they shared a common grounded conductor.

But he said "A 120 volt circuit . . . ", not a MWBC.

 

[Smart $]

But he said "A 120 volt circuit . . . ", not a MWBC.

My point of view is, if a conductor is directly connected to a source terminal dubbed Neutral, that conductor is a Neutral conductor. This is no different than calling a "hot" a Line conductor.

Besides, it has a neutral voltage [or near neutral at greater distances] when it is of a grounded Neutral system. Take for instance a 480/120VAC, 1? control transformer where one of the secondary outputs is grounded. Such terminal is commonly referred to as Neutral... and I agree that it should be called Neutral. In my mind the term "neutral" refers to voltage to ground?the ultimate neutral?not phase balancing.

 

[roger]

cnL4, a word of advice, it is not proper forum etiquette to edit posts after they have been replied to.

Roger

 

[iwire]

My point of view is, if a conductor is directly connected to a source terminal dubbed Neutral, that conductor is a Neutral conductor.

By that line of reasoning you better also call it the grounding electrode conductor as it is also connected to that.

This is no different than calling a "hot" a Line conductor.

Well it was and still is a line conductor, the neutral stopped being a neutral when the circuit became a two wire it is not neutral anymore.

 

[Smart $]

By that line of reasoning you better also call it the grounding electrode conductor as it is also connected to that.

The GEC and EGC's are not circuit conductors by my line of reasoning.

Well it was and still is a line conductor, the neutral stopped being a neutral when the circuit became a two wire it is not neutral anymore.

It is still a neutral in the sense it has far less voltage potential to ground than the other circuit conductor. Additionally, the line conductor is the source of a voltage which has a sine waveform. The Neutral conductor (in a grounded system) provides no voltage to the circuit and has no voltage waveform (except that which is a result of voltage drop). It simply sinks the current that passes through the load. The term "neutral" does not necessarily mean that it serves only as a means to balancing out-of-phase currents.

Anyway, I do not want to continue this debate, as it has been hashed and rehashed already. Consider "When in Rome..." as the end all of the debate.

 

[roger]

In my mind the term "neutral" refers to voltage to ground?the ultimate neutral?not phase balancing.

Slang terms (i.e. neutral for the white conductor) in the field are fast ways of communicating and are fine, I don't change my terminology from "screw driver" to "lock nut tightening device" when I'm using it with my side cutters to tighten a lock nut.

The fact is, in this thread we don't care about Potential Difference from one point or conductor to a terminal or another conductor, we are discussing current flow and it's associated heating of the conductors.

In a two wire circuit, (what ever someone wants to call either the grounded or ungrounded conductor) they are both current carrying conductors on each side of a resistor, and they both contribute heat within a raceway or cable sheath so there is not a neutral in the circuit.

The simple illustration below is an example of a true neutral which would not contribute any heat within a raceway or cable sheath.

Roger

 

[Smart $]

Slang terms (i.e. neutral for the white conductor) in the field are fast ways of communicating and are fine, I don't change my terminology from "screw driver" to "lock nut tightening device" when I'm using it with my side cutters to tighten a lock nut.

Neutral is not a slang term. Perhaps misused on occasion, but far from slang, nevertheless.

The fact is, in this thread we don't care about Potential Difference from one point or conductor to a terminal or another conductor, we are discussing current flow and it's associated heating of the conductors.

Prior to this point in the thread, I have not discussed anything with you. "You" can comment however and whenever you wish, but please do not tell me what "we" are discussing, when in fact we have not discussed anything. This is far from the first time a thread diverted from the main topic, and it certainly will not be the last.

 

[iwire]

Neutral is not a slang term. Perhaps misused on occasion, but far from slang, nevertheless.

It's slang when it's not used properly.

Prior to this point in the thread, I have not discussed anything with you. "You" can comment however and whenever you wish, but please do not tell me what "we" are discussing, when in fact we have not discussed anything.

Actually we (as in the rest of us) have been discussing derating and the neutral.

If you want to have a conversation with yourself about some irrelevant subject that is your choice.:smile:

 

[roger]

Prior to this point in the thread, I have not discussed anything with you.

Oh yes you have, you got into this thread, so by default your posts are actually part of a group discussion..

"You" can comment however and whenever you wish,

I know that, I've been a member here for a long time.

but please do not tell me what "we" are discussing, when in fact we have not discussed anything.

smart, you may have missed the beginning of the thread so let me tell you ( I know you asked me not to but I'm going to anyways ) what we are discussing.

From the OP

Their position is that in a 20 amp branch circuit, with a dedicated neutral, the neutral is not a CCC

I know that CCC may be a little confusing but it stands for Current Carrying Conductor.

It seems that you think voltage is part of the argument the OP is having with the Engineer, this is not the case.

Besides, it has a neutral voltage [or near neutral at greater distances] when it is of a grounded Neutral system.

This is far from the first time a thread diverted from the main topic, and it certainly will not be the last.

I'm glad to see you're catching on to the way the forum works, but be aware of the fact that you may be steered back on topic just as you are now.

BTW, I thought you didn't wish to debate this any further? :wink:

Roger

 

[Smart $]

Actually we (as in the rest of us) have been discussing derating and the neutral.

Good. Go back to what you are supposedly doing, because the post I am responding to surely contradicts what you are saying.

If you want to have a conversation with yourself about some irrelevant subject that is your choice.:smile:

Well, I could if Roger and you would butt out :grin: