Neutral as a Current Carrying Conductor

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necnotevenclose

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I'm working on designing an office building and I generated a question that I cannot answer. Based on the majority of the load being non-linear (lighting & computer equipment) it is my understanding that I will need to include the feeder neutral as a current carrying conductor and derate my feeder ampacity. Does this seem to be correct?
 
Yes, if non-linear loads really are the majority of the load.

Does the building have air conditioning and other HVAC loads?
 
iwire said:
Yes, if non-linear loads really are the majority of the load.

Does the building have air conditioning and other HVAC loads?
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Yes. The building has (4) HVAC units controlled by VFD's and a few fan's controlled by mag & manual starters.
 
feeder neutral

feeder neutral

My question is, when is a neutral not a current carrying conductor? As I understand it ,this would only occur when no power is being used on either power conductor otherwise a neutral Carry's 1/2 the unbalance load of opposite phases and is always a current carrying conductor. Derating a feeder would begin when you have more than three current carrying conductors in a conduit, your feeder ampacity would not begin to be derated until 4 current carrying conductors excluding your ground. tell me if I missed something here.review 310.15 (4)(a)this applies to a single phase circuit and is kind of a play on words 310.15 (B)(2) I believe is your answer
 
dennis schaffert said:
My question is, when is a neutral not a current carrying conductor? As I understand it ,this would only occur when no power is being used on either power conductor otherwise a neutral Carry's 1/2 the unbalance load of opposite phases and is always a current carrying conductor. Derating a feeder would begin when you have more than three current carrying conductors in a conduit, your feeder ampacity would not begin to be derated until 4 current carrying conductors excluding your ground. tell me if I missed something here.review 310.15 (4)(a)this applies to a single phase circuit and is kind of a play on words 310.15 (B)(2) I believe is your answer
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A neutral from a Wye system is not considered a current carrying conductor until the non-linear or harmonic loading exceeds 50% of the load. In most cases this will never happen therefore you would not need to count the neutral as a CCC.
 
neutral

neutral

As I understand it by Maxwell's equations current is one coulomb equal to 6.25x 10 to the eighteenth power of electrons moving at a given speed past a stationary point in a conductor and is the equivalent of 1 amp regardless of wave form. because a nonlinear wave form is a distortion or a different wave shape or more likely a secondary carrier wave its influence would generate an induced current and generate over heating in a conductor. But in either case the flow of electrons in a neutral conductor remains the amount equal to the potential applied voltage meaning the neutral always carries current.
 
Yes, the neutral almost always carries some current, but the code rule about current carrying conductors and derating is about heat from the conductors. With a 3 wire single phase system the two cases that produce the most heat are with one circuit fully loaded or with both circuits fully loaded any other combination produces less heat. The heat produced is the I^2R losses in the conductors. If you are using the same type of conductors for all 3 conductors of the circuit the R is a constant, so with 20 amps of load on a single circuit you have two conductors producing 400R watts of heat for a total of 800R watts. You have the exact same thing with both circuits loaded to 20 amps. If you load one to 20 amps and one to 19 amps, you have one conductor with 400R watts, one with 361R and the neutral with 1R watt for a total of 762R watts of heat. There is no need to count the neutral as a current carrying conductor for derating because any time the neutral is actually carrying current the total heat will be less than when you have 20 amps of load on one or both of the hots.
Don
 
necnotevenclose said:
Yes. The building has (4) HVAC units controlled by VFD's and a few fan's controlled by mag & manual starters.
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3? HVAC units?

necnotevenclose said:
Based on the majority of the load being non-linear (lighting & computer equipment)...
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What are you numbers... i.e. calculated loads?

Are you counting all the lighting or only the lighting which uses electronic ballasts? ...or does all the lighting use electronic ballasts? :rolleyes:

I've never considered traditional-ballast lighting as a non-linear load. Should I?
 
dennis schaffert said:
But in either case the flow of electrons in a neutral conductor remains the amount equal to the potential applied voltage meaning the neutral always carries current.
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I quite agree that the neutral probably carries some current in most situations.

The question is not 'does the neutral carry any current at all?', but rather 'do we count the neutral as a current carrying conductor for purposes of applying 310.15(B)(2)(a)?'.

I wrote the following, which was accepted as a FAQ answer:
http://forums.mikeholt.com/showpost.php?p=664225&postcount=38

-Jon
 
winnie said:
I wrote the following, which was accepted as a FAQ answer:
http://forums.mikeholt.com/showpost.php?p=664225&postcount=38

-Jon
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The following quote from the FAQ is somewhat misleading.

1) A single phase feeder or multi-wire branch circuit consisting of two 'hots' (ungrounded conductors) and a single 'neutral' (grounded conductor). In this case, the neutral carries only the unbalanced current of the two hot conductors, we would count a total of two current carrying conductors.
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This particular MWBC is system specific, meaning that in a single phase 3 wire system it would only count as two CCC. But if the same MWBC originated in a 3 phase Wye system the neutral would carry about the same current as the two phase conductors and would be considered 3 CCC's.

I'm assuming that you meant that the 3 wire MWBC originated in a 3 wire single phase system but the wording doesn't support that conclusion.
 
infinity said:
This particular MWBC is system specific, meaning that in a single phase 3 wire system it would only count as two CCC. But if the same MWBC originated in a 3 phase Wye system the neutral would carry about the same current as the two phase conductors and would be considered 3 CCC's.
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I make that exact point a couple of paragraphs down, and I differentiate between single and three phase systems. So I am comfortable letting the wording stand as written.

However I would not be offended if you come up with better wording and submit that to George to improve the answer! I've never written anything that is _perfect_ and am happy to see improvements.

-Jon
 
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