Derating neutral for nonlinear loads

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rlMutch

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NEC 310.15(B)(4)(c) says that the neutral wire of a 4-wire, 3 phase wye circuit shall be counted as a current carrying conductor where the major portion of the load consists of nonlinear loads. I have 2 questions:
1) Since the word "circuit" is used, does this paragraph not apply to feeder and service neutrals???
2) What does major portion mean? Anything over 50%?
This is my first post, so I hope this works. Thanks!
 
1. It applies to feeders. Services have a separate set of rules. The word "circuit" is not the same as the two word phrase, "branch circuit."

2. There is no answer in the code. There is also no standard way of interpreting what a term like "50%" would mean. Is that 50% of current, or of real power, or of reactive power, or of the total number of components served without regard to how much each component draws, or something else entirely? Does it refer in some way to Total Harmonic Distortion (THD)? I'm not sure.

3. Welcome to the Forum.
 
Thanks Charlie, I guess I meant to ask what "major portion of the load" means. I suppose you could calculate it in such a way that you could determine a percentage of nonlinear to linear loads??
 
Calculate what? That is exactly the problem with the wording of this section. You could calculate currents, I suppose, both for the linear portion of the loads served and the harmonics. But is current the thing that you should be calculating? Or should you be comparing power instead of current? I just don't know, and the book doesn't say.

It is generally accepted that if you are supplying certain types of loads, ones that are known to be suppliers of harmonic currents, then you do count the neutral as a current-carrying conductor. You don't need to do math. If for example the circuit supplies fluorescent lights or office computers, then count the neutral.
 
It is generally accepted that majr portion means 50% or more. As Charlie has indicated is that current or power? I usually just assume (and we know what that means) that if the nonlinear current loads approach 50% of the total current load I do not reduce the neutral. Except on an exam, you are never "wrong" to not reduce or not derate if you are allowed. and usually on an exam, the wording of the question makes the intent fairly obvious.
 
In designs for data processing centres,we normally assume the neutral will be carrying load current(predominantly 3rd harmonic).In many installations,we have measured neutral currents approaching 2 times the phase current( note:this is even with all 3 phases perfectly balanced).As a result we will "double size " the neutral for all these design applications.

As a result, even 50% non-linear load could still dramatically increase the neutral current loading.

Hope this helps and gives you some insight into the problem.
 
ghostbuster said:
In designs for data processing centres,we normally assume the neutral will be carrying load current(predominantly 3rd harmonic).In many installations,we have measured neutral currents approaching 2 times the phase current( note:this is even with all 3 phases perfectly balanced).As a result we will "double size " the neutral for all these design applications.

As a result, even 50% non-linear load could still dramatically increase the neutral current loading.

Hope this helps and gives you some insight into the problem.
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So I'm assuming that you would be using MWBC's in a data center and not individual neutrals for each circuit? Around here I have never seen a data center design that didn't specify a separate neutral for each circuit conductor.
 
infinity said:
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So I'm assuming that you would be using MWBC's in a data center and not individual neutrals for each circuit? Around here I have never seen a data center design that didn't specify a separate neutral for each circuit conductor.
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I believe that to be a result of the system designer not being fully aware of neutral currents resulting from harmonics loading... so they play it safe and spec individual branch circuits. IMO, "double-size" neutrals work just as well, while saving some money in both installation and operational costs.
 
charlie b said:
1. It applies to feeders. Services have a separate set of rules. The word "circuit" is not the same as the two word phrase, "branch circuit."

If the word "circuit" is a generic term, and feeders would have to comply with this requirement, why wouldn't service conductors have to as well? We use Article 310 to determine the ampacity of service conductors don't we??

(I guess I didn't do the quote thing right)
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rlMutch said:
If the word "circuit" is a generic term, and feeders would have to comply with this requirement, why wouldn't service conductors have to as well? We use Article 310 to determine the ampacity of service conductors don't we??
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Sorry I confused you. I was thinking of the limited circumstance of service conductors being provided by the utility in accordance with utility rules. If we design and install conductors from, let us say, the utility transformer to our service panel, then 310.15(B)(4)(c) would come into play.
 
100 conductors how many 2' do i need

100 conductors how many 2' do i need

I have to 42 cir pnl side by side one feeding other. all entrence pipe comes throu one pnl how many 2" do i need. All wire is #12.
 
KROC2004 said:
I have to 42 cir pnl side by side one feeding other. all entrence pipe comes throu one pnl how many 2" do i need. All wire is #12.
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You need to provide more information. What size and what type of circuits are these #12's feeding?
 
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