Neutral Derate with Nonlinear loads involved

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iceco1

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Article 220.61 (C) (2) States that there shall be no reduction of the neutral or grounded conductor capacity for that portion consisting of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system.

My question is regarding load calculating the size of a neutral conductor when there are nonlinear loads involved.

Article 220.61 (B) (2) states that the portion of the unbalanced load in excess of 200 amps where the feeder or service or feeder is supplied from a 3-wire dc or single-phase ac system, or a 4-wire, 3-phase: 3-wire, 2 phase system, or a 5-wire, 2-phase system shall be permitted to have an additional demand factor applied.

Does the whole calculated neutral amount above the 200amps fall into the permitted reduction of 220.61 (B)? Or do I need to take out the calculated nonlinear neutral load and not de-rate that portion, then take the first 200amps of the linear load at 100% and any additional linear load above 200amps at 70%? Then add those figures to get my calculated neutral load?
 
"310.15(B)(4)(c)
On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current carrying conductor."

Then you would review 220.61 and proceed with your calculations as you have stated. So, I would say yes to your question with one consideration to think about.
Calculating which portion of circuits/feeders that have nonlinear loads may be a bit confusing and hard to do/prove. Most likey installing a full sized neutral may be the case.
 
I understand if the major portion of the neutral load is nonlinear than it shall be considered a current carrying conductor and should not be derated. I guess then, if the major portion is not nonlinear, then that portion (nonlinear) should be taken at 100% and the linear load above 200amps can be derated at 70%. Does that seem correct?

I'm preparing for a Supervisors test where this may possibly come into effect.

Thanks for your help.
 
The way I understand it is that if you have a significant amount of harmonics (non-linear loads), then you don't talk about derating the neutral at all. You take the entire calculated neutral load at 100%,+ whether or not it is above 200 amps, and select a conductor with an ampacity at least that high.
 
charlie b said:
The way I understand it is that if you have a significant amount of harmonics (non-linear loads), then you don't talk about derating the neutral at all. You take the entire calculated neutral load at 100%,+ whether or not it is above 200 amps, and select a conductor with an ampacity at least that high.
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this is my understanding as well
 
In fact, most experienced designers will increase the circuit ampacity to 200% and size the conductor according. This is to help reduce the added circuit resistance from the non-linear loads.
 
in my experience, even with the buildings almost filled with computers, the worst I have encountered is a 15% derating of the transformer.

oversizing your conductors 200% is good if you have VSDs as your sources of harmoncs. some older models have current harmonics to the tune of 100% THD.
 
Upsizing neutrals when ANY harmonic loads are present is a good idea regardless if it is from a VFD, UPS, battery charger, retifier, etc.

The concept is to reduce turbulence or resistance from the non-linear load.
 
So if there is not a major portion of nonlinear loads, then you can derate above 200 amps at 70%. Is major portion defined? Does it mean that the nonlinear loads must be above 50% of the total to count as a major portion?
 
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