Non-Linear Load

[augie47]

Student in class asked a question concerning non-linear loads and my answer leaned toward "yes" but that may not be correct.
Assume a 3 phase MWBC.
Phase A feeds load consisting entirely of non-linear lighting loads.
Phase B&C, however, feed non-linear loads.
Would the neutral be considered a CCC for derating purposes ??

 

[jumper]

Based on 310.15(B)(5)(c) I want to say no.

I do not see the MWBC as having a majority of non linear loads as only 1 phase of 3 has them.

 

[Fitzdrew516]

Based on 310.15(B)(5)(c) I want to say no.

I do not see the MWBC as having a majority of non linear loads as only 1 phase of 3 has them.

Agreed. Like most codes a lot of things are up to interpenetration, but I definitely wouldn't consider this a "majority" of the load.

 

[Haji]

Derate when nonlinear load carried by neutral exceeds 50%.

 

[jumper]

Based on 310.15(B)(5)(c) I want to say no.

I do not see the MWBC as having a majority of non linear loads as only 1 phase of 3 has them.

Let me clarify my position:

If we assume that all three phases serve only L-N loads, Gus did not mention using any of the exceptions, then:

A phase is a nonlinear L-N load, with B and C being linear L-N loads.

I am not seeing how the shared neutral could have a majority of a load with harmonics causing it to be considered a CCC.

Am I missing something?

 

[gar]

150826-0930 EDT

The original post indicated all three phases had non-linear loads, but the answers assumed that phases B & C were linear. I suspect the non-linear for B and C was a typo and should have been linear.

.

 

[augie47]

150826-0930 EDT

The original post indicated all three phases had non-linear loads, but the answers assumed that phases B & C were linear. I suspect the non-linear for B and C was a typo and should have been linear.

.

Thank you ! I was a typo.. only one is non-linear, hence the question,

 

[jumper]

Gus, in your OP you said you leaned toward "yes", why?

 

[augie47]

Gus, in your OP you said you leaned toward "yes", why?

Ignorance !
Simply because I felt the non-linear load would have an adverse effect on the neutral current
regardless of what was taking place on the other two phases. I pictured an unbalanced load adding to the non-linear situation resulting in a high neutral current but that is purely from an unlearned standpoint.

 

[Carultch]

Student in class asked a question concerning non-linear loads and my answer leaned toward "yes" but that may not be correct.
Assume a 3 phase MWBC.
Phase A feeds load consisting entirely of non-linear lighting loads.
Phase B&C, however, feed linear loads.
Would the neutral be considered a CCC for derating purposes ??

Out of ignorance, I would be conservative when in doubt. If this were a practical in-the-field example, I would count the neutral as a CCC in this example for derating purposes.

Ok to do that for practical examples in the field, but gotta be careful when taking an exam.

And unfortunately, I see this rule as kind of vague. If I measure every detail of the loads with an oscilliscope, what measurable characteristic should I look for to know whether or not "a major portion of the load is nonlinear"?

 

[Carultch]

Ignorance !
Simply because I felt the non-linear load would have an adverse effect on the neutral current
regardless of what was taking place on the other two phases. I pictured an unbalanced load adding to the non-linear situation resulting in a high neutral current but that is purely from an unlearned standpoint.

As I understand it, the intent of this rule is to remind us that harmonic currents may not "cancel out" on the neutral conductor. Especially the 180 Hz third harmonics, and its larger multiples thereof, which accumulate instead of cancel, onto the neutral.

The idea is whether or not the four wires (ABC & N) all can generate more heat collectively, than the maximum possible heat generated by only the phase conductors, at full ampacity. And if that is the case, neutral should be counted.

 

[Smart $]

Okay, I see non-linear typo has been covered, but you did not mention amount of loading on each phase. 50% is based on total load. If A were 16A while B & C were both 7.5A, the neutral would be considered a CCC. Please note that circuit loading vs. circuit ampacity does not enter the determination.

 

[Carultch]

Okay, I see non-linear typo has been covered, but you did not mention amount of loading on each phase. 50% is based on total load. If A were 16A while B & C were both 7.5A, the neutral would be considered a CCC. Please note that circuit loading vs. circuit ampacity does not enter the determination.

Is it realistic to calculate this in terms of KVA in total?

If KVA_nonlinear / KVA_total is greater than 50%, then neutral counts as a CCC. No matter how the linear and non-linear is distributed among the phases.

Also, once a load classifies as nonlinear, should its entire KVA be counted in the nonlinear total, no matter what the degree of non-linearity?

 

[augie47]

Smart: This was a question asked by a student in a class so use numbers you wish
In his description, Phase A would be a fully loaded non-linear load supplying lighting, Phase B & C are linear supplying receptacles.
Assuming he is needing to derate for ambient, fill, etc., he asked does the neutral count.

 

[Carultch]

Smart: This was a question asked by a student in a class so use numbers you wish
In his description, Phase A would be a fully loaded non-linear load supplying lighting, Phase B & C are linear supplying receptacles.
Assuming he is needing to derate for ambient, fill, etc., he asked does the neutral count.

I sure hope your student doesn't write the exam questions in his future. Please thank him for offering us the interesting challenge.

"Linear supplying receptacles". Not sure that this is as fixed of a load nature as one might think. In the past, there used to be a lot more linear loads, like motors and incandescent lighting. Now that technology has changed, we've reduced the KVA and increased the nonlinearity of a lot of receptacle loads (fluorescent lights, LEDs, power supplies, computers).

 

[Haji]

Smart$:
I think, if the neutral ampacity is not exceeded, nonlinear load may exceed 50%. Only when the neutral is loaded to its full ampacity, does the restriction on the part of the nonlinear load not exceeding 50% apply. What do you think based on the code? Thanks.

 

[Smart $]

Is it realistic to calculate this in terms of KVA in total?

If KVA_nonlinear / KVA_total is greater than 50%, then neutral counts as a CCC. No matter how the linear and non-linear is distributed among the phases.

Also, once a load classifies as nonlinear, should its entire KVA be counted in the nonlinear total, no matter what the degree of non-linearity?

Being realistic has little to no influence on Code compliance.

Your second statement is how I interpret the written word.

Level of calculation scrutiny is not dictated by NEC. That duty falls upon AHJ...

 

[Smart $]

Smart: This was a question asked by a student in a class so use numbers you wish
In his description, Phase A would be a fully loaded non-linear load supplying lighting, Phase B & C are linear supplying receptacles.
Assuming he is needing to derate for ambient, fill, etc., he asked does the neutral count.

Tell him that should Phase B & C loads be entirely linear, the neutral would not count.

Then tell him with Phase B & C supplying receptacles their loads are unknown. The load served at any point in time could range from entirely linear to entirely nonlinear. Then ask the student to make the wisest choice... :angel:

 

[Smart $]

Smart$:
I think, if the neutral ampacity is not exceeded, nonlinear load may exceed 50%. Only when the neutral is loaded to its full ampacity, does the restriction on the part of the nonlinear load not exceeding 50% apply. What do you think based on the code? Thanks.

With nonlinear loads combining on the neutral at a level somewhere between the arithmetic sum and a typical vector sum, how do you determine whether the neutral ampacity is exceeded before you put it in?

 

[don_resqcapt19]

Can anyone show me a documented case of neutral conductor overheating from non-linear loads in someplace other than a data center?

This is a case where almost all of the published information on the problems that may be caused by non-linear loads come from the people that have something to sell you to solve the problem.