My Students are having trouble with this

[wwhitney]

So how much THD is required for a load to be considered "nonlinear" for the purposes 2023 NEC 310.15(E)(3) anyway?

If the load's current waveform has say 10% THD, and if that harmonic distortion consists solely of 3k-order harmonics (k any natural number), then the neutral current on a balanced 3-phase set of 3 such loads would be 30% of the line current. The heating would 3.09 times the heating from one line conductor, rather than 3 times the heating from one line conductor if the neutral currents perfectly cancelled. Seems like a small enough change that an 80% ampacity factor (with according 36% reduction in heating) should not be triggered.

Cheers, Wayne

 

[ramsy]

So how much THD is required for a load to be considered "nonlinear" for the purposes 2023 NEC 310.15(E)(3) anyway?

Over my head, and perhaps not needed to follow NEC rules for Nameplate Amps, rather than Watts, per 220.18B.

 

[ramsy]

IMHO, less thermal imaging reports would follow overheated wire if more electricians paid attention to 220.18 for inductive ballast & motor loads, where current rises with voltage drop.

 

[electrofelon]

So how much THD is required for a load to be considered "nonlinear" for the purposes 2023 NEC 310.15(E)(3) anyway?

If the load's current waveform has say 10% THD, and if that harmonic distortion consists solely of 3k-order harmonics (k any natural number), then the neutral current on a balanced 3-phase set of 3 such loads would be 30% of the line current. The heating would 3.09 times the heating from one line conductor, rather than 3 times the heating from one line conductor if the neutral currents perfectly cancelled. Seems like a small enough change that an 80% ampacity factor (with according 36% reduction in heating) should not be triggered.

Cheers, Wayne

Exactly, they need some qualifier on the degree of non-linearity. Then they say "majority" of the load, so I can have 49% of "very" nonlinear loads and not have to count it, but if I have 51% of "slightly" nonlinear loads and then I have to count it? . Just seems to be vary vague and arbitrary and not thought out.

 

[Cindysparky]

The book can be checked by NEC equation 310.15(B) without engineering supervision

I'-I*Sqrt((Tc-T'a)/(Tc-Ta))

I′ = ampacity corrected for ambient temperature
I = ampacity shown in the tables
Tc = temperature rating of conductor (°C)
T'a = new ambient temperature (°C)
Ta = ambient temperature used in the table (°C)

for #12 THHN @ 49°C or 120°F

I' = 25*Sqrt((75-49)/(75-30)) = 19 Amps

Teacher's book agrees 210.19(A)(1)(a) requires (15.5 * 1.25) = 19.38, which exceeds 19A

Teacher's book errors using 25A from T 310.16 based on 30°C. Table 310.16 is not valid for 49°C ambient.

Ok so I picked up on the error of 25A. I was like: why aren’t they taking the 90 degree column of the table 310.15(B)(16)??? But then I realized they were limiting the amperage based on the 75 degree terminals of the equipment…. The first comparison takes into account temp and current carrying corrections (but ignores continuous load) and the second comparison takes into account continuous load and equimpment limitations From 110.14(c)

 

[Cindysparky]

IMHO, less thermal imaging reports would follow overheated wire if more electricians paid attention to 220.18 for inductive ballast & motor loads, where current rises with voltage drop.

Thanks for this code article. Just opened up the book

 

[Cindysparky]

Where are you hearing that "electric discharge lighting" is nonlinear? Even in the old days, I dont believe say a magnetic fluorescent ballast would be nonlinear. Certainly inductive, but no non-linear.

It doesnt apply anymore because electronic power supplies are pretty good now and are power fator corrected and designed to be generally linear loads. A simple rectifier (or crude power supply contain one) would be non-linear because it only draws current at certain times in the cycle.

So, I’m guessing they correct these things with capacitors in design?

 

[electrofelon]

So, I’m guessing they correct these things with capacitors in design?

Yeah, my thesis is pretty much all power supplies these days are power factor corrected enough to not be consisted a non linear load

 

[Cindysparky]

Exactly, they need some qualifier on the degree of non-linearity. Then they say "majority" of the load, so I can have 49% of "very" nonlinear loads and not have to count it, but if I have 51% of "slightly" nonlinear loads and then I have to count it? . Just seems to be vary vague and arbitrary and not thought out.

Yeah, my thesis is pretty much all power supplies these days are power factor corrected enough to not be consisted a non linear load

That’s so cool