220.61 FEEDER / SERVICE NEUTRAL LOAD CALCULATION

[Cybatrex]

Help. I'm practicing for my exam and I believe on my practice test the answer is wrong here.

Feeder/Service Neutral Calculated Load:

What is the feeder/service neutral calculated load for the following service:

• 200 amp Line to Line loads
• 200 amp Line to Neutral Nonlinear Loads
• 200 amp Line to Neutral Linear Loads

200 Amp Line to Line = 0 Amps
200 Amp Line to Neutral (Non Linear Load) 200A @ 100%

• NEC 220.61(C): Nonlinear loads (like computers, electronics) contribute 100% to the neutral current because harmonics increase the neutral current.

200 Amp Line to Neutral (Linear Load) = 200A X 70% = 140A

• NEC 220.61(B): Linear loads contribute 70% of their load to the neutral. For line-to-neutral linear loads over 200 Amps, you apply the 70% demand factor.

SO MY ANSWER SHOULD BE 340A, CORRECT?

Sorry about the caps
My selection of answer are

 

[jaggedben]

I think you are ultimately correct, although the answer they want is clearly 400A.

They are thinking:
You can't take a demand factor on any of the linear load because it is not in excess of 200A.
You cannot take any demand factor on the non-linear load.
Therefore you cannot take any demand factor.

The counterargument would be:
The total line-neutral load is 400A.
The non-linear load can be considered as the first 200A.
Therefore you can take a demand factor on the remaining 200A without taking any demand factor on the non-linear portion.

Note there is another flaw in the question, which is that the code actually only requires the neutral to be sized for the largest load between the neutral and any one ungrounded conductor. The question doesn't provide any info on this aspect.

FWIW the language in the little blurbs you're quoting (e.g. "Linear loads contribute 70% of their load to the neutral.") is neither entirely correct nor the exact language the code uses. I don't know what the reasoning behind the 70% demand factor is, but unbalanced linear loads actually contribute 100% of their load to the neutral.