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.

 

[wwhitney]

And a third issue is that the prohibition on counting nonlinear loads in 220.61(C)(2) only applies to 3P4W wye systems. Nothing in the question specifies that this isn't a high leg delta or a single phase system.

Cheers, Wayne

 

[Elect117]

NEC 2020

220.61(B)(2) "That portion of the unbalanced load in excess of 200 amperes where the feeder or service is supplied from a 3-wire dc or single-phase ac system; or a 4-wire, 3-phase system; or a 3-wire, 2-phase system; or a 5-wire, 2-phase system"

The unbalanced part is not in excess of 200A so no reduction can be applied to the unbalanced. The annex has an example where the unbalance is 242A and they do 100% of the first 200A and then apply the 70% to the remaining 42A and get a solution of 229A. It is example D4(a).

Without telling you the voltage of the system, I don't know how you would assume that the nonlinear load is on a 3PH 4W wye.

 

[wwhitney]

The unbalanced part is not in excess of 200A so no reduction can be applied to the unbalanced.

Actually, we have been given no information about how the 400A of L-N loads (200A linear plus 200A non-linear) is balanced. Worst case they are all on the same line, and the unbalance is 400A.

Most likely the question meant to say that this is 200A of linear L-N loads from _each_ line to neutral, likewise 200A of non-linear L-N loads. And then further specify that the supply system is 3P4W wye. In which case we end up with having to face the ambiguity that jaggedben pointed out.

Cheers, Wayne

 

[don_resqcapt19]

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



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.

That assumes that all of the line to neutral load is on the same leg or phase. You only need to count the maximum line to neutral load on the highest phase or leg....but they did not provide that information.

 

[wwhitney]

Slightly confused about the following question:

Say I have a 120/240V supply, and I have 400A of load A-N and 400A of load A-B. So conductor A gets sized to 800A, and conductor B gets sized to 400A. [Maybe I get a fused disconnect with different size fuses on each leg.] Does 220.61 let me size conductor N to 340A, or must it be 400A?

Downsizing N doesn't make any sense, as there's no possible cancellation of current. If we can be sure the N load won't exceed 340A, then we know the A load won't exceed 740A. But there's certainly no allowance in the NEC to reduce the conductor A size to 740A.

Cheers, Wayne

 

[jaggedben]

Does 220.61 let me size conductor N to 340A, ?

That sure seems to be what it says, but don't ask me why it should be allowed.

 

[Cybatrex]

I want to thank you all for your candid replies as I have read them all. What I am learning about these questions is not to over think them. I'm starting to feel like the more code you learn, the more complex we make the questions. It's best to keep the question to its simplest form. I've been really studying to pass this exam in the next 2 months. Thanks for all the replies.

 

[Dennis Alwon]

I want to thank you all for your candid replies as I have read them all. What I am learning about these questions is not to over think them. I'm starting to feel like the more code you learn, the more complex we make the questions. It's best to keep the question to its simplest form. I've been really studying to pass this exam in the next 2 months. Thanks for all the replies.

Anytime you want more confusion come on over. haha