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Stephen2awesome

Complex Imaginary Test 1, Problem 69 error?

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Problem:

"Of the rated current listed below, which is the largest allowable for an overcurrent device to protect a 300kcmil, TW, 60degree Aluminum conductor feeding a 277V load. Assume conductor is in a steel conduit.

A. 150 A
B. 180 A
C. 200 A
D. 225 A

"

NEC says 300kcmil, TW at 60 degrees Aluminum, it is rated for 195 A. NEC 240 says, next highest standard device rating. I got 200A, C.  The book says the conductor is rated for 190 A and chose B. Is this an error on them?

 

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There is a nifty rule called the round up rule. This rule basically states that for anything 800A and below, you can use the next available standard size for the OCP device if your conductor sizing does not fall on a standard size. See NEC code 240.4 (B).

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2 hours ago, fsk0408 said:

There is a nifty rule called the round up rule. This rule basically states that for anything 800A and below, you can use the next available standard size for the OCP device if your conductor sizing does not fall on a standard size. See NEC code 240.4 (B).

Yes, that is why I chose 200A. CI chose the 180A.

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1 minute ago, Stephen2awesome said:

Yes, that is why I chose 200A. CI chose the 180A.

I misread your original post. My mistake. 

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I also got 195A and so I too feel it should be 200A. Any other expert advise? I do not want to miss on NEC!!!

 

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21 hours ago, rg1 said:

I also got 195A and so I too feel it should be 200A. Any other expert advise? I do not want to miss on NEC!!!

 

It should be 200A. their amps that they got does not match the NEC tables.

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The NEC ratings for Fixed-Trip Breakers is 15, 20, ..., 125, 150, 175, 200, 225, 250, ....., 6000

If you got 195 then you should pick 175, remember if you are sizing protecting you round up to the lower value, if you are sizing transformer, conductor, etc, you round up to the higher value.

In this specific question the options given are 150, 180 (Instead of 175 per NEC), 200 and 225, then you should pick 180.

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10 hours ago, FPar said:

The NEC ratings for Fixed-Trip Breakers is 15, 20, ..., 125, 150, 175, 200, 225, 250, ....., 6000

If you got 195 then you should pick 175, remember if you are sizing protecting you round up to the lower value, if you are sizing transformer, conductor, etc, you round up to the higher value.

In this specific question the options given are 150, 180 (Instead of 175 per NEC), 200 and 225, then you should pick 180.

Your logic makes absolute sense and I would go with this- round off towards safer side should always be a choice but the question here is interpretation of NEC and selection as per that. Would you please read the paras of NEC and prove concurrency  between the logic and  NEC. 

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20 hours ago, FPar said:

The NEC ratings for Fixed-Trip Breakers is 15, 20, ..., 125, 150, 175, 200, 225, 250, ....., 6000

If you got 195 then you should pick 175, remember if you are sizing protecting you round up to the lower value, if you are sizing transformer, conductor, etc, you round up to the higher value.

In this specific question the options given are 150, 180 (Instead of 175 per NEC), 200 and 225, then you should pick 180.

Can you cite the code section? for conductors I am referring to NEC 240.4(B), which states that the OCPD can be higher standard rating.

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1 hour ago, Stephen2awesome said:

Can you cite the code section? for conductors I am referring to NEC 240.4(B), which states that the OCPD can be higher standard rating.

I do not know if there is a code legislation for this, @FPar may let us know. But there is a convention; while deciding a protection setting, it is prudent  and safer to go for lesser setting provided it does not start giving nuisance tripping while energizing a transformer or starting a motor. This is only a trick of the trade, I do not see it is legislated. I myself  use this trick, when I am involved. But for PE exam,  it is a good practice. Because there you can change the setting if it does not work - NCEES does not allow us to change the answer if it is wrong!!!!

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Please accept my apologies for not answering earlier. Yes it's "a trick of the trade" as @rg1 said in the previous post and I personally do it all the time.

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Can someone explain the discrepancy with the code though?  Specifically why 240.4(B) would not apply to this question?

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Can someone please explain why we size down in this problem? I was getting 200A based on NEC 240.4(B).

Also, is there a way you remember when to size up or size down in NEC Code Problems?

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The CI answer is incorrect. 

Under normal design considerations without a specific circumstance or exemption listed that would prevent the next largest OCPD from being used as indicated in 240.4(B)... 200A is the correct answer.  It is the largest OCPD that can be used with the information given.  If they wanted the answer to be 180A, the answer should have been restricted in scope with additional information provided in the question.

What the others above are talking about is a good design practice of not using 2404(B) for sizing conductors / protection for *motors and transformers specifically* as it provides a more conservative / robust design and alleviates most of the common nuisance issues allowing the designer to expedite the design process. 

With that said, the absolute correct way of protecting a transformer / motor and its conductors is by using the time current curves of all the components plotted against the trip curve.  But in the office... this is rarely done for small size units.  

(In regards to the answer the NCEES would want to see... there is no reason to expect they would want you to ignore code from another section unless specifically stated in the problem.  Code allowing it trumps "good design"...  But that is just my 2 cents.  Good Design is subjective, as for instance, what if the primary concern for the project was economic in nature?)

 

(Edit)

Edited by Szar

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13 hours ago, Szar said:

The CI answer is incorrect.

Don't say that to the owner.

Guy is very douchy if you bring up incorrect information on his books

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