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Complex Imaginary Volume 1 #69


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#1 stbtigerr

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Posted 18 March 2012 - 06:19 PM

I answered 200A since that would be the next standard rating above the 190A rating for the conductor in the NEC. The solution says it should be less than or equal to the conductor rating and the answer is 180A. Is there something in the problem that does't allow you to use the "next standard breaker size" rule?

#2 Wildsoldier PE

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Posted 18 March 2012 - 07:00 PM

I answered 200A since that would be the next standard rating above the 190A rating for the conductor in the NEC. The solution says it should be less than or equal to the conductor rating and the answer is 180A. Is there something in the problem that does't allow you to use the "next standard breaker size" rule?


There is something in the code that says that as long is below 800Amp you can select the breaker size below not above standard size....thats why he choose 180A instead of 200A. If it is more than 800A you have to go or equal or above.

#3 stbtigerr

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Posted 18 March 2012 - 07:55 PM

The code (240.4(B)) says that up to 800A you can go above the cable rating if there is no standard trip rating that corresponds to the cable rating. In this case, 190A isn't a standard trip rating, but 200A is.

#4 Wildsoldier PE

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Posted 18 March 2012 - 10:01 PM

You are right....my bad is the wire you can go below not the breaker.

#5 stbtigerr

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Posted 20 March 2012 - 10:35 PM

FYI, I emailed CI and they replied with:

Because 240.4(B)(1-2) are not specified in the question. Therefore - them NOT being included in the problem - we cannot assume that it meets these conditions. Especially for code questions, you cannot assume that conditions are met that are not specifically communicated. Because if it is not specified, it is just as likely to NOT meet these conditions.

#6 Wildsoldier PE

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Posted 20 March 2012 - 11:43 PM

FYI, I emailed CI and they replied with:

Because 240.4(B)(1-2) are not specified in the question. Therefore - them NOT being included in the problem - we cannot assume that it meets these conditions. Especially for code questions, you cannot assume that conditions are met that are not specifically communicated. Because if it is not specified, it is just as likely to NOT meet these conditions.


Forget complex the way you answer the question is the correct way!

#7 afacemire

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Posted 22 March 2012 - 11:50 PM

The complex imaginary problem is correct as stated.

Generally speaking, you want to size your cables to have a greater ampacity than the required load ampacity.
You want to size your OCPD's to protect the devices downstream AND the cable itself.

Any exception to these rules is just that, an exception. See the PPI Practice Exam problem 100 for a WAY better explanation.
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#8 whitebuffalo

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Posted 08 April 2012 - 07:15 PM

CIs answer to this question I don't feel is adequate. First of all the table they reference is not in the 2011 NEC. The correct table 310.15(B) (16) has a New value of 195A. I would assume that 240.4(B) applies here because I have never seen a 195A breaker.

#9 stinkycheese

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Posted 08 April 2012 - 11:02 PM

I also chose 200A here because of how incredibly unlikely it is that 240.4 B (1) voids the "round up" rule. Why would you have a 195A conductor feeding a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads? That being said, since the load isn't defined you can't assume that the load has certain characteristics. So I begrudgingly agree with CI.




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