NEC Question about Conductor Protection - Complex Imaginary Exam 1 Question 69

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akyip

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Hey guys,

I'm re-practicing code questions from the practice exams I have, and there is one problem that I'm a bit confused about.

Complex Imaginary Exam 1 question 69 asks:

"Which is the largest allowable size for an overcurrent device to protect a #300 KCMIL, TW, 60-degree aluminum conductor feeding a 277 V load? Assume the conductor is in a steel conduit."

I can easily find the ampacity of this #300 KCMIL, 60-degree conductor using NEC Table 310.15(B)(16) --> the ampacity is 195 A.

Now, the given solution states that OCPD sizes normally cannot exceed conductor ampacity. So the answer here is 180 A OCPD size.

Now, I know that normally NEC Section 240.4 - Protection of Conductors normally DOES NOT allow using the next standard OCPD size above the conductor ampacity. BUT, there is one section in particular that I think may apply in using the next standard OCPD size above the conductor ampacity.

"NEC 240.4(B) Overcurrent Devices Rated 800 Amperes or Less: The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided ALL of the following conditions are met:

(1) The conductors being protected are not part of a branch circuit more than one receptacle for cord-and-plug connected portable loads.

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse of a circuit breaker without overload trip adjustments above ins rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes."

From what I see here, NEC 240.4(B)(2) and (3) definitely apply here, since 195 A does not correspond to a standard OCPD size and the next standard size up is 200 A, which is less than 800 A.

For condition (1), the problem only states that the conductor feeds a 277 V load. Can I not just assume that this is just one load or one receptacle?

What are your thoughts on this?

Thanks for any input on this!

 

jd5191

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I don't have an answer, but wanted to ask if you have tried Complex Imaginary's code drill book and if you would recommend it? I have a plan for studying the engineering topics but no plan for studying/prepping codes and no experience with code books in general.

 

akyip

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I don't have an answer, but wanted to ask if you have tried Complex Imaginary's code drill book and if you would recommend it? I have a plan for studying the engineering topics but no plan for studying/prepping codes and no experience with code books in general.
Uh, I honestly wasn't aware Complex Imaginary also had a code drill book LOL. So I can't answer that question... Someone else who is familiar with this will have to.

This question was from their practice exam book.

 

Dothracki PE

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@jd5191 I have not tried the complex imaginary code practice, but I have bought this book below and so far seems good NEC practice, even though it is technically an electrician study book. It is on the list of recommended references from electricalpereview.com.

https://www.amazon.com/dp/1946798002/ref=cm_sw_r_cp_apa_i_OulOFbJPB1JPG?_encoding=UTF8&psc=1

I know that engineerproguides.com also has a code practice book you can buy as a PDF for a decent price and had been updated recently. 

 
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DilutedAr18_PE

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Hey guys,

I'm re-practicing code questions from the practice exams I have, and there is one problem that I'm a bit confused about.

Complex Imaginary Exam 1 question 69 asks:

"Which is the largest allowable size for an overcurrent device to protect a #300 KCMIL, TW, 60-degree aluminum conductor feeding a 277 V load? Assume the conductor is in a steel conduit."

I can easily find the ampacity of this #300 KCMIL, 60-degree conductor using NEC Table 310.15(B)(16) --> the ampacity is 195 A.

Now, the given solution states that OCPD sizes normally cannot exceed conductor ampacity. So the answer here is 180 A OCPD size.

Now, I know that normally NEC Section 240.4 - Protection of Conductors normally DOES NOT allow using the next standard OCPD size above the conductor ampacity. BUT, there is one section in particular that I think may apply in using the next standard OCPD size above the conductor ampacity.

"NEC 240.4(B) Overcurrent Devices Rated 800 Amperes or Less: The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided ALL of the following conditions are met:

(1) The conductors being protected are not part of a branch circuit more than one receptacle for cord-and-plug connected portable loads.

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse of a circuit breaker without overload trip adjustments above ins rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes."

From what I see here, NEC 240.4(B)(2) and (3) definitely apply here, since 195 A does not correspond to a standard OCPD size and the next standard size up is 200 A, which is less than 800 A.

For condition (1), the problem only states that the conductor feeds a 277 V load. Can I not just assume that this is just one load or one receptacle?

What are your thoughts on this?

Thanks for any input on this!
The question is vague enough that I would argue that you could reasonably assume that it is a single load especially when the question is asking for the maximum overcurrent protective device. I use the NEC everyday and I find that many of the people who write practice tests with NEC questions don’t know the intricacies of the code. There are plenty of instances where things are misapplied. Where a table will indicate specific requirements to use it and the question will not meet these requirements, but the solution will use it. 

 

akyip

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The question is vague enough that I would argue that you could reasonably assume that it is a single load especially when the question is asking for the maximum overcurrent protective device. I use the NEC everyday and I find that many of the people who write practice tests with NEC questions don’t know the intricacies of the code. There are plenty of instances where things are misapplied. Where a table will indicate specific requirements to use it and the question will not meet these requirements, but the solution will use it. 
Hm, that complicates things... thanks for your input.

Another egregious thing that I saw in the Complex Imaginary practice exams was that one of the questions specifically gives one continuous-duty motor, but the solution does not include the 125% ampacity multiplier for single continuous-duty motors...

 

Gab PE

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Hey guys,

I'm re-practicing code questions from the practice exams I have, and there is one problem that I'm a bit confused about.

Complex Imaginary Exam 1 question 69 asks:

"Which is the largest allowable size for an overcurrent device to protect a #300 KCMIL, TW, 60-degree aluminum conductor feeding a 277 V load? Assume the conductor is in a steel conduit."

I can easily find the ampacity of this #300 KCMIL, 60-degree conductor using NEC Table 310.15(B)(16) --> the ampacity is 195 A.

Now, the given solution states that OCPD sizes normally cannot exceed conductor ampacity. So the answer here is 180 A OCPD size.

Now, I know that normally NEC Section 240.4 - Protection of Conductors normally DOES NOT allow using the next standard OCPD size above the conductor ampacity. BUT, there is one section in particular that I think may apply in using the next standard OCPD size above the conductor ampacity.

"NEC 240.4(B) Overcurrent Devices Rated 800 Amperes or Less: The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided ALL of the following conditions are met:

(1) The conductors being protected are not part of a branch circuit more than one receptacle for cord-and-plug connected portable loads.

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse of a circuit breaker without overload trip adjustments above ins rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes."

From what I see here, NEC 240.4(B)(2) and (3) definitely apply here, since 195 A does not correspond to a standard OCPD size and the next standard size up is 200 A, which is less than 800 A.

For condition (1), the problem only states that the conductor feeds a 277 V load. Can I not just assume that this is just one load or one receptacle?

What are your thoughts on this?

Thanks for any input on this!
The question is a little bit vague. To make this desicion, you must first know your load. The NEC will allow you to go with a 200 amps OCPD IF this is a dedicated circuit for just a specific load. However, if not then you must go with 180 amps.

In real world applications you as a designer must develop your style. 180A is a more conservative choice, some designers doesn't apply the exception that you have mentioned and just stick with the rule "my ampacity should be greater or equal than the breaker" even if the code allows you to size the ocpd a step higher that the ampacity of the conductor being protected.
 
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