Exam Prep Question Help

[Boiler Man]

Hello everyone. I'll start by saying I am new here. I am doing some master license exam preparation and have come across some questions I am struggling with. I came online to find some help and came across these forums. I browsed the products as well and now I wish I had known about Mike Holt's exam prep books before I got the one I have. I won't mention the name of it because I do not recommend it. I have already found a few questions that were wrong in the answer key.

Any who, here are the three questions I am struggling with.

***2014 NEC***

I believe I am correct on this question, and the answer key is wrong:
QUESTION 1.The ampacity of a No. 12 THHN conductor in a raceway with an ambient temperature of 28*C is ____ A.
My answer: 30A
Answer key: 31.2A
Other two answers: 15A and 20A

The answer I selected is 30A. The answer key says it is 31.2A. I'm assuming the question meant copper conductor, and based on 310.15(B)(16), the ampacity would be 30A. Using table 310.15(B)(2)(a), the correction factor for 28*C is 1.00 (26-30*C). According to the answer key answer of 31.2A, the correction factor would need to be 1.04 which would be in the 21-25*C range. I believe 30A would be the correct answer.



QUESTION 2. A 4/0 THHN copper conductor installed in free air on a 480V circuit at a temperature of 30*C will have an allowable ampacity of ____ A.
My answer: 260A
Answer key: 405A
Other two answers: 205A and 405A.

From what I read in the chapter, there were no examples of ampacity being affected by voltage under 2000v, or being in free air.



QUESTION 3. A TC cable with No. 6 THHN wire in a cable tray and with a spacing of one cable width between the multi-conductor cables will have an allowable ampacity of ____ A.
My answer: 75A
Answer key: 105A
Other two answers: 85A and 110A

This is I was just completely unsure of. Would love to have some guidance on figuring this one out.

 

[Smart $]

Welcome... :thumbsup:

Q1: You are correct.

Q2: Answer key is correct. For free air installation use Table 310.15(B)(17) [Hint: read the Table title]

Q3: You are correct. It appears the answer key is using Table 310.15(B)(17). However, 392.80(A)(1) requires us to use Table 310.15(B)(16) for multiconductor cables. The problem as stated forces us to assume there is not more than 3 current-carrying conductors in the cable. Cable spacing is a misleading parameter as it is irrelevant except under 392.80(A)(1)(c) which points us to 310.15(C) determination of ampacity under engineering supervision... which would likely establish an ampacity closer to the value in Table 310.15(B)(17) and the answer key... but you are not an engineer. :happyno:

 

[Boiler Man]

Welcome... :thumbsup:

Q1: You are correct.

Q2: Answer key is correct. For free air installation use Table 310.15(B)(17) [Hint: read the Table title]

Q3: You are correct. It appears the answer key is using Table 310.15(B)(17). However, 392.80(A)(1) requires us to use Table 310.15(B)(16) for multiconductor cables. The problem as stated forces us to assume there is not more than 3 current-carrying conductors in the cable. Cable spacing is a misleading parameter as it is irrelevant except under 392.80(A)(1)(c) which points us to 310.15(C) determination of ampacity under engineering supervision... which would likely establish an ampacity closer to the value in Table 310.15(B)(17) and the answer key... but you are not an engineer. :happyno:

Thanks a ton for the help. The further I get into this book the more it seems like I shouldn't be using it... Definitely didn't know about the free air table. The chapter made no mention of it, but I should have known to dig some more. A mistake I won't make again! Q3 just seemed like a poor question. I didn't know about 392.80, and it made no mention of it, but even so the question just didn't seem right.

I did four years of apprenticeship which I finished in 2012, but it was through Fairfax County here in VA and the classes were god awful. I didn't get a teacher who knew a thing about anything electrical any of the four years. I ended up teaching myself as much as I could.

Time to switch to a Mike Holt book!

Thanks again. I'm sure I will be back with more questions eventually.