Another Voltage Drop/Wire Sizing Question

[hankhill]

Studying for an exam, question is:

Due to voltage drop, a #2 ungrounded conductor must be increased to a 2/0; the original #8 grounding conductor must also be re-sized. What's the size of the new grounding conductor?

a. #8 AWG
b. #6 AWG
c. #4 AWG
d. #3 AWG

The correct answer is listed as d. #3 AWG


From NEC 250.122(B)
Increased in Size.
Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors.


I guess that I'm missing something because it looks to me like the correct answer is #4 AWG.

Table 8 Conductor Properties
#2 AWG Circular Mils: 66360
#2/0 AWG Circular Mils: 133100
#8 AWG Circular Mils: 16510

133100/66360 = 2.005

2.005 x 16510 =33114 (Minimum required)

#4 AWG Circular Mils: 41740 (#3 AWG Circular Mils: 52620)

Not sure what I'm doing wrong. Thank you for any help offered.

 

[charlie b]

I agree with your math. :happyyes: That said, the correct answer is #3. :? What is wrong here is that the statement of the problem has an error, and the person who figured out the "correct test answer" didn't notice the error. :happyno:

A #2 wire has an ampacity of 115. We were not given the breaker size, so I must assume a breaker size of 115 amps. That means that the minimum size EGC associated with the #2 phase conductors is a #6. So if the problem gave you an upsize from #2 to #2/0, it should have given you a #6 EGC, and asked you what to upsize that EGC to. If you do that math, in the same fashion you did, a #3 comes up as the answer.

 

[hankhill]

Thank you Charlie, I see exactly what you're saying. You looked several layers deeper into this question, while I assumed too much. I think that I also may have dropped too many numbers from the right side of the decimal point when I gave you the numbers. (Sorry, my fault.)

#2/0 AWG Circular Mils: 133100
#2 AWG Circular Mils: 66360

133100/66360 = 2.0057263

#6 AWG Circular Mils: 26240

26240 x 2.0057263 = 52630.258 (Minimum required)

#3 AWG Circular Mils: 52620
#2 AWG Circular Mils: 66360

Maybe #2 should be included as a possible answer for this question, I don't know. Thank you for clearing this up for me I really appreciate it!

 

[charlie b]

Maybe #2 should be included as a possible answer for this question, I don't know.

I really hope they don't! :happyno: In my view, 10 circular mils is insignificant. I think a #3 would satisfy the minimum requirements, and adding #2 as a possible test answer would only add to the confusion.

Thank you for clearing this up for me I really appreciate it!

You are welcome.