Complex Imaginary Test 1 Problem 40

[Wildsoldier PE]

I dont agree with this answer. But I could be wrong GA Tech review course discuss a problem similar like this but with different values the pages from the GA Tech review notes are NEC 21-23.

Zeff=.07(400f/1000ft) = .028

Vphase = 277.12V

Vdrop in the line from line to neutral = 70A x .028ohms = 1.96V <--- this is a line to neutral voltage drop

Vdrop line to line = 1.96V x square(3) = 3.39V line to line

V@ load = 480Vline to line - 3.39Vline to line=476.61 (choose 476V) Answer B

Book shows 478Volts as a correct answer.

I think the book forgot to convert the Line to N Vdrop to line to line Vdrop.

 

[EEVA PE]

I dont agree with this answer. But I could be wrong GA Tech review course discuss a problem similar like this but with different values the pages from the GA Tech review notes are NEC 21-23.
Zeff=.07(400f/1000ft) = .028

Vphase = 277.12V

Vdrop in the line from line to neutral = 70A x .028ohms = 1.96V <--- this is a line to neutral voltage drop

Vdrop line to line = 1.96V x square(3) = 3.39V line to line

V@ load = 480Vline to line - 3.39Vline to line=476.61 (choose 476V) Answer B

Book shows 478Volts as a correct answer.

I think the book forgot to convert the Line to N Vdrop to line to line Vdrop.

I got 476V also

 

[EEVA PE]

I dont agree with this answer. But I could be wrong GA Tech review course discuss a problem similar like this but with different values the pages from the GA Tech review notes are NEC 21-23.
Zeff=.07(400f/1000ft) = .028

Vphase = 277.12V

Vdrop in the line from line to neutral = 70A x .028ohms = 1.96V <--- this is a line to neutral voltage drop

Vdrop line to line = 1.96V x square(3) = 3.39V line to line

V@ load = 480Vline to line - 3.39Vline to line=476.61 (choose 476V) Answer B

Book shows 478Volts as a correct answer.

I think the book forgot to convert the Line to N Vdrop to line to line Vdrop.

I see the same symptom in Exam #2 , prob 40

 

[DK PE]

I dont agree with this answer. But I could be wrong GA Tech review course discuss a problem similar like this but with different values the pages from the GA Tech review notes are NEC 21-23.
Zeff=.07(400f/1000ft) = .028

Vphase = 277.12V

Vdrop in the line from line to neutral = 70A x .028ohms = 1.96V <--- this is a line to neutral voltage drop

Vdrop line to line = 1.96V x square(3) = 3.39V line to line

V@ load = 480Vline to line - 3.39Vline to line=476.61 (choose 476V) Answer B

Book shows 478Volts as a correct answer.

I think the book forgot to convert the Line to N Vdrop to line to line Vdrop.

I don't have this reference but it appears you are correct. My only guidance is be prepared to handle how this changes as

1. the load power factor is something other than unity

2. The conductors have an appreciable "X" component

 

[xd-data-ii]

I see the same symptom in Exam #2 , prob 40

What are you calling Exam #2?

It is not the variation 2 book. Number 40 in that is about a short circuit test on a transformer

 

[EEVA PE]

I see the same symptom in Exam #2 , prob 40

What are you calling Exam #2?

It is not the variation 2 book. Number 40 in that is about a short circuit test on a transformer

My variation #2 book problem #40 is not about a short circuit. We must have different version releases. I got mine in June. When did you get yours.

 

[Complex Imaginary]

Good catch, everybody! I've looked this over and I agree with everybody here. Errata for this question will be issued via email to all customers on 9/28.

Josh

 

[xd-data-ii]

I see the same symptom in Exam #2 , prob 40

What are you calling Exam #2?

It is not the variation 2 book. Number 40 in that is about a short circuit test on a transformer

My variation #2 book problem #40 is not about a short circuit. We must have different version releases. I got mine in June. When did you get yours.

last week. seems you must have older versions.

I think Josh mentioned this in another post, so your references to questions wouldnt apply to most people.

 

[ElecPwrPEOct11]

The new errata helps but is still not entirely clear since it says the correct answer is 477V. I get a value of 476.323V after carrying 3 decimals through the problem. This makes a hard choice between answer B ) 475V and C) 477V. Is the real test going to have answers that are so close and seemingly due to rounding?

I looked at how other people did the question and realized I didn't use the Zeff value in the book (which is easier but limited to 0.85 PF) but instead calculated it for the 0.85 power factor. Using the NEC values of R= 0.057 and X = 0.052 and note 2 of NEC Table 9, I get a Zeff of 0.0758. Carried through this makes Vdrop= 2.1224 (line to neutral), and 3.676 (line to line).

Strange that this Zeff of 0.0758 doesn't even match the 0.070 in the Zeff column of Table 9. At the end of the day I fundamentally have the right method, but the choice of Zeff and when you round the values seems to give different results. Any thoughts on how to avoid this problem during the test?