Practice test question.

[K2X]

What is the smallest recommended thwn copper conductor to power an 80 amp single phase load located 360ft from a 240 volt service if all lugs are rated 60 degrees c?

So i use Vd =2KIL/ cma, then cma=2kil/vd..

The recomended vd on the feeder side is 3% and 12.9 seems to be K on most of these practice questions so cma=2x12.9x80x360/.03x240=743040/7.2=103200.
So it looks to me like 1/0 with a cma of 105600 would be the answer but the answer sheet says #2. Am I missing something???


Thanks for the help..

 

[Chamuit]

Tell us your guess, first.

I came up with 1/0 for 3% and #2 for 5%.

Vd = 2 x K x I x D / CM
2 x 12.9 ohms x 80A x 360ft / 66360(#2) or 105600 (1/0)

 

[mfishmike]

1/0 or #2

1/0 or #2

I ran the numbers through my computer program as I usually do.

A 1/0 will give you a 1.4 % drop or 3.4 volts loss with the CMA of 105600.

A #2 will give you a 2.3 % drop @ a 5.4 volt loss. CMA= 66360; which satisfies the 3% allowance.

 

[infinity]

Since this is a branch circuit I would use 5% VD.

 

[yired29]

Since this is a branch circuit I would use 5% VD.

210.19 (A) (1) FPN 4 states 3% on branch circuit or a total of 5% on feeder and branch circuit combined. This means 2% on feeder and 3% on branch circuit or (215.2 (A) (3) FPN 2) 3% on feeder and 2% on branch circuit. In any case the NEC recommends keeping voltage drop from exceeding 5% on the entire circuit but not to exceed 3% on branch circuit or feeder.

 

[infinity]

210.19 (A) (1) FPN 4 states 3% on branch circuit or a total of 5% on feeder and branch circuit combined. This means 2% on feeder and 3% on branch circuit or (215.2 (A) (3) FPN 2) 3% on feeder and 2% on branch circuit. In any case the NEC recommends keeping voltage drop from exceeding 5% on the entire circuit but not to exceed 3% on branch circuit or feeder.

But if you think about it, the recommendation is for a total VD of 5% if you have both a feeder and a branch circuit. Since you do not have a feeder then why would you look for a VD of less than 5% if you only had a branch circuit?

 

[mfishmike]

Feeder or branch circuit?

Feeder or branch circuit?

But if you think about it, the recommendation is for a total VD of 5% if you have both a feeder and a branch circuit. Since you do not have a feeder then why would you look for a VD of less than 5% if you only had a branch circuit?

I failed my test by 5 points 10 days ago and I marked three of the questions as "questionable" and gave my reasons why. I felt that 2 of the answers would work, just like we're doing here. The folks that write these exams do so to make you think about the exact wording in the question and do everything they can to trip you up on some of them. This is probably one of the biggest reasons people miss the question. I experienced this in my USCG masters license exam. I passed that test the first time.

I just checked the wording on this example queston in post #1 and it asked for the "smallest" conductor, even though it might be coming from the main panel, that may be the reason for the #2 answer.

It's a tough test and I go back in tomorrow for round #2. I'm taking the Texas PSI masters exam. I'll keep testing until I'm out of $78 test fees and gas money.

 

[M_J_C]

K2X,

I'm doing this from memory, so please forgive me if this seems too obvious (or just plain wrong).

The calculation you're using for wire size begins with a "2" as a multiplier, which I believe is for a circuit that requires a grounded conductor (neutral) to complete the path, because the "L" (or "D" depending on whose book you read) is for distance in feet one-way, you need to double that distance for round-trip circuit completion. Therefore you would not double the length for a 240 volt 1 phase circuit. That is why your wire size was larger than the minimum required.

Duh, I just remembered whose web site I'm on, and verified my thought.
Look here: http://www.mikeholt.com/documents/freestuff/ElectricalFormulas.pdf

Read the notes on the bottom of the second page.