Size of Conductors-Capacitor Motor Connection

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Steve Temp

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What formula is correct to size my conductors for a 21 amp capacitor connected to the motor terminals of a 25 horse 208 motor, tried 135% of the cap, then tried 125% of the motor feeder @ 74.8 amps, divided by 1/3 as per 460-8, and connot find the correct result
 
It most certainly is a renewal exam question. Check out 460.8 and good luck getting the math to work out, I sure couldn't. I think this is an error in the test. For those of you who care to give it a whirl, the questions reads EXACTLY as follows:
The ampacity of the capacitor conductors of a 21 ampere rated capacitor connected to the motor terminals of a 25 horsepower, three phase, 208 volt motor shall not be less than: a) 26.50 ampere b) 30.85 ampere c) 33.35 ampere d) 35.85 ampere.

I worked this out based on the 135% of the capacitor rating AND the 1/3 of the minimum motor circuit conductors and I came up with 31.16 amps, based on the 1/3 of the motor circuit condutors 25 HP motor amps per table 430.250- 74.8 x 125% for minimum motor conductors= 93.5 amps/3= 31.16 amp. The other way is simply 21 amps x 135%= 28.35amps.

If any of you figure out where ( or if) I went wrong, let me know via a private message please.
Thanks
 
I am not comfortable with supplying someone with the answer to a renewal exam question, therefore I am closing this thread.

Chris
 
I have re-opened this thread after having a look at the numbers used in the question. It appears there may be a discrepency with some of the answers. I am OK with a discussion on how to apply the math in section 460.8 since the OPer has already posted the section.

Chris
 
raider1 said:
I have re-opened this thread after having a look at the numbers used in the question. It appears there may be a discrepency with some of the answers. I am OK with a discussion on how to apply the math in section 460.8 since the OPer has already posted the section.

Chris
Click to expand...

It's still a renewal question. Make up your mind. You moderators need to huddle up. :confused:
 
Out of curiosity, Why are threads,that ask questions from "Re-newal Tests"

closed? Or any test for that matter. Is'nt all questions on this forum

in one way or another a test question. Just people striving to find the

"Correct" answer to a problem. Real world or Exam.
 
chevyx92 said:
It's still a renewal question. Make up your mind. You moderators need to huddle up. :confused:
Click to expand...

Yes, it is a renewal question.

But the original poster has already posted the code section he was using, and there does seem to be some discrepency as to the math, so I felt a discussion on the math would be OK.

I am human and sometime change my mind.:)

Chris
 
IMHO it is all on how the test writers do their math.

74.8 x 125% for minimum motor conductors= 93.5 amps
93.5 x .33 (almost a 1/3) = 30.86 amps This is close to test answer (b) which is 30.85 amps
 
stevenje said:
IMHO it is all on how the test writers do their math.

74.8 x 125% for minimum motor conductors= 93.5 amps
93.5 x .33 (almost a 1/3) = 30.86 amps This is close to test answer (b) which is 30.85 amps
Click to expand...

93.5/3= 31.16 amps ( an exact 1/3)....not close to the test answer at all, but I think that you nay have solved the mystery as to how the test admins did the math to come up with the answer.
 
Remember, consider the actual feeder size vs the computed...

93.5 is calculated feeder size.
Actual feeder is #3 - 100A (75C column)
100/3 = 33.33


Answer = C (but not sure how they get 33.35)
 
wasasparky said:
Remember, consider the actual feeder size vs the computed...

93.5 is calculated feeder size.
Actual feeder is #3 - 100A (75C column)
100/3 = 33.33


Answer = C (but not sure how they get 33.35)
Click to expand...
That's the way I looked at it. However, (there is always a however in life), no matter how you do the problem, the math does not work.:roll:
 
rcarroll said:
That's the way I looked at it. However, (there is always a however in life), no matter how you do the problem, the math does not work.:roll:
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I'm trying to figure out where the missing 0.02 is.

Actually 33.33 is less than 33.3333333333, so I guess the answer is 33.34

Now I just need to find the remaining 0.01 :rolleyes:
 
It would be interesting

It would be interesting

It would be interesting to know who writes the renewal exams for Colorado. It seems that there is always one question that does not appear to have the right answer as a choice or it is a typo. One thing is for sure. The renewal exams makes you dig deeper into the NEC than you do on a daily basis and that's a good thing.
 
Colorado Code Question #17

Colorado Code Question #17

To all, yes it is the question on the Renewal Exam for the State of Colorado, I did provide the the section for which I was basing my answer including the math, but neglected to state I also used Table 430-250 for amperage of the 25 HP Motor. I also have Master Electrical Licenses in the State of Virginia, Washingto DC, and Maryland. This question baffled me,

I believe it would be in the best interest of this forum to provide help with all questions, people of this forum need some direction from time to time, and I do believe that is why Mike Holt set this up. Not looking for a quick, cheap answer, but as you can see, there is not a correct answer. Mike Holt has said to go for the answer that is closest to the real one, as you can see there are (2) that fit.

Steve
 
Steve Temp said:
...but as you can see, there is not a correct answer. Mike Holt has said to go for the answer that is closest to the real one, as you can see there are (2) that fit...
Click to expand...

There is one correct answer, and one that appears to be correct - if you do not read closely...
 
How to do the Math

How to do the Math

It is correct that it would be 74.8 * 1.25 = 93.5 then multiply by .33 it does not work out if you merely divide by 3. The answer is 30.85
 
rmbowen said:
It is correct that it would be 74.8 * 1.25 = 93.5 then multiply by .33 it does not work out if you merely divide by 3. The answer is 30.85
Click to expand...

What conductor did you select?
(you need 1/3 the value of the conductor selected, not 1/3 the value of the calculated ampacity)
 
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