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If it's a test question you would use the voltage given in the question. Same with a calculation in Article 220.
Cap.
- Thread starter Flight987
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That's why I edited my response, because using 480V didn't match the answer key and 460V did.
jumper
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I am getting senile. I used 460 and did even not attention to what I did. :slaphead:
jumper
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jumper
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Assuming your mean 80kVAR (not kVR or kVA)
Assuming you mean 3 phase, not 3/0.
3 phase Capacitor current = 3 phase VAR / Line to Line voltage * sq. rt. 3
so I = 80,000/460.1.732 = 100.41A
If you are connecting the caps at the motor terminals, it's best to use the votlage rating of the motor, 460V, because that is the "utilization voltage" and accounts for voltage drop.
If you are connecting the caps at the line equipment, i.e. with a capacitpor switching contactor, then use 480V.
jumper
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Where is the 135% here that 460.8 calls for?
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That's for sizing the conductors once you know the current. This was for determining the current.
jumper
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In cases, like this, I always play it safe and go with the larger wire. Bigger is usually better.
In cases, like this, I always play it safe and go with the larger wire. Bigger is usually better.
iceworm
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I know this is a test question and I agree the calc should be done with the given information. The goal in a test is to give the answer the examiner wants to hear. So, yes, I agree that using 460V for the calc is correct. I got all of that.
Now that we are by that, here is my comment/question - Consider actual design/installation issues (as opposed to a test question) - these are two I have actually seen:
If one had 600V rated caps and the installation system voltage was indeed at 480V, this pushes the kvars down to 64% rated value and the cap current down to about 80%.
The second was 7200V rated caps applied at 6600V. This drops the kvar rating to 84 % and cap current to 92%.
The wire size difference, saving a few bucks on copper, wouldn't matter to me, but I would tend to lower the OCP if I could. My thinking is if a fuse blows, there is no cap to save - It died. One just wants to get the fire out as quickly as possible to limit the damage.
The change in kvars could easily be significant and could well require a change to get the required power factor correction (or harmonic reduction).
So, the question is (for those that have an opinion) how would you handle this?
When I saw these two, in each case I noted what was done, and didn't particularly agree with the solutions. Since I wasn't getting paid to look at that part of the installation, I didn't say much about it. However, I am interested in what others think.
ice
I would look at that differently. If one were to use 480V instead of 460V, the kvars go up by the ratio of the square of the voltages, and curent goes up by the ratio of the voltages. So at 480V, the cap current would be 105A and 135% is 141A. With respect to the test question, it wouldn't change anything - wire size and OCP remain the same.
Now that we are by that, here is my comment/question - Consider actual design/installation issues (as opposed to a test question) - these are two I have actually seen:
If one had 600V rated caps and the installation system voltage was indeed at 480V, this pushes the kvars down to 64% rated value and the cap current down to about 80%.
The second was 7200V rated caps applied at 6600V. This drops the kvar rating to 84 % and cap current to 92%.
The wire size difference, saving a few bucks on copper, wouldn't matter to me, but I would tend to lower the OCP if I could. My thinking is if a fuse blows, there is no cap to save - It died. One just wants to get the fire out as quickly as possible to limit the damage.
The change in kvars could easily be significant and could well require a change to get the required power factor correction (or harmonic reduction).
So, the question is (for those that have an opinion) how would you handle this?
When I saw these two, in each case I noted what was done, and didn't particularly agree with the solutions. Since I wasn't getting paid to look at that part of the installation, I didn't say much about it. However, I am interested in what others think.
ice
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