When looking at a feeder, that needs derated for 4 CCC and then checked for voltage drop, what is the procedure; which gets done first? I would think they are independent with the VD calc being based on the 3CCC non-derated wire size. Compare that to the derated wire size for 4CCC and select the larger. Assuming they both calc to the same wire size, the difference would be the ground as derating does not change ground size but the VD calc will. I have done a search on this with no solid luck.
voltage drop calc for 4ccc feeder
- Thread starter mjmike
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voltage drop and ampacity adjustments are not all that directly related and need separate considerations.
If you size a conductor on VD alone you can often come up with a conductor smaller then allowed by ampacity tables in NEC for short runs.
For long runs ampacity tables alone may not allow for sufficient VD. You need to figure both and select a conductor that meets both needs.
If you size a conductor on VD alone you can often come up with a conductor smaller then allowed by ampacity tables in NEC for short runs.
For long runs ampacity tables alone may not allow for sufficient VD. You need to figure both and select a conductor that meets both needs.
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- Lockport, IL
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If you are going to have to derate for 4CCC, then do that first. VD considerations are optional, but that one is not. After you derate and pick your wire size, then do the VD calc on that wire size. That will tell you if you need to go bigger.
Thanks, do you then go back and resize the ground IF the derated conductor size is needed for VD?
Here is an example. Lets assume you are feeding a 225A panel. #4/0 is upsized to #250 for a derated amperage of 232A. The ground remains #4. When doing a VD calc, the #250 is suitable, but the original size of #4/0 would not. Therefore the ground would then need upsized to a #3.
Thanks
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That is a very good question. This has been a concern of mine over the past several cycles. I believe they finally clarified this in the 2014 NEC. Since a 4/0 would not have sufficient ampacity under the conditions of the installation, and since a 250 MCM is the smallest conductor that does have sufficient ampacity under the conditions of the installation, you don't need to upsize the EGC from the #6 value associated with the 225 amp OCPD.
To take it one step further, suppose this was a very long run, and you had to go larger than 250 MCM due to VD. Suppose you used 500 MCM. Now you do need to upsize the EGC. But you upsize it in proportion to the upsizing of the ungrounded conductors from 250 MCM to 500 MCM. To repeat the point, you don't upsize the EGC proportionally to the upsizing of the ungrounded conductors from 4/0 to 500 MCM.
To take it one step further, suppose this was a very long run, and you had to go larger than 250 MCM due to VD. Suppose you used 500 MCM. Now you do need to upsize the EGC. But you upsize it in proportion to the upsizing of the ungrounded conductors from 250 MCM to 500 MCM. To repeat the point, you don't upsize the EGC proportionally to the upsizing of the ungrounded conductors from 4/0 to 500 MCM.
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Although the physics agrees with your reasoning, the code does not.
You have increased the conductor size for ampacity reasons, not for VD.
But I would not object to a #3 EGC.
You have increased the conductor size for ampacity reasons, not for VD.
But I would not object to a #3 EGC.
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- Lockport, IL
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- Retired Electrical Engineer
The code does agree with me. At least the 2014 edition does. I have not increased the conductor size for ampacity reasons. That is exactly the point. I have chosen a conductor size that give me the minimum ampacity that I need for the circuit. I took the number of CCCs into account, when I selected the wire size. The 4/0 was never an option, since it did not have enough ampacity, when the CCCs were considered. So I am not increasing from 4/0. I am going bigger than 4/0 because 4/0 was not big enough. No increase in EGC size is needed.
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Sorry for the confusion. My reply was aimed at mjmike's post, not yours.
I agree 100% with what you say, I think.
I am a little confused by your statement that you did not increase the size of the conductor for ampacity reasons, you just had to start with the larger conductor. But I see what you mean. However it is correct that the adjustment required you to use a larger conductor than the basic three in a raceway table size. Some would call that an increase.
Charlie b, your post was very helpful and makes perfect sense. Re-reading it though, I do have a question as to why you are referencing a #6 EGC for 225A? Table 250.122 would required a #4 because a #6 is only good for 200A?
GoldDigger, I am also confused by your post. Can you please clarify? I got #3 EGC as follows: 225A = #4 EGC phases upsized from 4/0 to 250. in circ mills: (250000/211600) x 41740 = 49314. (250mcm/ 4/0) x #4 = #3.
Thanks
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- Placerville, CA, USA
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- Retired PV System Designer
All I am saying is that the Code requires that you upsize the EGC when you upsize the CCCs for VD reasons.
If you upsize the CCCs because of ampacity rating issues, then the Code says that you use the appropriate EGC size for that combination of circuit OCPD and conductor size and not add an additional factor for the upsizing of the CCCs because of number of conductor and temperature calculations.
thanks. I am in agreement.
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