Parallel Feeders

[buddy tew]

[B[/B]

this is a hard one so all you smart electricans and or engineers.... i need help!

i had a question asked of me the other day that i could not answer. the question was on parrellel feeders. i know how to figure voltage drop across the line but how does this compare to different links in the cables? if you have two parrellel 4/0 feeders 650 ft long that is fed from the service equipment and it is 480 volts / 3 phase / 400 amp , what problem would you have if one of the phases is shorter than the other. for example, (A) phase in conduit #1 is 650 ft. (A) phase in conduit #2 is 642 ft. what would the amp / voltage between them be, if any, and how do you figure this problem?

thanks,
btew

 

[chris kennedy]

[B[/b]

this is a hard one so all you smart electricans and or engineers.... i need help!

i had a question asked of me the other day that i could not answer. the question was on parrellel feeders. i know how to figure voltage drop across the line but how does this compare to different links in the cables? if you have two parrellel 4/0 feeders 650 ft long that is fed from the service equipment and it is 480 volts / 3 phase / 400 amp , what problem would you have if one of the phases is shorter than the other. for example, (A) phase in conduit #1 is 650 ft. (A) phase in conduit #2 is 642 ft. what would the amp / voltage between them be, if any, and how do you figure this problem?

thanks,
btew

Oh boy, here we go.

http://forums.mikeholt.com/showthread.php?t=91255

 

[coulter]

Don has an excellect post in the link Chris posted.

paraphrased: "A 1% change is length gives a 2% change in current." It's a parallel resistor algebra problem. Not too tough.

So if you are right up to the limit of the conductors, the long one is 1% under and the short one is 1% over.

So how much difference is too much? Well, how close is the load to the conductor ampacity?

If the load is 99% of the ampacity, then the cable lengths better be within 1%. This is not a code answer, it's an engineering answer that will keep the cables within their rated ampacity.

carl