Question how do all the contractors insure parallel feeders are the same in length?? For a medium voltage installation is there any information that indicates what the tollerence is (+/- 10%?) just curious as we currently have a medium voltage installation where paralleled feeders are being utilized on a 2400' run, and was curious as to how they keep track of it with all the manholes, splices, etc. and what is an acceptable difference between cable lenghts? thanks.:grin:
Parallel Feeders
- Thread starter Mike01
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I don't have a technical answer, but I would think that your raceways are following the same route and fairly close to each other, so your length difference even over 2400' shouldn't be very significant as in within a foot or two. What kind of load do you have that you are using parallel on a medium voltage run (5kV or 12kV)?
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- Illinois
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- retired electrician
Mike,
The voltage doesn't really matter. It is an issue of the impedance of the parallel paths making a current divider. The most current in on the path with the least resistance. In your case if the only change in the impedance of the paths is from the change in length, even if you assume a 25' successive increase in length of each of the sets starting at 2350' the currents would range from 342 amps on the 2350' run to 324 amps on the 2450' run.
The voltage doesn't really matter. It is an issue of the impedance of the parallel paths making a current divider. The most current in on the path with the least resistance. In your case if the only change in the impedance of the paths is from the change in length, even if you assume a 25' successive increase in length of each of the sets starting at 2350' the currents would range from 342 amps on the 2350' run to 324 amps on the 2450' run.
wow
wow
I never thought of it like that thanks don, where can i find information on how to perform that type of calculation, so I guess it really comes down to the load on the cables as that would be your weakest link, this must also have something to do with single pole interrupting ratings of mccb's. thanks again.
wow
I never thought of it like that thanks don, where can i find information on how to perform that type of calculation, so I guess it really comes down to the load on the cables as that would be your weakest link, this must also have something to do with single pole interrupting ratings of mccb's. thanks again.
- Location
- Illinois
- Occupation
- retired electrician
Mike,
I use a simplified method for the calculation. It uses the total length of all of the conductors for a single phase and the length of each individual run as a percentage of the total. Then you take the percentage length of the longest conductor times the total current to find the current on the shortest conductor. You use the percentage length of the shortest conductor to find the current on the longest conductor. Note the there are other things that change the impedance of the runs and a poor termination can make a bigger difference than the length. Also if the cables are not in conduit, the spacing and distances from conductor to conductor and set to set can also produce larger impedance changes than the length.
I use a simplified method for the calculation. It uses the total length of all of the conductors for a single phase and the length of each individual run as a percentage of the total. Then you take the percentage length of the longest conductor times the total current to find the current on the shortest conductor. You use the percentage length of the shortest conductor to find the current on the longest conductor. Note the there are other things that change the impedance of the runs and a poor termination can make a bigger difference than the length. Also if the cables are not in conduit, the spacing and distances from conductor to conductor and set to set can also produce larger impedance changes than the length.
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