Any ill effect of different length cable 1

[schan05]

I am running two of sets of same type medium voltage cable with same rated capacity, but, take 2 separate routing paths, resulting in different length and therefore different impedance, how will this impact its overall performance under normal opearting and fault conditions? thanks.

 

[schan05]

One important clarification: both sets are run in parallel from one source and to the same load.

 

[itsmoked]

How about some more info??

What frequency?

What is the load?

What is the voltage.

What is the current?

DC or AC?

What kind of cable?

How big is the length difference between paths?

 

[davidbeach]

First off, that would be a violation of the NEC 310.4. The real issue is that the current sharing will not be equal; the greater the difference in length ( in per cent of total length, absolute difference length is much less important) the worse the current sharing. If the loading will not cause the lower impedance path to exceed the cable ratings, there probably won't be an operational issue.

If you are under the requirements of the NEC, you need to find a different way of getting there so that the two routes meet the requirements of 310.4. Lengthen the shorter of the two runs to get the length up to that of the longer run.

 

[schan05]

load is around 420A, 4160V, 3Ph, 60Hz AC, 3-1/c, 750kcmil copper, 5.5kV rated copper tape shielded cable, the total legth is about 1000 feet, length difference is approximately 5-7%. Thanks.

 

[davidbeach]

Other than the NEC violation, it looks like you should be in good shape. The 750kcmil copper conductors can individually carry the load (with no concern for voltage drop) so you won't have any concerns about the shorter conductor becoming overloaded. The shorter conductor will carry about 53% of the load, and will be subject to 27% more heating then the longer conductor.

 

[itsmoked]

Hey Davidbeach won't the heating raise the resistance of the short cable thru heating until there is a balance? Or is that taken into account in your 53/27 numbers?

 

[davidbeach]

No, I did not take increased resistance due to increased heating into account. He said a 5-7% difference in length, so I took the middle of that range and said 6% difference in length, meaning that one had 53% of the total length and the other 47% of the total length. The short one would then carry 53% of the current and the long one 47%. (53%/47%)^2, based on relative I^2*R heating, is 1.27.

 

[itsmoked]

Gotch it thanks.

 

[schan05]

Thanks for the explanation.

 

[jraef]

Excellent response David. You get a cookie!