temp rating ques.

[wireman1]

if the wires from a pad mounted trans. are in pipe and are buried 40 in .deepcan the 90 dregee column be usedto detrmine the ampicty of the wire.for instance a 500 mcm cable thwn is rated at430 amps . does it matter that the cables are buired under ground in determing which column in 310 .16 you can use especially if the cables are landed on a 75 degree terminal in which case the ampicty in the 75 degree column for a 500 mcm cable is 380 amps. being buried under ground does not have any exception on the cables amps?

 

[cpal]

The weakest link in the installation will determine the final ampacity of the run. The length underground may have an increased ampacity above the 90 deg col if the temperature of the cable is determined to be above 30 deg C. See 310.15 A and B. and refer to the temp correction factors in T 310.16. Is the sytem under the guidance or a PE??

Also you might wish to crimp larger conductors or adapters at the point of termination and allow the UG to carry a increased ampacit IAW the 90 Deg Column.

I'm sure others will ring in on this one good luck

 

[Smart $]

It makes no difference in the termination temperature requirements of 110.14(C) when the cables are run underground.

By most industry interpretations, there is no workaround once the 500's enter the pad-mount transformer's termination enclosure. If you were to compression splice (rated 90?C or greater) the 500's upon entering the enclosure to say 10' of 600's and then land the 600's, you may find a sympathetic ahj... but don't count on it.

The only industry-accepted workaround is to splice in a separate enclosure where all terminations and equipment are rated 90?C or greater. While there is no industry set cable length for the larger conductor, I recommend not less than 10'.

PS: your 500's would have to be of the xxxx-2 type before you can use the 90?C column for ampacity correction and adjustment.