Cable Splicing

Cable splicing is possible using bimetal cable slices (or similar), but not ideal and not for all situations. 11kV 300mm2 is a rather serious distribution power cable (I am assuming for connecting the power grid to a large industrial site).

What do your local electrical standards say regarding your particular application and the use of cable splices?

I am assuming this cable is run underground rather than overhead. Can you provide more information.

Physically you can do it but electrically there's a world of difference between the ampacity of 300 Sq mm copper and the same size aluminum conductor since the conductivity of EC aluminum is 56% that of the same grade copper.

300 Sq mm is 604 mcm, for the same insulation and ambient conditions copper's ampacity is 355A, aluminum 285, as the temperature rises the disparity increases. For equivalent ampacity the cross sectional area of an aluminum conductor needs to be increased by 50%; i.e., you need a 900 mcm aluminum conductor to carry the same 355A as a 600 mcm copper conductor, all other things being equal.

In addition to the foregoing, you may need to recompute the voltage drop.

"can we splice and extend existing U/G, 11 KV, 3C, Cu conductors, 300 Sq mm with new piece of cable having same size but with Alunimun conductors." NO!

" Are thy matching with each other" NO!

"Ampacity vge drop, etc are OK ?" NO!

Consult: NFPA 70E: Standard for Electrical Safety in the Workplace® or applicable electrical codes.

Have a look at the scope of NEPA 70E

This is more apt Network Design Manual

Good point, but we don't know where the unknown poster is located.