Q. What is derating factor for cable laying when different size of cables are laid together?

Combining different size cables is not cause to derate. Derating is needed when the amount of current carrying conductors is greater than the ampacity of the standard 3 current carrying conductors will create more heat. Another reason to derate is if the wiring passes through an area with higher than normal ambient temperature.

Hi, sir tkrussel
How about direct burial cables is deration applied if 4 cables(3C-300mmsq xlpe cu) are laid in one trench? What will be the suggested spacing and depth of the trench to attain minimum deration?
Thanks.

Ambient temperature is 60deg centigrade.What is flexible cable size I have to use for 63A breaker inside the Low voltage switchgearpanel having IP-65protection.