Derating Factor

According to NEC-2008,
If more than three current carrying conductors are installed in a raceway or in a cable for a distance greater than 24 in or 600mm the allowable ampacity of such conductors shall be derated by the factors listed in artical 310.15(B)(2)(a) and Table 310.15(B)(2)(a).

conductors in ambient temperatures, exceeding 86 F are required to be derated according to the correction factor of table 310.16. The derating factors are in the ampacity correction factor chart below table 310.16 and are selected based on the ambient temperature srrounding the conductors and based on the type of insulation and the correction factors are based on the material, insulation and size of the conductors utilized.

→ British Standard 7671.

It depends upon the depth of burial, soil temperature, thermal conductivity of soil, grouping factors, etc. Better to refer to the cable manufacturer's catalogues for these de-rating factors and arrive at the net-derating factor.