As it is known, there are many factors affecting underground power distribution cables' loadings such as ambient temperature, cable depth laying, number of cable parallel circuits and thermal resistivity of the soil. One important factor usually ignored is the formation of dry zones around the underground power cables due to cable loading. Dry zones usually form around underground power cables under loading condition due to the migration of soil moisture content. In this paper the effect of dry zone formation on the underground power cables ampacity is investigated. De-rating factor for the formation of dry zone around underground power cables is suggested and calculated for different types of natural backfill soils. IEC 60287-1-3 is taken as reference. Experimental work is done to study the dry zone phenomena of each type of soil.