Based on the potential theory, an analytical method for calculating the temperatures of a group of power cables laid underground in parallel was developed. The technique is applicable in particular for the case of three-phase high-voltage cable lines laid in ducts. The analytical expressions are presented for effective thermal conductivity coefficients for elements of the duct taking into account conductive heat transfer, convection and radiation. Two algorithms are proposed for calculating the temperatures of cable cores based on the found average temperature of the element building duct. The developed technique provides a relative error of no more than 3% in estimating the temperature of the cable cores in the block when compared with the results of the FEM calculations. The method allows using standard spreadsheet software.