The present work is aimed at calculating the magnetic field around a current-carrying conductor positioned in air at a given height above a two-layer ground. The method is based on the successive imaging technique. A three-phase transmission model was constructed to simulate three-phase transmission line above a two-layer ground. The measured magnetic fields agreed reasonably with those calculated for one- and two-layer ground models. The proposed method of calculation is extended for magnetic field assessment in AC substations. The effect of magnetic ores in the top-layer of the ground on substation magnetic field values is discussed. © 2001 Elsevier Science B.V. All rights reserved.