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Existing EMS applications resort to static network models built upon constant resistances whose values are specified beforehand. However, the steady-state thermal balance of overhead conductors requires that ohmic losses be dissipated by convection and radiation heat, leading to conductor resistances being a function both of environmental factors and transmitted current. Each term of the equation involved in the thermal balance of overhead conductors is analysed allowing the influence of conductor current on conductor temperature to be separated from that corresponding to environmental factors. As a result, three approximate thermal models are elaborated, which are compared on the Spanish transmission system. Test cases are included showing that the influence of the selected model on transmission losses may be significant enough, particularly when the transmission system spans areas with diverse loading levels and weather conditions.