Power dissipated in the center conductor of a transmission line causes an increase in temperature, which is the determining factor in the power handling capability. Mathematical solutions for the thermal resistance of the center conductor relative to the outer conductor and the electrical solution of the characteristic impedance Z0 are similar; however dielectric loss is not included and, therefore, makes the similarity invalid. A simple technique is described that includes dielectric loss and preserves similarity, thus achieving an overall correlation between thermal resistance and Z0. The technique is applied to many important practical transmission line configurations.