The loadability curves of three different types of single-circuit overhead transmission lines are computed and compared in this paper, for line lengths up to 500 km. The examined solutions include traditional 400 kV ac three-phase lines, innovative 325 kV ac four-phase lines, and monopolar 500 kV HVDC lines with metallic return. The loadability curves are computed accounting for thermal limits, voltage drop, power losses, steady-state and transient stability.