The author describes a methodology for determining the loading limits of extra-high-voltage transmission lines, taking into account the system environment under which these lines operate. This methodology is based on the line loadability concepts developed for planning applications and extends their use to operating studies. For this purpose, specific line and system parameters are used to construct curves of thermal duty, bus voltage, and steady-state stability as a function of the power carried by the line. These curves, referred to as the operating loadability characteristics, not only make it possible to quickly identify the principal operating limitations to line loadability, but also provide a convenient mechanism for quantifying the trade-offs which exist between a given choice of system operating criteria and line loadability. >