This talk will discuss an algorithm to evaluate the margins to loadability limits of power systems without a full mathematical model. It will utilize the concept of Thevenin equivalents behind key transmission line terminals whose voltages and currents are available as phasor measurement unit data. Loadability will be described in terms of the thermal, voltage and stability constraints obtained from these Thevenin equivalents. The talk will begin with the origins of these loadability limits and the potential problems with their use. It will present some challenges associated with the computation of the Thevenin equivalent needed to apply the loadability limit criteria. It will conclude with a discussion of the cyber infrastructure needed to compute these margins.