In this paper, a new nodal loading model for use in voltage stability assessment of electric power systems is proposed, and the formulation of worst cases based on this model, as well as related numerical methods, are described. In this nodal loading model, called the "hyper-cone" model, a set of future operating points in a load parameter space is modeled. That is, the "vertex" of the hyper-cone is taken to be the current operating point, and the "thickness" of the hyper-cone represents the uncertainty of future loading. The worst loading case is the point, among the set of transfer limit points on or within the hyper-cone, at which the total load is smallest. In other words, in terms of the uncertainty of future loading, the worst case corresponds to the most conservative transfer limit. Efficient numerical methods to compute this worst case are shown, and these methods are demonstrated on some sample power systems including IEEE 118-node system.