The application of probability methods to the determination of generating capacity spinning reserve requirements has been illustrated in several publications (1,2,3). The basic difference between a static capacity reserve study and a spinning reserve study is in the time period considered. The basic statistics in the first case are the probabilities of finding the existing and proposed units in various states at some time in the relatively distant future. In the spinning reserve case, the probabilities of having load carrying capability within the next few hours are required given that the system resides in a particular state at time zero. In each case, the actual system load is a variable and can be described by a probability distribution.