We present a coherency measure and a network reduction algorithm. The rank correlation function was used to determine the mutual couplings of generators. Generators were grouped at two correlation levels. These are grouping tolerances whose values are unique for a given system. The first produces the most strictly coupled generators to be aggregated, and the second identifies the generators to be saved during reduction. The terminals of the generators in each group were extended to include load buses to partition the network. Previously reported techniques were used to reduce generator and load buses.