new method is developed for solving integrated transmission systems (as well as other types of extensive physical systems) without calculating the inverse of large matrices, or without using a large a-c analyzer. The physical system is torn up into several parts, each part is solved separately (either on an a-c analyzer or by a digital computer), and afterward the component solutions are interconnected into the solution of the original system by a series of transformations. The method is illustrated by establishing the individual and total loss equations of a power pool, whose component companies form several closed loops. A numerical example is worked out in a companion paper.'