The electrical design of distribution facilities is based on the loads they are to carry safely and permissible voltage variations, the final design, however, cannot be divorced from mechanic al, economic and other considerations . Several different designs may serve the same electrical requirements adequately ; each, in turn, may be modified by mechanic al considerations . The design ultimately selected must reflect economic considerations : specially, the design that results in that result in the least annual expense in supplying the load or loads in question.