It has been shown in Chap. 4 that designing a power transformer to meet a required set of specifications may be analytically reduced to the determination of the optimum electric loading, magnetic loading, core diameter and window height that would lead to the fulfilment of the transformer volt-ampere (VA) rating, maximum copper losses, maximum no-load losses and leakage reactance. It has also been shown that, through the analytical reasoning, all other design parameters may be deduced from the abovementioned optimum design values, materials specifications and design assumptions within limited pre-identified ranges. In this chapter, identification of the design problem is further elaborated and the impact of utilizing electromagnetic field calculations in the accurate design process and/or cost minimization is discussed.