Electric power system planning is an important aspect to anticipate the increasing electricity demand each year. Therefore, it is necessary to plan electricity distribution networks such as planning for transformers and distribution substations that refer to existing conditions of the area as well as anticipated load growth to maintain optimal service. The ratings and capacity of the transformer are determined by long computations and intricate algorithms that can produce erroneous results in the current design standards for distribution transformers and distribution substations. This paper proposes to design a simulation program to bridge the issue using Graphic User Interface which can determine criteria of the transformer such as the ratings, the capacity, and the quantity of distribution transformer and distribution substation demanded in an area. The designed can accommodate load growth in algorithm calculation and the determination of the transformer rating still refers to the existing standards which must not exceed 80% of the transformer capacity. So it can reduce losses that may occur as a result of load requests that cannot be served and also optimize equipment, especially distribution transformers.