The international standard to determine grounding conductor size can be applied for a wide range of items because it includes the specific physical characteristics, but it has a problem where the numerical formula is complicated. However the JIS and domestic standard numerical formula are simple but have many constraints. The new grounding conductor size selecting method is necessary in order to solve such problems. Therefore, in this study, we suggested an improved method for selecting the grounding conductor size and simple numerical formula with a numerical formula control modeling. and we verified reliability of paper used case study.