In the last decade, the installation of distributed generations (DGs) by many independent power producers (IPPs) has been being increased in distribution systems of South Korea by the active policy of government to expand power generations based on renewable energy sources. However, it is difficult to unlimitedly admit the penetration of DGs on a practical distribution system due to its stability and reliability problems. Therefore, it is required to reasonably regulate the allowable capacity of DGs in distribution system. This paper newly proposes a methodology and procedure to estimate the allowable capacity of DGs connected to a physical distribution system. All technical requirements derived for the estimation are established by considering protective coordination, short circuit current, and voltage stability. Then, they are evaluated by applying them to a practical distribution system in South Korea.