Traditional electric distribution systems are radial in nature, and are supplied through a main source. These networks have a very simple protection system, and its implementation is also very simple. Distributed generation (DG) is defined as energy sources connected to distribution systems. Presence of such energy resources in distribution systems is being extended rapidly because of its useful advantages, but such advantages can be achieved if the relevant concerns are deliberately taken into account. For example, penetration of DG currents results is not having a radial distribution network, and consequently losing protection coordination. This paper has analyzed and derived a set of equations of general characteristics of protection devices to represent such protection coordination and operating range constraints. This new approach is presented for determining the maximum capacity of distributed generation for maintaining the protection coordination. The proposed approach was implemented using MATLAB and Power Factory DIgSILENT on a simple distribution network and the numerical results is presented.