The integration of Distributed Energy Resources (DER) in the distribution network has plenty of advantages if their allocation and Penetration Level (PL) are done appropriately. Hence, the challenge of finding the best allocation and PL of DERs in large distribution networks is an important but intricate problem. This paper proposes a novel methodology to simultaneously determine the optimal location/capacity and PL of DERs based on both power losses and voltage deviation minimization, while constraints of voltage profile of feeders under light loading and short circuit capability of the CBs are met. Moreover, a Multi-Objective Mutation based PSO (MOMPSO) is presented that by introducing two modifications of dynamic inertia weight and utilizing a mutation operator improves exploration and exploitation searchability as well as convergence capability of the PSO algorithm. The proposed methodology is tested on a practical distribution network to evaluate its effectiveness in finding optimal location and capacity of DERs along with the feeders.