Mission critical network offers a reliable, delay minimized available communication network that accomplishes a mission for the community. Smart grid is an example of these networks which is a modernized electrical grid that uses communication technologies to gather and act on information. Delay is one of the main requirements of this network that needs to be carefully studied. Considering the vast geographical area being covered by Smart Grid, a good approach for decreasing delay is to divide the network into a number of subareas and place the relay nodes in each one. This placement should be accomplished in a way that communication delay for all nodes become less than an accepted maximum. In this paper, firstly, an Integer Linear Programming (ILP) problem formulated which cannot be solved for a large network like Smart Grid. To lessen the complexity of this model, we added a preprocessing phase on the input and changed the model to accept this new input. A heuristic algorithm also implemented and compared with the proposed method. The simulation results show that our method drastically decreases the solving time while keeping the optimality of the answers. In addition, it was found that the heuristic algorithm finds the sub-optimal answers for dense networks. So due to the inherent tree topology of Smart Grid and its optimality requirements, our method surpass the heuristic algorithm.