The radial topology of Nigeria Grid makes various parts of the network prone to blackout or system collapse on occurrences of line outage, hence the need for transmission network performance evaluation. Available Transfer Capability (ATC) is an index for transmission network performance. This research presents a hybridized Continuation-Repeated power flow structure for ATC computation that provides a good approximate alternative to determine the maximum loadability. Single line outage (N – 1) criterion is used to simulate the effect of line outages on ATC values thereby identifying overloaded transmission facility. Single line outage can result in an infeasible operating condition (system collapse) of the Nigeria 330KV power grid and tends to lower transfer capability. Application of ATC expected values among selected candidate buses is used as criterion to siting of new generation resource, a comparison is then made between this ATC level index and heuristic Genetic Algorithm (GA) based optimization method for optimal siting of generator. The results show that GA method is far more effective in locating the optimal candidate bus for the siting of new generator.