Neutral beam injection systems for heating the plasma of a fusion research device utilize a deflection magnet to separate the unneutralized residual ions from the neutral particles and steer them into an ion dump. Performance of the separation magnet is crucial in that its failure will cause serious damage to beamline components. A technique using wire orbit simulations was successfully applied to test the performance of the modified 180° separation magnet for DIII-Big Dee neutral beam injectors. It simulated the stable ion trajectories, and showed the fringe field effects and the proper range of operating magnet field strength to be determined.