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In this article, we address the overhead hoist transport allocation and dispatching (OHTAD) problem in large-scale, 300-mm automated material handling system (AMHS) management. We propose a simulation-based, two-phase approach to solve the problem. At first, a discrete-event simulation model is built for each OHT loop. Without considering the OHT service requirements, the best OHT dispatching policy that efficiently controls OHT transports is selected among several OHT dispatching rules. For a given set of service requirements to an OHT loop, the required number of OHT vehicles in the loop can be calculated from the simulation results of this loop. However, as the total number of OHT vehicles in the fab is limited, the optimal number of OHT vehicles for each loop is then determined by leveraging the total requirements on OHT services from all the other loops. Our OHTAD objective is twofold: to meet the transport requirements of throughputs and to minimize carrier delivery times.