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As electricity markets emerge, power exchanges and wheeling transactions become a common operating practice in most power systems. This paper considers an interim system which transfers power between two neighboring systems and presents a methodology to estimate the maximum secure wheeling transaction that the interim system is capable to support. By reactive or active power rescheduling actions, the interim system can significantly increase the size of transfer capability, thus offer support services to accommodate the wheeling transaction. The method utilizes an Optimal Power Flow algorithm, in order to calculate the maximum active power that can be transferred, subject to security constraints, as well as the support services provided by the host system. An AC-OPF model is used and a variety of formulations are examined to model the host system actions. The method is intended for application in real time procedures, utilizing the intra-day electricity market processes. An illustrative study case of the methodology is presented for a 26-bus power system supporting wheeling transaction between two neighboring systems.