Available transfer capability (ATC) is an important indicator of remaining transfer capability in the transmission network for its commercial use in a competitive electricity market. In this paper, an optimal power flow based approach has been proposed for multi-transactions deregulated environment. The ATC has been obtained with unified power flow controller (UPFC) and Sen Transformer (ST) for intact and line contingency cases. The impact of ZIP load model has been evaluated on ATC with both devices. The main contribution of the paper is the comparison of ATC obtained with UPFC and Sen Transformer for intact and contingency cases with constant P,Q load model and also taking impact of ZIP load model. The results have been determined for intact and line contingency cases taking simultaneous as well as single transaction cases. The results have also been obtained with DC/AC PTDFs method for comparison. The proposed method have been applied for IEEE 24 bus RTS.