Multi-objective optimizations by means of 3D finite element models result in very high computation burden. To have an affordable computation cost, the output space-mapping technique is applied with a new method where the scalar correction of the model outputs is replaced by a set of corrective functions. This method is used for the bi-objective optimization of a transformer and allows finding the complete Pareto optimal set in less than two days on a laptop. Index Terms-Space-mapping, Multi-objective optimization, 3D finite element model, Transformer.