In the field of hairpin stator technology, increasing demands are currently being made on the semi-finished product of insulated flat copper wire. In particular, the focus is on the electrical requirements against the background of increasing voltage levels to 800 V and more. The test procedures described in the standards IEC 60317 & 60851 for verifying the properties of insulated flat copper wires only partially map the requirements from the point of automotive industry. An example for insufficiently considered properties lays in the correlation between wire and insulation resin. In addition to electrical and thermal benefits, impregnation helps to mechanically stabilize the winding and protect it from ambient factors. Adhesion between the winding and the impregnating resin is a key parameter here, but is not considered in material pre-selection today. The adhesion of the impregnation resin to the insulated wire is essential to ensure the lifetime of electric motors. This paper describes a method for determining the adhesion of the impregnating resin to the insulation of the wire. It could be shown that there is a correlation between the material of the wire insulation and the impregnating resin in terms of adhesion. Further on the described method can be used for an application-oriented specification of insulated copper flat wires to ensure a consistent composition of the insulation material.