Analysis of AC copper losses in electrical machines is still a challenging task. Due to high utilization of high power density automotive drives, stray fields in the slots cause a significant copper loss increase compared to DC copper losses. Especially at large cross-sectional areas of the wires up to several times the DC-losses may occur in copper. This paper presents a new method of identifying these additional copper losses in electrical machines directly through measurement and without any finite-element co-simulation, which has not been shown in literature so far. Therefore, a motor sub-assembly consisting of three stator-teeth of an automotive machine and a rotor back iron are manufactured. Different adaptations of the motor sub-assembly are analysed using finite-element software to ensure proper flux-paths and AC copper losses close to the motor. Finally, measurements are performed and discussed.