This paper shows that a magnetic coating can reduce AC copper loss caused by the proximity effect based on a measured value. A solenoid coil is used for the measurement. This reduction was quantified by supplying external magnetic fields to conductors placed in a central region of the solenoid coil and measuring AC copper loss as changes in the coil's resistance. AC copper loss in aluminum flat wires was minimized by applying a magnetic coating to the wires. The experimental and FEM results confirmed the reduction. The AC copper loss was reduced by a maximum of 59% due to applying magnetic coating. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.