Sure, everyone has different skills when it comes to cable laying, but larger than 4x5 mm2 through an empty conduit over 15 meters I consider very difficult. For me, 5x4 mm2 would be the largest reasonable cross-section and I would NOT put it in a pipe, as mentioned. Voltage drop should not be a problem at 15m and 32A, even though I have not calculated it now. If necessary, one has to be satisfied with 24A.