Which cable for bathroom stereo ceiling speaker?

[hifiphones]

I am fitting a single stereo speaker in the bathroom, and the cable lenght will be 2x 30metres.

What should be the cable diameter? will 1.5mm2 be enough, or should I fit 2.5mm2? When I saw the 2.5mm2 cable I thought that it will be an overkill for one speaker of 6.5" size...