I did some PCB's  for power applications a while ago, and we handle up to
30A in tracks with 70 micron copper and 10mm MINIMUM width. However, where
possible we fill out areas to act as heatsinks, i.e. some places tracks are
30mm wide.

It is not only the track width that has an impact, but also the length,
since the resistance is a function of the whole track area, and this
influences the actual heat dissipation (IIR).

Tom Theron