Our 2002 Islander 270 was 'pre-wired', but I found out from the factory that the wiring was form the helm area to the bow. When I inquired where the power would come from, I was told that they simply take it from the general power available at the helm.
It made little sense to me to install a windlass that has a 25-amp breaker, fed from a supply with a 40-amp breaker. That's the kind of installation that causes your electronics to shut down when you raise the anchor.
So, I installed new, separate, feeder cables from the battery area aft to the console, to provide the power for the windlass. I used a surface-mount 40-amp breaker aft (as required within two feet of the source), and still installed the 25-amp breaker locally at the helm.
I made this choice with the smaller Horizon 500 windlass, which uses a 25-amp breaker. It would seem really strange if using a larger windlass, with the usual 40-amp breaker, to feed it from the general helm supply. JMHO.
Brian