The sizing of the wire that you need will depend on the capacity of the pump that you are using, if it is 800 GPH or below, then you need to be looking for a wire gauge of 16. Between this figure and 2000 GPH then you will require a wire gauge of 14, this is naturally the capacity that the vast majority of electronic bilge pumps have so it’s more than likely that you’ll need 14 gauge wire.
If you’re going for anything bigger than that then for anything from 2000 to 3500 GPH you’ll need 12 gauge wire and for anything above that you’ll need 10. This is vital to ensure the correct working of your bilge pump.