Pardon me if I am wrong Diesel,.. I agree with the formula and calculation of current based on W=ExI (W = Watts, E = Volts and I = Amps), using a 60W globe in a 12V system hence 5A. If you're talking headlamps, don't forget to multiply by 2, which gives a total current draw on hi-beam of 10A based on your figures.
Relays are required to enable switching high current with a light (small) switch. It seems that you are just using the heavy cable as a feed to the fuse box and hence would not necessarily need a relay. That said, Brock is correct if you do want to switch power off when the bike isn't running. In that case you would need a minimum of a 35A relay.
The size fuse that you would put on the 30A cable depends on the overall anticipated current draw on the cable. If all devices fed by the cable have a combined max. current of 20A then a 25A fuse should be suitable. Your maximum constant current for that cable is obviously 30A in which case you go up to a 35A fuse to protect that cable if need be. Don't fit a fuse size that equals the anticipated maximum current draw as you may find yourself continually replacing fuses.
I don't think I can agree totally with Whizz's comments about surge currents etc. Headlights never equal a dead short as the electrical resistance always exists in the globe filament. The resistance may increase ever so slightly once the globe is illuminated but it is always there. A nominated Voltage can only push a limited amount of current through a set resistance. In this case, using nominal figures. a 12V globe pulling 5 Amp would have a resistance of around 2.4 Ohms by my electrical theory and calculations.
I am curious to know why Brock suggests that the relay should be powered from the tail light circuit. I would have thought any live, switched feed accessible would be acceptable.
Good advice too from Brock about checking on total capacity of your alternator v's total current required by accessories and the bike itself.
Cheers
Dennis