If you're running 12 volts 10 amps or less might be OK - but if you're on 6 volt IMHO and experience, you'll need more to allow safely for the surge when you turn the headlamp on. If, for example, you're running a 36 watt bulb, plus tail etc lights, say 40 watts plus all told, then that's nigh on 7 amps constant - but far more more in 'surge current' when you first hit the switch and the filament warms up. The fuse might take it, or it might not. After a couple of failures with 10 ampers, I've gone for 20 amps on my 6 volt bike - it'll blow well enough if there's a fault, but doesn't plunge me into darkness when I turn the headlight on or swap from main to dip beam - a constant source of possible surges with the traditional switches employed. Even with the bare glimmer of 25 watt bulbs, which are as much use as a bedside candle to a blind man, 10 amps can blow. Max surge can, I'm told, amount to three times rated requirement - so for 25 watts plus 5 watts tail plus 2 watts speedo - 32 watts, that's 6.33 amps, which could become way more than 10 amps for a long-enough millisecond.
With the 12 volts conversion installed on another bike, I use household 13 amp fuses (notional load 156 watts) because I have gazillions of old plugs with them in. Modern blade fuses are fine, so are the old spring-loaded cylindrical in-line jobbies, although they need the blobs of solder/tin that touch each end of the fuse to be clean - and cleaned periodically, especially in winter. One thing's for sure - a bike without at least one properly fused circuit is a hazard waiting to go bang - especially if the wiring is not accessible and rippable-out when things start to sizzle! Hope that helps, Groily
