Think in terms of "work". That is the effort required to do something. OK, now think about Volts as "pressure" current as "volume". With twice the pressure you need 1/2 the volume to do the same work. With 1/2 the pressure you need 2 time the volume to do the same work. Fuse only sees the volume. So....6 volt head lights= 20 amps each. 12 volt head lights=10 amps each. 6 volt blower motor=40 amps and 12 volt would be 20 amps. Modern cars are 12 volt and head lights get pulled together. So 2 head lights at 12 volts =20 Amps. Are we having fun yet? This is not too hard just take your time and pay attention to the requirements of each item you are fusing. 6 volt wires are just fine for 12 volt systems. 12 volt wires might be too small for 6 volt systems. From 6 to 12 volt keep the wires and you will be OK. 12 to 6 volt change might take some wire changes. But most don't go from 12 to 6 unless they are restoring. Cigarette lighter will pull more amps than your phone charger. put in a 15 amp just to be safe. Heater fan, 30 amp. Turn signals, 15 amp. back up lights, 15 amp. Something like that. This is all for 12 volt systems. Double if you are 6 volt.
