[rotorwrench]

It's all about amperage as was mentioned. The only reason they associate them with 12-volts is to indicate a low voltage DC electrical system. 12-volts is the most common automotive voltage in the common era. 1956 was a long time ago as much as I hate to admit it. Short stop circuit breakers use a bi-metallic internal switch that is rated at the max safe amperage of the wire it protects. If a short to ground happens, a 6-volt system will be only slightly slower to trip but we are talking fractions of a second. All it has to do is trip before the wire burns up. If you don't think it will work then it's easy to test. Just short one with power on it to ground and see what happens.

Higher voltage systems can use smaller gauge conductor wire. A 12-volt system can use 18 gauge wire for average light duty circuits. 24-volt systems can used 20 gauge wire. Circuit breakers are sized with slightly higher amp ratings than fuses for the same gauge of wire since it will trip faster at overload. Wire is sized depending on what amperage is needed in the circuit. It can be larger but it can't be smaller. With fuses, they can be a lower rating on the average wire size but it can't use an amp rating that exceeds the rating of the wire or it won't protect the wire. Protection of the wire is the whole point of using circuit protective devices.