[bobbecky]

John brought up the main breaker in a house panel, and I think most people, including most electricians don't understand the purpose of fuses and breakers. First, the main breaker in a house panel is not intended to protect the house wiring, but is really there to protect the utility upstream from a failure in the main panel. The various breakers in a panel are intended to protect the panel from a circuit failure. So, in our RV's, a fuse or breaker in the 12 volt cable from the battery would be there to protect the battery from a failure in the cabling from the battery to the panel with the individual fuses. The small fuses in the panel are there to protect the panel from a failure in the 12 volt circuits. So, in sizing a fuse or breaker at a battery, the weak link is probably the cable, and then the battery. Most batteries can provide enough current to melt even a 4/0 copper cable pretty quick, before the battery were to fail. I would say, size the fuse as large as necessary to handle the maximum current that a 4/0 cable can carry without melting, and you will be good.