I would have a main circuit breaker and an individual one on each circuit you are testing. You will almost be setting up the same way as a household circuit panel is connected. You need to be real careful on the battery side, especially if you are using a full size automotive battery. They are capable of producing a lot of current. If it were me I would be more apt to use a smaller battery like you would find in an alarm box. They are still 12V but only produce about 8-10 amps. They are a lot smaller and a lot safer and will allow the uses of smaller wire and lower current rated parts.
Artificial Intelligence is no match for Natural Stupidity