Can you overload the junction box behind the battery. In another words if you have to many circuits drawing power thru it will the fuseable link burn out? I will have the following hooked up there, Electric fuel pump, stock front harness, Vintage A/C, MSD box, and the fuel injection system.
If I need to add another buss bar where should I pull the power from. I have the stock snap ring battery terminal cables so no way to attach a second lead to it for the second buss bar. Thanks
Yes you can burn out a fusible link if too much current is pulled through it. Adding things past protection devices can make the protection device (fusible link, fuse, circuit breaker, etc) activate since the original circuit was probably designed without these adding things being considered.
It would be nice if the factory over designed the electrical system to allow whatever we wanted to add to be supported by what was on the car but they have no idea what will be done and they are only going to worry about making there systems work so when added electrical devices are added to the vehicle a person must properly install these things so they do not interfere with what is already in place.
Adding small things to the stock electrical system like relays and small additional current demanding devices normally will work just fine but when a person adds larger things like electric fans, higher amperage draw electric fuel pumps, electric water pumps, higher amperage draw ignition systems, audio amplifiers, and such you need to look at the whole picture.
Let's say a person wants to add an electrical device that draws 20A of current. To power this you could get this supply of power from a spot on the car. If it is past an existing 30A fusible link or fuse and this existing circuit is already drawing 20A, then adding another 20A to it past the fusible link or fuse will blow the fusible link or fuse as you are asking for 40A to go through 30A of protection.
You could go before this fusible link or fuse but you would need to add either another fusible link or fuse to protect this added circuit.
What a lot of people overlook is the ground path on the vehicle. Normally you have a large ground wire from the battery negative terminal to the engine block and a small one from the battery negative terminal to the fender (and sometimes there are additional braided wires from the engine to the firewall). The small wire from the fender to the battery negative terminal (forgetting for a minute about the braided wires from the firewall to the engine) is the limiting factor for other added things grounded to the car body. This wire while short is made to design what is already grounded to the car body. Add in new high amperage devices and ground them to the car body will stress this connection and become a restriction. Electrical devices need a good power source and also a good return path back to the power supply.
When I install audio amplifiers in cars and the amplifier requires a 4 gauge power wire I also add a 4 gauge ground wire from the battery negative terminal to the body of the car (and not the frame or subframe as these are normally electrically isolated with rubber mounts). Too many systems are hindered by getting a good power to the device but fall short in the ground return path.
On your car with the snap type battery terminals, you could get power on the other end of the starter cable but be careful with your wires are run so they do not get shorted before they pass through a fusible link or fuse block. I'm guessing since you are wanting to add an electric fuel pump, Vintage A/C, MSD box, and a fuel injection system you are not having this car seen as an original or restored car so why not get rid of the snap ring terminals and replace them and related wiring to support what you want to add ?. If you do get new battery terminal clamp ends, get the ones that have threaded studs on them to where you can make the wire connections to them with soldered on ring terminals on the ends of your wires. I've seen other battery terminal clamps that have set screws in them to clamp down on a stripped end of a wire but these to me seem like they are more prone to corrosion down the road.