House bank negative wires

[millsrj]

Hi, I have a cruising boat that I am installing a new LifePO4 house bank on. I have 4/0 cables leaving the battery and going (through fuse/shutoff for POS, through shunt for NEG) to 600a busbars. All good thus far.
My question is what happens from the NEG busbar to connect to boat common ground. Does that wire need to be 4/0 as well or can it be 1/0 which would be the same as the positive wire from the panel. I was thinking that all current going to the POS busbar would stay local and head back to the battery via the NEG busbar and 4/0 black wire.
Second is the connection point to common ground. I have a busbar in front of the engine, about 10ft away. I have the NEG posts of the starter batteries, I have the contacts ON one of the engines and I have the NEG bar that is behind the panel (about 9ft away). I have included a pic of the busbar behind the panel. Not sure that it can handle a 4/0 lug if that is needed.
Thanks

 

[Skypower]

You have a three bank charger. That might already have a common negative. You need to check. If it is you might as well connect a larger lead to the ship’s ground and hence it’s galvanic bonding. After shock and safety, galvanic corrosion is my major electrical concern. Every ship is different and dependent on hull material and equipment(new?). The fall back question is, how was it rigged before and did it work before with healthy/correct alloy anodes. If it was poor chances are you might improve on it. The first thing I always check on a boat is the shore power cable. With normal equipment running I put a clamp meter on it. The line(s) and neutral should cancel each other out, if not you are losing current into the water through your bonded parts. Note! This may not be your boat’s fault! It could be a “hot” harbor or another vessel nearby. I clamp my neighbors shore power lines. Unfortunately you won’t believe how common this is. I’ve requested to move my boat to another slip before.