...Chassis gronding wire can be no smaller than one wire GA below the chargers DC negative...
Interesting. Of the four inverter/chargers we have owned (Trace, Prosine, Outback, Victron), none of their chassis ground lugs could take one AWG size below the 4/0 DC negative wire connecting them to the batteries. About 6AWG is the maximum that will connect to the ground lug.
Chassis ground needs to be able to carry the full current of the charger, in a failure/fault, and thus it needs to be bonded to your main DC neg bus or at the ships single ground point (most often the engine).
This has always been a bit of confusion to me. How does the main DC neg bus differ from the battery DC neg post? On our boat, there is a 2/0 wire from the negative battery terminal to the negative bus at the panel.
Our boat also has engines electrically isolated from the water, with the indirect exception of the raw water feed. There is no electrical connection through the transmission/prop. We also have no external ground plate. The engine block is connected electrically directly to the starting battery, and to the house DC negative only through an ACR when charging and the batteries are combined. The alternators are isolated ground leading directly back to the house battery.
Both the panel negative bus and the closest engine are 20' away from the inverter/charger. So why can't I save a 20' run of large cable and connect the chassis ground to the negative battery terminal, which is only 3' away from the inverter/charger?