The house bank is amidships and the forward bank is at the aft end of the V-berth, hence the 15ft run. The main reason for the initial question was to find the maximum amperage that would be pulled from the house bank to the forward bank once the bow thruster was running. This would dictate the cable size I would need to install.

The reality is that I have a forward bank which weighs 106lbs and weight does not appears to be an issue. I have installed 1/0 cable between the house and the forward bank and this is more than sufficient to run the windlass directly from the house bank if I ever wanted to eliminate the forward bank. It is also big enough to supply the full amperage to the forward bank (125Amps) from the house bank/alternator.

This issue will be if I install the bow-thruster that I have recently acquired. Your last line is spot on; "For so short a run you are better off with cable rated for the load back to your main battery bank. " Can someone directly me to a formula to indicate what the max current flow will be between the forward bank and the house bank assuming the bow-thruster is still trying to pull 300amps from the forward battery. Based on battery internal resistance and cable resistance this should be less than the full draw by the bow-thruster.

This is an interesting article about connecting batteries which might help explain my question;

http://www.smartgauge.co.uk/batt_con.html

Thanks again

Gerry