You should run the Xantrex string calculator , short answer is I suspect you will only have one option, when I've changed out ST2.5's , I just wire-nutted all the strings in series in the BOS ( balance of systems box, AKA the wiring compartment of the GT inverter ). If your conduit is to large for the BOS, add one of those grey PVC work boxes from home depot and do the wiring in there and they send the single string to the GT

http://www.xantrex.com/support/gtsizing ... p?lang=eng

This is the users manual

http://www.affordable-solar.com/admin/p ... 120951.pdf

And more about string voltages

http://www.xantrex.com/web/id/2671/docserve.aspx

Got the replacements and installation was done within a couple of hours.

Now I want to connect the RS232, Is there a max length of the cable from GT to the PC?

Good to hear your installation has gone well, I have used a full roll of cat5 cable ( 500ft ) without issue, I just soldered on DB9 connectors on each end