Wiring in portable generator

I plan to shut off the breakers on the selected circuits. This I believe would isolate these circuits from the common bus bar?

http://www.electrical-online.com/simple-diagram-of-a-generator-transfer-switch/

Utility power in would be your shore power cord and load would be your panel. (where the shore power cord was originally)

I believe you would be back feeding into your shore power cord. You need a transfer switch. Your shore power cord will go onto one set of terminals, your gen cord will go on a set of terminals, then you go from the set of terminals on the transfer switch to where the shore power cord was in the panel. Mine lets the generator run for 30 seconds before closing and putting power to the panel.

I would add that when you unplug the gen and the bare terminals of that cord are out in the open there is no way that shore power will back feed to them.

No. All the breakers are fed through a common buss bar in the panel. They cannot be separated out. The easiest way is to just plug it in.

B.O.