I also agree with Bjarney - all the IG work in the world will not help if some clown has set things up already, which create ground loops!
Along with an attempt to reduce the possible noise being brought in from utilizing the common equipment grounding schemes (transmission of noise via conduit systems), the complete IG design criteria should focus around TVSS devices, Isolating equipment with different characteristics, and to attempt to eliminate the possibilities of ground loop currents.
For simple situations, (where the connected equipment is not crucial), adding the IG bus in an existing subpanel will work fine. Be sure to derive the IG "Feeder" from the point of system ground bonding (where the grounded conductor for the system of use, is bonded to the grounding electrode system).
The "IG Feeder" may be as small as #12 cu (if no larger than 20 amp OCPD on IG circuits), or could be #6 cu - which is normally used on my designs and projects.
After this, things become a little more in-depth, but not to the point of flat-out sillyness! (don't get me started on some of the crazy crackpot IG "Interpretations I have seen from some quack EEs!!!).
Feel free to ask more Q's, or contact me directly via E-mail. This IG stuff deserves a much more detailed section in the Technical Reference Area.
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!