If I set the "way back" machine to when IBM wanted IGs (maybe before some of our readers were born) all we wanted was a wire that originated at the MDP and did not land in any other load side panel. Typically these were simply spliced with a wirenut in intermediate panels and it could be a single wire sized to the largest downstream EGC until it split out to the various branch circuits. As long as it was a "tree" radiating from the ground bus it was OK with us.
Over a quarter century ago IBM decided it was not worth the money and stopped "officially" recomending it. The legend still lives on tho.
