Tom... to find the common GEC size according to table 250.66 note#1, you add the equivalent area for multiple sets of SEC and base the size of the common electrode conductor to this figure. I don't agree with redsy's post which describes one #4 to electrode with a tap off the #4 to the 2nd enclosure.
What type of building are you feeding with parallel 3/0 copper conductors?
markp, yes, this tap method is permitted by 250.64(D), but not a required method for grounding AC premisis wiring systems.
It is also an accepted practice to run seperate GEC to the electrode from multiple service disconnecting means installed according to 230.40 ex#2, sized according to the service entrance conductors feeding each disconnect...
shortcircuit
