DLinton:
I have never seen such a reference.
Although in my area at one time we were allowed to separate the the grounding electrode conductor into two parts and use two separate terminals, for example if it was a duplex with two 100 amp panels and a 200 amp service the No.4 wouldnt fit into the smaller lugs of the 100 amp panels. This was suggested to me by the city inspector, however, we have county inspectors for the rural area, and when I tried that out in the county he called to say, that he gigged the job, because I had separated the conductor as I did and said to me, You know Mark they make 'Listed Lugs' for that get a couple, this was about 6 years ago now.
As I said I have never seen a reference for that.
-Mark
