Steve, before we go any farther, do you think a GEC plays any part of opening an OCPD in a fault condition for services covered under the NEC?
Let's look at a Rod only installation, the GEC would only need to be a #6 CU even if we were talking about parallel (or more) 2000 kcmil's' 250.66(A)
Table 250.66 "Ground Electrode Conductor for Alternating-Current Systems" does not give any exceptions for oversized conductors, nor state the size in terms of current, but specifically requires GECs based on conductor size.
and 3/0 is the largest CU GEC required of any size conductor or parallel sets of conductors.
Even when a "Grounded Conductor" is brought to the service equipment, Article 250.24(C)(1) still references back to table 250.66, which requires it as a % of the conductor size with a few exceptions that won't apply on a 100A service.
That has nothing to do with a GEC, it is for clearing a fault and opening the OCPD, in otherwords the Grounded Counductor is an EGC (back to the source) in this instance, and is sized per table 250.66 since 250.122 could not be used due to this wording over the first column "Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)", we (speaking for our installed componets) would not have an Automatic OCPD at the POCO's transformer.
[This message has been edited by Roger (edited 08-17-2006).]