Tom,
While the code doesn't require the grounded conductor to be brought to the building, it does require an equipment bonding conductor to be run into the building unless all of the building equipment grounding conductors are brought to the transformer. The equipment bonding conductor is sized per Table 250.66 using the size of the transformer secondary conductors. This is the same size that is required by 250.24(B)(1)for a grounded service conductor. The rule for SDS is not as clear as for services, but I think that the code really requires the same thing.
Don
