Do not have a code book in front of me, but the reference is either 250.26, or 250.32 for your application.
It gives you two options for a sepearate structure.
A: is to supply the structure with 4-wire feeder (L1, L2, N, + EGS) just like any other sub-panel. The exception is you have to supply a ground electrode at the new service and bond it to the panel ground. But the neutral bus is not bonded, that bond exist at the main panel:
B: Is more restrictive. Absolutely no metal paths can exist between the two structures. If that condition you can run a 3-wire feeder (L1, L2, & N). With this option you set a ground electrode and GEC, and bond the neutral to the GEC just like setting a service.
As I stated earlier I never specify option B unelss I am 110% certain no metal paths exist like new construction. I use option A 95% of the time. By going Option A route it does not matter if a existing metal path exist or not, it is always safe.
You need to either add the N-G bond if the conditions allow, or add a EGC to your feeder. Either way a ground electrode is required.
