I've been helping my son rebuild a house (tear down, new construction) and I'm doing most of the electrical. He also has a pole barn that had it's own 200 amp service (believe it or not) with it's own meter. He wanted to consolidate that into one service.
At first, I suggested we just feed the pole barn off a breaker in the new panel in the new house. But an electrician friend of mine said then it would be treated like a sub-panel, requiring a 4 wire feed. I called an inspector and also said that it would need a 4 wire service, even though the pole barn already had it's own grounding system. I've never understood that logic if the neutral was floated (not bonded to the grounding system).
The reason for not wanting to convert to a 4 wire service for the pole barn is it would have required trenching in a difficult area and the service conductors were direct bury, not in conduit.
Others here have said outbuildings require their own grounding electrode system. I'm interested in hearing from other parts of the country how this is handled.
We did solve the problem by going with a class 320 service, which of course treats the panel in the pole barn as a main service panel.
Best Answer
In every part of the country, any subpanel requires a 4-wire feed. That's been code for well over a decade.
Additionally, any outbuilding (not connected by a breezeway) requires a Grounding Electrode System of its very own.
The reason is because the wire takes human-made current back to source, but the ground rod takes nature-made current back to source. Dirt doesn't conduct well enough to do both.
Retrofitting ground isn't that big a deal
In a 3-wire outdoor subpanel feed, you can just add the ground wire. It needs to get from A to B, it is not required to run the same route or lay in the same trench.
Rigid Metal Conduit or IMC only needs a 6" burial depth (but it's expensive), and it qualifies as the ground path. Lay a 1/2" steel pipe and yer done. Use it for comms later :)