Seperate building

[kh1965]

If your feeding power to a building from the main service at the house a four wire feeder is required. Ground comes from main service. Isn't that correct?

 

[Dennis Alwon]

Yes you feed it as you would any other sub panel except that you need to drive ground rods at the building and connect the grounding electrode conductor to the ground bar. The neutral would be isolated from the grounds. You also need a main breaker or meet the 6 throw rule

 

[infinity]

As Dennis states your separate structure will require a grounding electrode system (GES). If you have 20' of 1/2" or larger rebar in the footing that would qualify as an electrode and must be used for or as part of the GES.