If you run a #6 bare copper GEC, you can run it exposed on the surface unless it is subject to severe physical damage (250.64(B). Although the minimum size grounding electrode conductor is typically #8 copper for a 100 amp service (it's actually determined by feeder size, not amperage), the NEC requires that conductors smaller than #6 be physically protected (usually conduit). It is almost always easier and more cost effective to satisfy the NEC requirement by supplementing one ground rod with an additional one (250.53(a)(2). The measurement must be performed with a dedicated ground resistance meter, something that very few contractors have. Now, this is the correct wire size for a sub panel that is close to the electric device you want to run. for 100A breaker (80A computed) 1 will come in at 2.99 drop. This is not required in the United States, but its a viable rule of thumb. The only way that you can get by with a single one, is if you can prove by measurement that the ground resistance is 25 ohms or less. With the 80 NEC rule, the best wire size for 100 amp service is 1 AWG. The Canadian Rule (dont permit more than 3 voltage drop at 80 of breaker trip, which is the max provisionable) requires a wire size 'bump' around the 175 mark on distance. If you are using rod type electrodes, you really need to drive two of them, at a minimum of 6' apart. The chart below gives an idea of the approximate wire size needed for power to travel through a line. 4 AWG to 1 inch is the safest wire size for a 100 amp service panel. Depending on the service panel, different wire sizes are available. The size of the wire determines how much electrical current it will carry, as well as its resistance. A minimum of 4-gauge wire and 1 1/4-inch conduit is allowed for 100-amp service. When it comes to electrical wire, it is important to choose the correct gauge for your needs.

The only exception is if the separate building is fed with just a single branch circuit (2014 NEC 250.32a). What size wire do I need for 100 amp service 250 feet.

Jason, A separate building always requires a grounding electrode system in addition to the equipment grounding conductor that you mention.

Service entrance wire size for 100 amp