Typically you will use 3 gauge wire 75 C 3 wire 120240 volt if in the USA. As far as driving a ground rod at a subpanel it is far better to ground to a cold water pipe where it enters the house as long as the pipe is not plastic. A 100amp service can use 4 cu or 2al and Section 310-15 b 6 says the feeder conductors dont have to be larger than the service entrance conductors.
Three-conductor cable comprises four wires two hot wires a ground wire and a neutral.
A single phase circuit would have 3 conductors a 3 phase circuit would have 4 wires. The cable must contain one or two hot wires depending on your needs one neutral wire and one ground wire. 2 2 THHNTHWN-2 Wire for 100 amp sub-panel. - Click here for pricing on 3 gauge THHN wire.
