A#3 bare copper conductor is used for a ground wire for 100 amp service.
A 200 amp service panel will require a # 4 bare copper ground wire.
Yes, a smaller ground wire for a ground can be used. There is a rule (table) in the code book that states the size for ground wire to be use for a specific service conductor size.
A bare #3 copper conductor is the minimum size wire to be used.
A 100 amp residential service requires a size #8 copper wire, it should be insulated in green.
A ground wire for a service distribution has to be copper. A #6 copper conductor can be used up to a 200 amp service.
Yes, there are ground wire gauges. The approiate size ground wire must be matched to the size service you are installing. For instance a 200 amp serivce must be grounded with a # 4 bare copper ground wire.
A minimumbare copper wire that can be used to ground a 200 amp distribution service is a #6.
#6 bare copper wire.