If you are using a motor circuit protector or magnetic only breaker for the short circuit and ground fault protection then 250.122(D) lets you size the EGC based on the overload protection rating. In your case, it appears that you are using a thermal magnetic breaker and you must size based on the breaker rating, however as greg pointed out 250.122(A) says the EGC is not required to be larger than the circuit conductors, but that does not change anything for your example as the circuit conductors will be #8 and Table 250.122 calls for a #8 EGC for a 100A OCPD.
Don
