In this case the size of the conductors will determine the size of the EGC.
250.122(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.
It is a 100 amp panel but you are running 1/0 awg I imagine for voltage drop reasons.
If we figure the normal size for a 100 amp feeder is 3 awg with a circular mil area of 52,620, and now you increase to 1/0 awg with a circular mil area of 105,600 the ratio between these two conductors is essentially 2.
Table 250.122 shows a 8 awg ground for a 100 amp breaker.
8 awg has a circular mil area of 16,510.
16,510 x 2 = 33,020 circular mils.
The smallest conductor that is
more than 33,020 circular mils is a 4 awg with a circular mil area of 41,740.
So even though Table 250.122 shows an 8 awg ground for a 100 amp over current device we have to follow 250.122(B) and increase the size in proportion to the ungrounded conductors.
In this case that means a 4 awg ground with the 1/0 copper ungrounded conductors.
For extra fun,
What size grounding conductor would be needed on a 30 amp circuit using 4 awg as the ungrounded conductors?
Think of this as a long circuit for site lights around a building.
Bob
Edit: I agree with Roger and Ryan, I also would pull an EGC.
[This message has been edited by iwire (edited 11-12-2003).]