Re: residential ground conductor
The size of the grounding electrode conductor is based on the size of the conductors that will supply the fault current, not the rating of the service disconnecting means. If you use Table 310.15(B)(6), a 200 ampere service can be supplied by 2/0 cu, therefore, the GEC is based on Table 250.66 and for metal water pipe or building steel grounding electrode, it can be No. 4 Cu. If it is a ground rod(s), regardless of the size of the supply conductors, the GEC is not required to be larger than No. 6 Cu. If the supply conductors are No. 2 or smaller, the EGC for a ground rod(s) can be as small as No. 8 cu. according to 250.66(A).