The minimum size equipment grounding conductor for a 100A, 120/240v single phase underground dwelling feeder whose ungrounded conductors have been increased from #3 AWG to #1 AWG
#4 AWG ref: Table 250.122, Table 8 CH9 83690 cmil/52620 cmil = 1.59 #8= 16510 cmil x 1.59= 26250 cmil which = #4