Table 250-66 is based on "the largest ungrouded service-entrance conductor". Now which one is considered service-entrance conductor, the 500 kcmil coming into the CT or the 3/0 coming into the individual 200A panels. I can see your point TE. Along the same line, what if we go out of CT can right into a 400A Main disconnect and then from load side split off into the two 200A panels - one next by and the other in basement. Now the Main bonding jumper will have to be at the Main disconnect with 4 wires feeding each panel (GND bus separated from Neutal bus). According to 250-66 we need 1/0 for EGC based on 500 kcmil coming into the Main disconnect. # 4 or 1/0 ...that is the question...
