a friend of mine is telling me that i am wasting time and money sizing larger than 3/0 copper for gec and bonding jumper based on 250-66 for parallel conductors over 1100kcm copper but i have always sized to 12.5% of the total circ mils? isn't that the correct method?
The GEC and the Main Bonding Jumper serve two different purposes, that is why the MBJ is required to be sized larger than 3/0 for services with Service Entrance Conductors larger than 1100 Kcmil.