Sizing Common GEC Conductor for 4 service entrance panels

[Constantin]

I'm sizing a Common GEC for an installation that consists of 2 sets of overhead service conductors:
One drops down to a triple meter bank and then splits into 3 sets of 150A service entrance conductors that terminate in 3 separate 150A main breaker panels.
The other set of overhead service conductors drops into a meter and then goes to a 100 amp panel.
This is a 3 family dwelling unit with 150 amp panels for each unit and a 100 amp house panel on its own service drop.
All panels are in the same room in the basement.
Al SEU Cable is used.

250.64(D)(1) states "...the common grounding electrode conductor shall be sized in accordance with 250.66, based on the sum of the circular mil area of the largest ungrounded conductor(s) of each set of conductors that supplies the disconnecting means."

So I need to sum the circular mil area of three 2/0 and one #2 conductors.
Ok, according to Chapter 9 - Table 8 - Conductor Properties, I have: 3*133100 circular mils + 66360 circular mills = 465600 circular mils.
This result puts me between 400 and 500 kcmils and according to Table 250.66, I need to use 1/0 copper for the Common GEC.

Is this right?
I know I won't have to run a copper conductor larger than #6 AWG to the grounding rod but do I have to run 1/0 copper to the water pipe?

Thank you

 

[Dennis Alwon]

I didn't check the math but that is the correct method unless the sum of the conductors in the riser are larger.

 

[infinity]

So I need to sum the circular mil area of three 2/0 and one #2 conductors.

Why would you add the two? You have two sets, size the GEC to the largest and tap off to the other.

 

[Constantin]

Why would you add the two? You have two sets, size the GEC to the largest and tap off to the other.

I thought I have to add the areas of the largest conductors of the service entrance cables for all 4 panels if I want to run a common GEC for all 4 panels.