I am in the process of upgrading the 70 amp panel on a 9 units building to a 100 amp and changed the wire to 1/o ready to connect to the meter of each unit. My question is what is the best way to configure or reconfigure the 400 amp incoming service to maximize the efficiency and cost effectiveness?

Currently the 400 amp service comes into a main disconnect box with a 350 amp fuse serving 2 meter banks of 5 meters and disconnect boxes with 200 amp each.

Thank you for looking into question and providing guidance and advise.

Attached is the diagram of the existing system.

Your question is too vague. Be more specific.

What size will the incoming service entrance be?

The size of the incoming service will remain 400 amp through a 3/o wire conductor.

Would the existing main circuit breaker/disconnect box still be adequate to deliver adequate power to the existing meter banks if the 70 amp main disconnect box for each unit is replaced with a 100 amp main disconnect box?
Or
Do I need to replace the main circuit breaker to a box equivalent in functionality to ezm1400cb 400A and use multi gang meter stack to replace all meters and disconnects as proposed?

Please see proposed set up and current main breaker box info attached for illustration.

Thank you

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While there is nothing wrong with having a 400 amp main supply 9- 100 amp panels, I question that the 400 amp is adequate.

If I were to install new modular metering, I would install a new main disconnect.

Be sure to confirm with your utility that the metering you chose is approved. Also confirm with the utility that 10 KAIC is acceptable.

If the existing main does remain, use lugs approved to accept two conductors.

Actually grounding the bonding bushing with a grounding conductor is required.