Is it ok to install a 200A/3P breaker in 800A MLO panel to feed an existing 400A panel which had 400A/3P main breaker (the 30 days peak amp. metered for 400A panel is 150A)? The circuit to 400A panel will be good for 200A.
It’s the load that is to be served that is important, which it seems you understand by stating the 30 day peak load. So yes. No problem for a breaker to feed a panel with a main rated higher than the feeder breaker supplying the panel.
