Meter/Panel Combo question

[kwired]

I know it is early and I may need more coffee but, if there is an OCPD- 200A and then feed to a 150A MB panel, why would you not be able to use 1/0 CU feed from the 200A to the 150A. Wouldn't the 150A MB protect the Feeder conductors?

The 150 A overcurrent protection is on the load end of the conductors. That makes it a tap conductor. Overcurrent protection on the supply end can be higher than the ampacity of the conductor but must meet requirements of 240.21.

240.21(B)(2) is likely the rule that will be most looked at in the installation by the OP.

(2) Taps Not over 7.5 m (25 ft) Long. Where the length of the tap conductors does not exceed 7.5 m (25 ft) and the tap conductors comply with all the following:
(1) The ampacity of the tap conductors is not less than one-third of the rating of the overcurrent device protecting the feeder conductors.
(2) The tap conductors terminate in a single circuit breaker or a single set of fuses that limit the load to the ampacity of the tap conductors. This device shall be permitted to supply any number of additional overcurrent devices on its load side.
(3) The tap conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

Part 3 will prohibit using cable type wiring methods unless otherwise approved by the inspector