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I would see a problem with placing 300 amp's worth of main breaker on a feeder sized for 200. Use the breaker.


Not feeder, service entrance conductors. [Linked Image]

This is done quite often in multi family dwellings, as long as the conductors are rated for the calculated load.

230.90 Exception No. 3.

Bob

Bob Badger
Construction & Maintenance Electrician
Massachusetts