SEC's & Main breaker

Correct me if I am wrong. I was reading on the electrical education site on line and it was talking about The main breaker being rated at 100 amps , it does not matter that the SEC’s are rated at 200 amps. I thought that I was told that that would have the breaker undersized in that configuration. I thought the breaker had to match what was stamped on the side of the insulation on the cable. Am I wrong? Can somebody please help me

The SEC must at least be the ampacity of the breaker,

If the breaker is 100 amps, but the conductors are rated for 200, the service is only a 100 amp service.

I realize that your question is hypothetical but I would say that in almost every case you would not be able to fit 200 amp conductors into the terminals on a 100 amp CB.

The size of service is rated based on the weakest link. In your case it is a 100 amp service. If you find a new 200 amp service installed in place of a 150 amp service and wiring wasn’t changed, it is still a 150 amp service with undersized wires feeding main breaker.