I meant melt or burn. Speaking of that i bet the 20A breaker enclosure would be rated for 400A in that panelboard.
Anyways why in your opinion it would not melt or burn in disco case?
There is no special rating for a breaker that is based on the rating of the upstream OCPD. The same exact breaker is installed in a panel with a 100 amp main as in one with a 400 amp or even larger main.
If what you are trying to say is correct, there would be no way to ever make a 240.21(B) installation, because based on what you are saying the load end OCPD equipment would have to have a rating equal to the upstream OCPD.
As far as it not melting or burning, the fuse in the disconnect limits the running current to the rating of the fuses. Even 250 amps of current on the 4/0 feeder conductors would not reach a temperature that exceeds 90°C. You can run 629 amps through those conductors without their temperature exceeding 200°C. While that will damage the conductor insulation, it is no where near a temperature that would cause the metal parts of the disconnect to melt.
The only things that will damage your disconnect would be poor connections made by the installer, or some damage that causes a high current short circuit or ground fault. With those types of fault there would be little to no difference in the damage at the disconnect between a 200 amp or a 225 amp upstream OCPD.
