I was visiting my brother's house during some basement remodeling, and
noticed that the electrical contractor had added a 100A loadcenter to feed
the new basement lights, etc.

The new 100A loadcenter is fed via a 30A double-pole breaker in in one of
the home's original two loadcenters. This struck me as being odd. After
consideration, it would seem that (as long as wires are properly sized) it
is not unsafe electrically but what does the electrical code specifiy?

Also, I would anticipate nuisance tripping of the 30A breaker feeding the
sub-panel, since there are several 15A and 20A breakers in the sub-panel and
even if a few are run near max it would exceed the 30A breaker rating.

Thoughts, anyone?