I fly CLT on a very regular basis. The general rule of thumb for connection times is that the airline can only issue a ticket with a short connection provided there is sufficient time between flights that allow the airline to transfer checked luggage from one aircraft to another on that connection. This is referred to as published connection. The common answer by airline staff at airports when asked about short connections is that your baggage often makes it to the next plane before you and your going to be ok, In CLT I believe this often is the case but certainly not at every airport.

As a rule in Charlotte (and most domestic flights generally) aircraft will discontinue boarding 10 minutes prior to departure (Hence the 15 minute at the gate rule). So with that being said your 30 minute connection is actually 20 minutes and provided you don’t stop at the Starbucks on the way....you should be able to walk from the end of C or A etc to E or D terminal in that time.

I avoid 30 minute connections in general as there is no margin of error should a delay occur, and generally 40 minutes or more is very doable in CLT, and if 30 minutes was my only option bar a 3 hour layover then I would take the risk as CLT's weather and other factors generally make flights more on time than at other airports.