John Steinke,
When did you start picking circuit sizes based only on the OCPD?
In general terms, the NEC requires you to determine the load current requirements, then make adjustments for continuous or non-continuous, then chose a conductor(taking into account any applicable deratings and temperature limitations), and finally choose the OCPD.
For multi-outlet branch circuits, only, you are not required to determine the load and may size the circuit conductors based on the OCPD instead.
Overcurrent protection is a term that covers both overloads and short circuits. OCPDs applied per the NEC protect against both conditions.
