Step 1 - load calculations
The NEC does not address the issue of transfomer sizing. All parts of 450 simply talk about the full load rating of the transformer. It is up to the manufacturer to decide and state if a device is rated for continuous or non-continuous loading.
Step 2 conductor sizing
Conductors are sized based on the load. for example 215.2(1) and 210.19(A)(1)
non-continuous: 100% for loading up to 3hrs
continuous: 125% for loading longer than 3hr
combination: a total of the above loads
Step 3 overcurrent protection selection
Overcurrent protective devices are sized based on the chosen conductors, for example 240.4, or using the same general rules, for example 210.20.
The exceptions to 210.19(A)(1) (conductors) and 210.20(A) (protective devices) do not require the 125% factor when the "entire assembly" is rated for 100% operation. 100% rated assembly are very rare.
No where in the code does it say breakers are only rated for 80%. For normal conductor (full load current) protection the NEC treats and sizes breakers and fuses identically.
