(1) Start with primary protection only, 450.3(B).
(2) Then use 240.21 for the secondary conductors.
(3) Now go back and look at 450.3 and see if the OCPD chosen using 240.21 will allow (but not force) you to change the transformer primary device.
Don't forget that a main device in a panelboard can also be used to meet the 240.21 reuirements.
