Where is this required? ...assuming you mean with a secondary-side OCPD. The whole transformer, primary and secondary is always required to be protected. The matter of how is all that can be questioned.
There is no such thing as secondary conductors after the first OCPD!
I was just referring to the fact that if the secondary is not single phase two wire or three wire delta any primary protection that allows full use of the transformer rating does not protect the secondary windings.
With a single phase three wire winding, for example, you could have a fault on only one side of the secondary which pulled twice its rated current without tripping the primary OCPD.
With reference to secondary conductors, there is the argument that even the wires from the secondary terminals of the transformer to the first OCPD would need to be oversized compared to the secondary rating in order to be protected by the primary OCPD. The reasoning being that if they were not accordingly oversized and were only protected by the first OCPD on the secondary side they would be an illegal tap (secondary conductors) of a tap (the secondary winding).
Not everyone agrees with this, of course. :happyno: