This is the way I have always thought of it:
Think of the Pri. protective device as actually providing short circuit protection should the transformer fail. It's the device that takes the transformer off line so that it doesn't affect the upstream distribution system. Sometimes the rating of the pri. protective device seems to be unreasonably high but it really isn't intended to provide overload protection for the transformer as that's the purpose of the secondary OCPD.
The transformer will be tied to the 8 amp load and the A/C unit load of 15amp.
The A/C unit is powered with 480V 3 phase, that same 480V is then tied to the 1kVA transformer, to provide 120V single phase 8 amp.
If I use 3.5A fuse on the primary, I am afraid startup and operation of the A/C unit will open the fuse and the 8 amp load not turn on.
Thanks