Assuming SPD is surge protection device.. when it finally fails, and goes dead short - what is the item that will open to save the whole supply from going off ? That is the rating those cables need to match.  If the SPD is internally fused at say 50A then that would be a good place to start. If it is not then I'd not expect a direct bus connection, rather connection via a fuse or similar,
M.

Does the SPD manufacturer not specify the rating of the protective device?

when selecting cables to run to an SPD from a busbar should they be rated to the upstream protective device?

I'd say, applying general rules, yes; but since the SPD is a negligible load and unlikely to create an overload  you'd only have to provide fault protection, rather than overload - which usually means substantially smaller conductors will be acceptable (like a 1.5mm² c.p.c. on a 32A ring).

   - Andy.