20A or 30A fuse in 200A fuse clips -- is nesting reducers OK?
Probably best to take another route

I spoke with a Cooper Bussmann tech support rep (via chat) and they recommended against it.  In my case, the alternatives include:


Using a 30A fuse block with a suitable fuse and a 200A, Class R dummy fuse for the tap pole, which is probably the simplest approach as it allows the use of a fused switch, although it does introduce a (short) tap conductor into the circuit.
Using field-fitted fuse blocks of the appropriate size to mount fuses in a nominally non-fused switch.  Again, this requires a short tap conductor, but allows more flexibility with fuse types (including the use of Class T fuses, which don't have reducers).
Going to Class J fusing, as Mersen/Ferraz Shawmut makes a 200A->30A Class J reducer although it's not clearly a catalog item, and most North American fused disconnects can be reconfigured for Class J fuses simply by moving the load-side base to a different position.  While Class J fuses are more expensive than Class R or Class T fuses, this makes for a clean installation with no tap conductors roaming about or fuse blocks lurking in odd places.