Well a 100W TPO radio will be somewhere around 40-50% efficient give or take, so maybe 16 - 20A or so at full snout, and if you are running RTTY or FM you are running full snout all the time the key is down.
Of course SSB will be a fraction of that RMS, but still higher then you would expect as efficiency drops as you reduce the power, typically current falls as the square root of the fractional power.
A 30A fuse on a rig that might hit 20A seems quite appropriate to me, I would probably even be ok with a 40A part, but 30A is a more common automotive part.
In an automotive application wire size is far more typically limited by voltage drop then current rating, so while the fuse will not protect the rig, that is not its function, it is there to stop a car fire if there is a short circuit.
Even the **EXPENSIVE** 'FF' semiconductor protection fuses only sometimes manage to fully protect the semiconductor, and those semis are typically huge geometry IGBT or Thristor parts not RF power sand with multiple emitter bond wires just waiting the blow open.
73 Dan.