Well, I don't think that's it exactly. A good ground fault or short circuit will blow the 100 amp fuse just as surely as a 20 amp fuse. The real danger is in not being able to limit the load current in the #12 wired circuits to 20 amps or less (ignoring the 80% rule for the moment). Try running 40, 50 or more amps thru #12 wire and watch it heat up to dangerous levels, then we'll get that ground fault when the insulation melts off the wire.
Also - I doubt this installer has any liability insurance - or any prior experience with electrical work.
Radar
