I noticed that a black wire coming off a 30A fuse was not hot. Unscrewed the fuse, it was fine. Went to screw it back in and noticed the rejection base said 20A on it. The center of the rejection base is of course hot. Anyway, I knew everything in the house was working without this wire hot so I set out to remove it. The surprise came when I traced the corresponding white wire, it went to the fuse right next to the one that had the black not hot wire. This white wire was hot and had a 20A fuse. I was rather expecting it to go to the neutral bus. The ground went to the ground bus, 12/2 NM wire. What is going on here?
I did notice according to the outdated schematic that these 2 fuses were formerly part of a 220V heater circuit, the heaters were removed before I got the house.
So it looks like this circuit was essentially a white hot, proper ground and a black functioning as a "pseudo-neutral" since it was basically attached to a dead fuse screw.
I have not been able to follow this to see where it goes but it seems like it could go to a functioning 110V outlet etc.
Of course I removed both fuses since this seemed seriously unsafe.

 

This sounds like an old 240 volt circuit that was improperly re-purposed or abandoned.

It was common before not to re-identify a white conductor as a hot in a cable assembly. The Code has been changed to avoid this confusion.

 

So basically someone took the heater out, used one of the hots as hot and just made the other hot not hot by putting in a 20A rejection base and just screwing in a 30A fuse just so there would be a fuse in that slot. Interesting, lol.

 

I suspect the "other" end of the cable is disconnected and therefore the "circuit" is incomplete. The 30 ampere fuse in the 20 ampere adapter was likely an attempt by someone not knowing the circuit was abandoned.

I suggest that you remove both the white and black wires and then check to see if anything in the house is not working.

 

You are correct, I checked and everything works. I removed the cable from the box and taped it up. Now I have 2 open slots. At least one of those is getting wired directly to the garage.