Get a piece of 'Greenfield' Its a wound metal tube that is flexible and used for
covering wire. You can get a short piece and run it in the wall or outside.
I would get wire sized for the 50 amp circuit. No change necessary if you
go from 30 to 50 amp. Wire difference is small compared to doing it over again.
You can use separate wire - that's what was used before romex (along with conduit.)
Need 3 wires.