Where in the codes doe it state what kind of sheathing is required for wiring through floor joists? I am going to pick a wire to run 220. I assume that single wires coated with plastic is not to code, when I say single wires I am referring to.typical house wiring, white and yellow Romex NM-B Cable, ie the three wires inside it, a bear copper wire and two covered wires and paper. When running wires through joists what is the code configuration? Is sheathing always required, I would assume it is.
If you mean where passing through a floor, the Romex cable must not only have sheathing but shall also be enclosed in rigid metal conduit, intermediate metal conduit, electrical metallic tubing, Schedule 80 PVC conduit, Type RTRC marked with the suffix -XW, or other approved means and extending at least 6-inches above the floor. NEC 334.15.
If you mean running through floor joists (horizontally) then of course, the wiring requires sheathing. If there isn't sheathing, then it's not an approved wiring method, per NEC.