I have a 60a subpanel on the side of my house for the pool equipment. There are only two conductors and a ground. Originally the only things that were probably connected were the pool pump and heater, both 240 volts. But the lack of a neutral prevents the extra spaces from being used for 120 volt circuits.
And at some point, that's what happened. There's a 120v receptacle under the panel. Apparently, whoever installed this realized the problem, and at least was smart enough not to use the subpanel grounding conductor as a neutral.
About 20 feet away from the panel, there is another circuit feeding the a/c condenser, which does have a neutral for some reason (its unused by the condenser). So, whoever installed the receptacle ran a single 12 ga conductor (in conduit) from the a/c disconnect to the subpanel and used that for the receptacle's neutral.
Yes, it's infinitely better than using the subpanel's ground conductor as a neutral, but it makes me cringe and I want to add another 120v circuit or two so I want to fix this the right way.
Seems to me I have two choices. One, replace the entire feed to the subpanel with a 4/3 cable, or run a single #4 conductor from the main panel to the subpanel for the neutral. Since you can't buy single conductor Romex, I assume I'd have to run it in conduit. The second option would be much cheaper, but is it against code or a bad idea for some other reason? Could I run the single #4 conductor in 3/8 flex conduit rather than PVC? It would be entirely in the attic.