Any electricians available? Wire size question...

I'm not sure he SAID he didn't use the neutral. A floating neutral not only creates a shock hazard it will quickly show up or damage electrics. If you have, say just one 100w lamp burning on leg, and a big load, say, a stove burner or 110 electric heater, the lamp will get very bright and MAY blow out. Replace the lamp with a TV, radio, computer, and you can get expensive.

Subpanels can be a tricky deal. It pays to check local AND US code and do it right.

Some places (I don't remember "our old well" and I mean OLD) you could get by with two legs and a neutral, then ground the box with a second ground stake AT the well house. Not sure you can do that anymore.

Neutral and ground are "the same thing" at only one point.........where the main service comes into the main box. You have a three wire service, with the main box grounded by a rod. The box, the rod, and the service drop neutral / ground are all bonded at that point ONLY. From then on, the neutral is treated as "insulated," even though tied to the box.

Another danger is crap like old commercial buildings, where you might find, say, a small 1 1/2 or 2 ton (24000 BTU) AC unit, 240 single phase, no neutral as no 120V is used. The conduit might have been used as ground. "Now" 20-30--60 years later, after remodel, damage to building, who knows, the conduit might be damaged or loose, BREAKING THE path to ground. VERY dangerous. Now, with a poor ground at the AC unit (example) and no or poor ground, a leakage from say, the compressor, blower, or control transformer to the unit ground WILL CREATE a possibly lethal condition.

This situation would have been made safer by a SEPARATE green wire ground run in the conduit.