Use the existing wire assuming it is of adequate size and no 115V is going to be connected. Simply remove the white wire from the Neutral bar, put black tape on at the last 10" of the white wire at each end to identify it as a non neutral. Then connect it to a two pole breaker, the same one as the black wire. A better solution would be to connect another piece of wire (black or red, it doesn't matter) to the white wire and pull it in while you pull out the white wire. If this doesn't work then pull them all out and then pull in the correct wires.

As the proud owner of a ca. 1825 farmhouse, I understand a lot of the challenges and adventures that come with playing "this old house", and it sounds like you have your share too! That said, I have no idea what it might take to rework your plan to take power from the main panel instead of that unused two-wire ungrounded circuit. Any chance you could pull a new cable there with an equipment ground and maybe a bit more ampacity? The lack of a ground at the point of hookup is definitely a problem and it all but guarantees that any "solution" you try will violate modern electrical codes. I think I understand the ground rods providing some protection against lightning and other surges but not being conductive enough to create overcurrents that will trip a breaker (though surely strong enough currents to trip the GFCI I plan to place on the barn branch circuit). Actually, equipment grounds have little or nothing to do with the operation of a GFCI. Because a GFCI works by monitoring the hot and neutral and only trips when they are out of balance, a GFCI doesn't even need a ground wire to function properly. But you should also keep in mind that, while a GFCI protects against ground faults, it doesn't provide overcurrent protection. Thus, in the case of a short circuit, the heat can start a fire while a GFCI continues to feed power! "Actually, as a side note, I once measured an impedance of about 10 ohms in a ground rod circuit. Indeed not low enough to generate the 20amp overcurrent to trip a breaker at 120V." Yep, you're spot on there and, actually, 10 ohms is better than average! With this in mind, you may have wondered why I recommended two ground rods. It's because recent code editions allow a single ground rod where the impedance is 25 ohms or less. If a single rod flunks this test, a second rod is required and many single rods do, in fact, flunk. Don't ask me why (practicality, I'd assume), but the two ground rods together don't have to pass the test--they're just presumed to be OK. So it ain't all pure science, if you catch my drift. :-) "I am starting to get the feeling that unless I establish a (green) equipment-ground wire in the house circuit and connect it to the main panel, there is no way to take electricity to the barn and still meet code." That's my take too. "I assume that by equipment-ground you mean the wire that is normally green in color but sometimes grey or bare copper. Yep. (While gray is technically designated as an alternate color for neutrals, those old green ground wires do sometimes look gray.) 1. Maintain separate neutral and ground bars in that panel. (You may have to buy a small ground bar for a couple of bucks or, if there are two bars and they are connected, they need to be disconnected.) There is only one bar in the small barn subpanel. But no problem, say I buy a small separate ground bar. If you still have the packaging materials, they'll probably say somewhere the correct part number with the notation "sold separately." (Thanks, folks.) It's best to use the designated part although, in truth, those short ground bars are fairly interchangeable. 2. In your house, connect the equipment ground wire to the ground bar. In a main panel, there is often only one bar, serving both neutrals and equipment grounds. (That's fine in a main panel. In a subpanel, however, they should be separated.) There are no equipment-ground wires in any of my house wiring. That is, there are no green, gray or bare copper conductors anywhere. All wires in my house are 2 conductor wires, just one white and one black (hot+neutral). The only ground (bar) is inside the main panel where the neutral from the utility pole attaches. See, for now, I was planning to feed the barn circuit from an existing, unused, 20amp two conductor (black+white) house circuit. This house circuit was originally used to feed an old radiant space heater in one of my bathrooms (that bathroom is the closest room to the barn anyway so tapping into that circuit for the barn is most convenient). I disconnected the old radiant heater a long time ago for safety reasons and since that was the only appliance on that circuit, this circuit is now unused and available, I assume, to feed the barn. But it is strictly a 2-conductor wire (black+white only). Sigh, I feel your pain. (Really! No sarcasm intended.) 3. At the barn, connect the equipment ground feeder to the subpanel ground bar and land your branch circuit ground wires there too. In other words, all the (green) equipment-ground wires go to the subpanel ground bar. However, there is no existing (green) equipment ground wire coming from the house. If one is needed I would have to install one on a separate route and run it all the way to my houses main panel. True, true and true. (Not to get too picky here, but technically speaking, the EG is supposed to run in the same cable or raceway, btw.) 4. Drive two 8' grounding electrodes (ground rods) at least 6' apart and connect them to the panel ground bar with #6 copper wire. Again, from what I understand, unless I install that (green) equipment-ground conductor from the house main panel to the barn, then I should probably not ground/earth the barn subpanel at all. IMO, earthing the barn panel is a good thing in any event. What is to be avoided is a "false equipment ground". (That's a made-for-purpose term meaning that if the EG's are connected only to a grounding electrode, it may create a false confidence in addition to a code violation.) Somewhat better (although not fully code compliant!) would be to bond the neutral and the grounding electrode at the barn's ground bar. (That is what is done with a so-called three-wire 120/240V outbuilding service--the neutral effectively serves to feed both grounded conductor and grounding conductor.) But note that, as ronnatalie has correctly pointed out above, this approach is not strictly applicable in your case because there are other metallic connections (e.g., the phone wire) between buildings. Now that would take another long-winded discussion, but suffice it to say that the code only permits a neutral-EG bond when the house and outbuilding have no other connections. That may be a technical glitch, but it's a potential violation nonetheless. If I do run the (green) equipment-ground wire from the barn to the houses main panel then I should probably ground/earth the main panel only using rods implanted somewhere close to the main panel. If your house really has no earthing system, that would be a wise improvement in any case. It might also be useful to evaluate your plumbing as a potential source of earthing. If your water service is metalic when it enters the house, and runs at least 10 feet in the ground, you should also connect the water supply at the point of entry to your main electrical panel. With an old house, seemingly simple projects do tend to mushroom, don't they?

The ampacities have above have nothing to do with distance. It's the standard capacity for 90C wiring derated due to having more than three current carrying conductors in the conduit. The amount of load you can run depends on how much voltage drop you can tolerate in the barn. Since it seems that the wire size is fixed, it doesn't seem like it matters. You still use a 60A breaker. To compute the voltage drop you'll need the ACTUAL load drawn and the distance (and the #6) wire size. You can google to find several calculators that will let you know what kind of drop you're looking at.

12/2 UF, 100 feet is $73.71 locally. 20A. Using sheathed (NM, UF, etc) cable inside conduit gets weird, but you're allowed to do it where protection from damage is required/desired, and, if you ask me, any spot underground fits that condition. On the shed end, wire all your outlets first, ignoring the light for now. Aim to make a loop around the shed, with the last outlet being the one closest to the switch. You apparently know how to wire outlets since you mistakenly wired your switch the same way. All in parallel. Black to black, white to white, etc., around the whole shed. Black goes to the side with the small slot, white to the other, connect your grounds together and pigtail to the green screw. After you've wired up all your outlets, add another piece of 12/2 coming off the one closest to the switch. Run it TO the switch. Run another piece from the switch to the light. Inside the switch box, wirenut the two white wires together and connect/pigtail your grounds as well. When you've done this you'll have two black conductors remaining. One goes to each screw on the switch. If the switch has a ground screw, attach it. You connected the panel end correctly: hot to screw on breaker, both other wires to neutral/ground bar.