If you are rewiring an old farm and there are several circuits running 240V on a 12-2 or 10-2 wire using the ground as a neutral for what applications is this ok?
You might want to look a little closer at this. Because you said it was a farm, I have the suspicion you're looking at 220v motor circuits. 220v motor circuits, like 220v water heater circuits, do NOT have neutrals. Both sides of the line are hot, and the bare wire is ground like it's supposed to be. This is completely legal, and there's even a plug for it: the NEMA 6 series. (NEMA 6-20 for 20a service, NEMA 6-50 for 50a service and so on.) My favorite part of Leviton's 6 series receptacles is the marking on the box: "Industrial grade." You ever hear of a residential-grade metal lathe? All you can run off this are things that use 220v, and not 110/220 things like dryers and ranges. You could wire a water heater, something with a single-phase 220v motor, or a welder this way. If you need 110v in the barn, you're going to have to move around a bunch of stuff in the box to get the grounds onto a ground bar, or you'll need a separate panel for 110v. If you asked this question because you have no experience of working on the latest household power circuits and you hope to get enough information on this site to be able to do the job properly and safely, it is best to warn you not to rely on getting accurate information from this site about such a potentially dangerous subject.
By asking this question you probably aren't quite ready to take on this particular task.
Really, don't do this one yourself. Electricity is far too dangerous to handle if you have not been trained how to do this work. That you asked this question here shows that you don't already know your local Wiring Codes or Regulations so don't know how to choose the correct wire sizes and types to use for the lengths of run from circuit breakers to power outlets.
How to do this job depends entirely on the Wiring Codes or Regulations for the locality (Town/State) and on the exact location of the appliances you wish to hook up. If they are situated anywhere that is subject to water splashes or spray - such as in any room supplied with running water pipes, like a kitchen, bathroom, shower room, etc., or in a pool-side area - in many places nowadays it is actually illegal to attempt to do this kind of work unless you are already a licensed electrician.
For your safety and that of anyone else who may to use the appliance, ask a professional licensed electrician to advise you or to do the work for you.
As far as I know there is NO applications where a ground is used for anything but a ground.
There is a huge section of the NEC (National Electrical Code) that covers all the rules that cover the "ground" and I can't find any that allow it to be used as a "neutral".
Don't scrimp on the cost of wire, you don't want a burned down barn and a insurance company that won't cover it because you didn't follow the rules.
The plain answer is that you can't! There is no application where you are allowed to use the ground wire in non-metallic sheathed cable as a neutral! The neutral [an identified conductor] MUST BE INSULATED!
(LIFE SAFETY WARNING! [disclaimer] Electricity is dangerous! You can be injured or killed! Improper installations can cause fire, injury and death! Should you be doing this yourself?) This is one of those questions - if you know this little, you shouldn't be doing what you are planning...
No disrespect intended, but this is SO basic that it suggests you have not studied the subject at all! There are many good reference books and course books to study from.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.
If you do this work yourself, always turn off the power
at the breaker box/fuse panel BEFORE you attempt to do any work AND
always use an electrician's test meter having metal-tipped probes
(not a simple proximity voltage indicator)
to insure the circuit is, in fact, de-energized.