Second out building
NOOO!!! Definitely not. That would be MUCH WORSE than just using the 3-wire feed with ground and neutral bonded. Here's the problem: The equipment grounding conductor (EGC, the wire that goes to the ground prong of a receptacle, or connects to the metal frame of a piece of equipment) has one main purpose. It exists to prevent the metal parts of an appliance from becoming electrified if a hot wire inside shorts out to the metal parts. It does this by tripping the circuit breaker. As soon as a hot wire contacts the grounded metal, lots of current flows through the hot wire, to the metal part, through the ground wire to the panel, across the neutral-ground bond, and back to the utility transformer via the service neutral conductor. Since way more current flows than the circuit breaker rating, the breaker trips and prevents the metal part from staying electrified. This is obviously important for safety. It also obviously requires a neutral-ground bond. If the panel is a subpanel with a 4-wire feeder, then the "ground" conductor in the feeder provides the return path for the fault current so it can reach the neutral-ground bond in the main panel.
What happens if there is no neutral-ground bond? The hot wire contacts a grounded metal part. Current flows back to the panel through the ground wire, where the ground wire is connected to all the other circuits' ground wires and to two ground rods - but not bonded to neutral. Each ground rod has an impedance of several tens of ohms (let's say 30), so both ground rods together are about 15 ohms. 120V divided by 15 ohms is 8 amps. So about 8 amps flows into the earth through the ground rods. The breaker obviously does not trip because that's not an overload. Now the ground rods, the earth around them, and every other piece of metal equipment connected to the grounding system are all electrified with 120V. If the plumbing system is bonded, then the plumbing is hot too. And it will all stay electrified until someone turns off the power (the metal case of the panel is hot, too).
So you can see that the neutral-ground bond is REALLY important. A "grounding" system which is connected only to grounding electrodes but not bonded to neutral is much worse than no grounding system at all. Ground rods do not ever clear faults. They have way too much resistance to earth to ever be able to reliably trip a circuit breaker. They exist only to protect against indirect lightning damage and to stabilize the system voltage with respect to the earth. All of the electric shock protection provided by the "grounding" system is actually provided by BONDING of metal parts to the system neutral, not by connecting things to ground rods.
If you insist on not running new wire, then the ONLY way to use what you've got is to run two hots and neutral, with a neutral-ground bond at the subpanel. This used to be code compliant up until a few years ago. It was probably code compliant when the feeder was installed, and there's no real need to change it. The four-wire feeder requirement fixes the big set of problems that occurs if that neutral conductor in a three-wire feeder ever comes loose.
Last edited by mpoulton; 09-24-2012 at 02:27 PM.