Let's take a look at NEC 1999 which provides an allowance for a no-safety-ground-wire situation.
This is NEC 1999 here. Not current Code.
250.32. Two or More Buildings or Structures Supplied from a Common Service.
(a) Grounding Electrode. [deleted: short version: you need ground rods].
(b) Grounded Systems. For a grounded system at the separate building or structure, the connection to the grounding electrode and grounding or bonding of equipment, structures, or frames required to be grounded or bonded shall comply with either (1) or (2).
(1) Equipment Grounding Conductor. An equipment grounding conductor as described in Section 250-118 shall be run with the· supply conductors and connected to the building or structure disconnecting means and to the grounding electrode(s). The equipment grounding conductor shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. Any installed NEUTRAL shall not be connected to the equipment grounding conductor or to the grounding electrode(s).
(2) NEUTRAL. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, and (2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved, and (3) ground-fault protection of equipment [GFPE] has not been installed on the common ac service, the NEUTRAL conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded.
This is NEC 1999 here. Not current Code. I placed the word NEUTRAL where Code refers to that (it normally uses an obtuse and very confusing phrase).
250.32(B)(2) is the interesting part. It's describing the cases where you're allowed to bootleg ground off neutral.
- Obviously it's not allowed if you have a ground wire
- It's not allowed if there's another "metallic path" available
- It's not even possible if the feeder has GFPE (GFCI) protection.
That seems like it gives you a pretty good roadmap.
If you're willing to install a GFCI, that'd do the trick. Note that the 1999 Code calls for a "30ma" GFPE, which is actually a weaker version of a GFCI designed only to protect equipment not humans. It's the same thing as a "whole house RCD" found on most of the houses in the UK and Europe. When you're doing a whole building, the "5ma" human-protection GFCIs are just too sensitive and will have nuisance trips. You'll need to call a real electrical supply to find a GFPE breaker. It's not adequate for human safety, so you still need GFCI receps at relevant places.
However, xHHN means conduit.
It's not legal to direct-bury xHHN wire, which strongly suggests you have a continuous conduit between A and B.
If that's true, see if the conduit is not collapsed (i.e. see if the wire is pullable). If the wires are able to move, then add a ground wire and be done with it. Because of the age of the conduit and the risk of collapse somewhere, I would start by trying to fish in just a ground wire. But the normal way is to pull all the wires out, add the extra wire, and pull them back in.
Meanwhile, address what's the deal with a 60A breaker on THHN wire.
- #8 Cu or #6 Al THHN is good for 50A and takes a 50A breaker. Ground is 10Cu/8Al.
- #6 Cu or #4 Al THHN is good for 65A and takes a 70A breaker. Ground is 8Cu/6Al.
You can put a 60A breaker instead of the 70A if you really want to, but that doesn't get you off the hook for using the larger ground wire.
If conduit fill is at a premium, a copper ground can be bare.
If you find aluminum wire and it's AA-1350 alloy, you should probably change that to AA-8000 alloy, but I'd stick with aluminum. I generally prefer aluminum for large or long feeders like this, unless conduit fill forces my hand into copper. (I had to do that once with 1000 kcmil - $1000 for a 15' run of wire.)