I don't believe you can run 50 amps over 8awg, most of the charts I'm seeing limit you to 40 amps. You have to upgrade the wire to 6awg to get all the way to 50 amps. Since you have to upgrade the wiring, you should also get a 6/3 line with a ground. If you're burying the wire, make sure to get one rated for burying and consider conduit, sand, and warning tape. Make sure the ground and neutral are not bonded in the sub-panel, only in the main panel.

You can use a 3 wire feeder to supply a separate building, if...

• The installation was in compliance with a previous edition of National Electrical Code (existing premises wiring).
• An equipment grounding conductor is not run with the supply to the structure.
• There are no continuous metallic paths bonded to the grounding system in each structure (bonded water, or gas piping, other conduit, etc.).
• Ground-fault protection of equipment has not been installed on the supply side of the feeders.

National Electrical Code 2014

Article 250 Grounding and Bonding

II. System Grounding

250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s).

(B) Grounded Systems.

(1) Supplied by a Feeder or Branch Circuit. An equipment grounding conductor as described in 250.118 shall be run with the supply conductors and be 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. The equipment grounding conductor shall be sized in accordance with 250.122. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).

Exception No 1: For installations made in compliance with previous editions of this Code that permitted such connection, the grounded conductor run with the supply to the building or structure shall be permitted to serve as the ground-fault return path if all of the following requirements continue to be met:

(1) An equipment grounding conductor is not run with the supply to the building or structure.

(2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved.

(3) Ground-fault protection of equipment has not been installed on the supply side of the feeder(s).

If the grounded conductor is used for grounding in accordance with the provision of this exception, the size of the grounded conductor shall not be smaller than the larger of either of the following:

(1) That required by 220.61

(2) That required by 250.122

Changing from a 30A breaker to a 50A breaker can only be done, if you also change the wires to 6 AWG. In which case you'll have to follow current codes, and install 6/3 with ground. Breakers (and fuses) are always sized to protect the wire connected to them, so you can't change the breaker size without also changing the wire size (unless you're going down e.g. 50A to 30A).

However, depending on what you're doing, you may not have to change the breaker at all. If the planned circuits in the structure are not going to be fully loaded, you may well be able to supply the subpanel with a 30A breaker. Just because the subpanel has 50 amperes worth of overcurrent protection, does not mean the supply breaker has to be 50A. Whether or not you actually need a 50A breaker on the supply, depends entirely on what the subpanel will be powering.

You can use 8awg on 50amps. Refer to NEC 310.15 (B) (16)) Personally I use 6awg, but it can be done

You need four wires no 6 for 50 amps to 60. You might get away using three sixes and a eight if Your running 6 THNN copper. THNN 6 copper will carry the current up to about 180 feet. 8 will not and is rated about 45 amps at a short run. And a ground Rod with a fifth wire by building with a ground rod to panel box on new building 8 feet in ground. But the code enforcement wants to see all equal wire size if not you can mix a 8 in on neutral.

8/3 would be too small for a 50amp circuit. If your running above ground to a sub panel it should be 3 wire and a ground rod at location of sub panel ,bonded with neutral as if it was a main service. If your running under ground then it should be 4 wire. Seperate neutral and ground bars in sub panel. No ground rod. Confirm the conduit isn't metallic and bonded. Confirm the two buildings are not grounded to water pipes.

I'm visiting from the future.

In the future, we change NEC to allow retrofitting of ground wires on almost any circuit. This will happen in NEC 2014, which due to a number of other insane requirements (hot tip: buy stock in companies that make GFCI devices) many states will really drag their feet to adopt, so you may not see it adopted until 2018 or so.

There's no such thing as a "grounded neutral wire", that's a wish-burger such as "driver-friendly alcohol". Neutral is never grounded except right at the N-G equipotential bond inside the main panel. 3-wire subpanel connections are throwbacks to the age before any grounding at all. They're just attaching local grounds to the neutral using the same logic used to justify the same on dryers and ranges: "it's too expensive to force a fix, so choose the least bad option". The practice is now barred, which tells you what they really think.

Ground rods alone are not sufficient because during a bolted ground fault (hot-ground short), the ground needs to be able to return hundreds of amps to assure a rapid breaker trip. Including dirt in the grounding path limits you to about 4 amps at best. It's not going to happen.

A solitary ground wire can be buried. Not sure on the burial depth, but if you bury Rigid conduit, that only needs 6" of cover, and the pipe itself is the ground path, so you're done. Expensive stuff, but you can trench it with a garden trowel, which means no worries of destroying other utilities.