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.
Well, I would not say you're doing anything stupid. You have some very good questions.
First,
- If you did decide to direct bury the wire the minimum depth for direct burial is 24", not 18".
- At least three #6 ( black, red, white) and one #10 insulated ground ( green coating).
- Anything <= 60amps just requires a #10 insulated ground with green coating.
Second, Consider voltage drop:
- Load: 30 Amps @ 240V Single Phase.
- Length of run: 80 feet
- Wire Size: #6 Copper
- Voltage: 240V
- Voltage Drop: .81 %
- Voltage At End of Circuit: 238.05
That is less than 3% which the NEC recommends for a feeder. Very Good !
Note: I would hesitate to install the ground rod if you have a in-ground pool in line of the transformer.
Correction: This being a feeder would require a grounding electrode!
Best Answer
Summary: a ground rod will not substitute for the neutral/ground connection back to the main.
To be clear, your third wire is actually providing two functions: neutral and safety ground. The power disturbance is due to the failed neutral. 120 v loads on only one leg do not have a return path and so will not work. If you have loads on both legs, they will conduct through each other but, depending on the ratio of the loads, the voltages will vary between nearly 0v and nearly 240 volts and may destroy equipment or cause a fire.
The safety ground has multiple functions but significantly is to prevent shock with metal-cased, three-prong equipment. If a hot wire were to come lose and contact the metal case, excessive current would flow to ground and trip the breaker on the hot. If a GFCI is in the circuit, the trip will come with much less current. This prevents fatal voltages from appearing on the case.
In this case, because the neutral and safety ground are combined and have become disconnected, you are actually in worse shape. Voltage will conduct through switched on loads and actually energize grounded objects (metal-cased 3-prong equipment, outlet and switch screws, etc.) providing a great shock hazard.
This is why the electrical code has been changed to require a 4-wire feed to sub panels.
A ground rod would mitigate this to some degree but dry dirt is a poor conductor. It is a poor safety ground and a lousy neutral. You absolutly need a working neutral/ground back to the main panel.
At a minimum, I would fix the open conductor and add a ground rod at the garage. You should also consider upgrading your feed to 4 wires to separate the neutral and ground, as current code requires.