I have an old ITE main panel in the garage with 600 amp service which has 3 - 200 amp breakers, each feeding a sub-panel in the house. All 3 ITE sub-panels in the house have been fine for over 5 years since we renovated.
Last week, the voltage on the left side bus of one of the sub-panels started fluctuating from 120v to 32v, and now it goes from 0v to 2v to 32v to 0v, etc. The right side bus is fine, so every other breaker pair works (1&2, 5&6, 9&10, etc) and every other breaker pair 3&4,7&8, 11&12, etc) has little or no voltage.
The lugs on the bus bar are screwed on tight, and are not loose. A voltmeter on each of the 120v feeds displays 120v on the right bus bar and 0v or 32v on the left bus bar.
There is a junction box midway between the garage and the house with Polaris taps, so I was able to check the voltage in the J box. Both 120v feeds checked out at 120v coming into the taps and 120v going out of the taps. I removed the taps, cleaned any corrosion off the wires, and replaced two of the taps (on the bad 120v line and the neutral). This leads me to believe that the issue is the copper wire between the J Box and the sub-panel, which is under the foundation.
The only thing I can think of doing is running new wire in the attic from sub-panel (west) to sub-panel (east), as it will be near impossible to run new conduit and wires from the garage to the house without tearing up the driveway and running conduit on the exterior of the house.
As one busbar in sub-panel east is fine and has 120 volts, can I just run 120 v from west to east by adding a 125 amp breaker in west to provide the missing 120V on east, which already has 120 v, a neutral and a ground? Or do I have to run two 120 V wires and a neutral from the west panel to the east panel?
Alternatively, as there's 15 - 20 amp breakers in east that have power, and perhaps there's not enough amperage in west to supply west + all of east, would it make more sense to add another small sub-panel next to the east panel and move circuits off the old east sub-panel to the new east sub-panel which is fed from the west panel, so I need less amperage from east, such as 80 instead of 125?
If circuits aren't all used at the same time and most of the time, only a few circuits are used concurrently, such as a 30a cooktop and two lights, or 1 or 2 TVs, a computer and a few 10 to 60 watt lights, can the 200 amp panel support a 125 amp breaker to power the east panel?
Any ideas or solutions would be much appreciated.