Power loss on a sub-panel

Question

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.

Answer 1

I have a rule: Nobody does things for no reason. 600A is a crazy amount of service, and if the reason for it isn't obvious, figure it out before you change anything. Once you understand that...

Your principle of feeding one subpanel off another has merit. However you must bridge over all the wires together - two hots, neutral and ground. You never pull over a hot and have its partner wires return via a different route, For a variety of reasons. This means if one wire in a cable run is dead, the whole cable run is useless and must be taken out of service entirely.

But I wouldn't focus on the cross-connection yet. I'd try to save this setup. I'm betting it's a termination problem not yet found - it's unusual for wires to just fail.

The bigger problem is: This work was all installed at once, right? Whatever damaged one thing could well be damaging others. Did you find corrosion on the connections you serviced? They're all that way. Don't wait til they fail, because a neutral failing is especially bad news.

As a temporary workaround, like, to get you to the weekend when you can fix this, you might move your 120V loads to the breaker spaces that are working. If you need more breakers, either share breakers (when legal) or buy duplex breakers.

I find almost all such problems are with terminations, perhaps in a splice box you haven't found yet. Don't just tighten lugs, de-energize it and take it all apart and really give it a once-over and fastidious cleaning. If it's aluminum wire (AA-8000-series is legal and OK), apply the anti-oxidation goop. Only then, look at the cable itself.

Somebody who puts in 600A service doesn't cheap out on the installation. There's a good chance your cable run is in conduit, which is designed to make cable replacement easy. On the other hand, it surprises me that cable would fail in conduit, so I'd be concerned maybe something penetrated the conduit, damaging both conduit and wire. Any wire damage seems vanishingly unlikely under a foundation, far more likely in a more accessible place (perhaps a recently accessed place - had any diggers around lately?)

And by the way, it's possible to splice underground direct-burial cable.

If the cable is in conduit, easily replacing wire is the whole point of conduit. They spent extra money as insurance for this situation - use it! Validate that the conduit is physically intact (not collapsed). The techy way is to energize the bad wire (only) and use a detector to find the wire break (by looking for EMFs). That tells you how far down to send a borescope (a plumber will have that if an electrician doesn't).

A low-tech way to inspect conduit is to disconnect the wire bundle at both ends and pull it an inch and see if it gives more than the normal resistance. The "Alexander the Great"** method is to pull the entire bundle, inspect, maybe borescope, fish and pull it back in, and see what happens. Mind you, this is not to continue the old wire bundle in service - but merely to use it as a "test dummy" to test whether the conduit is pullable. If there are any defects in the wire, replace all of them unless it is extremely clear what went wrong.

If the conduit is intact and the wire is bad, this is routine: pull and replace the wires. That job is easier with specialized tools an electrician will have those on his truck.

Wires don't just randomly fail in conduit. This for sure: whatever killed the one phase threatens the others. You'll want to know what that is.

Answer 2

You can't hack this. If one of the feeder wires is bad, you'll have to replace it. All the feeder wires ("hots", neutral, and ground) must run together in the same raceway, from one panel to the other. You can't run a single "hot" alone, and you can't steal a "hot" from somewhere else.

Answer 3

Transpose the feeder Hots at the Main then retest at the sub. If the Voltage is 0v to 32v, the same as before you reversed the hots out of the main feeders breaker then it’s not the Wire