Broken feeder cable?

Ninjaman

Garage running Underground feeder 4/4/4 AL from main panel off dual 80 amp breaker.
Yesterday I found half the garage panel was inoperative.
Tested lugs, one of the feeders is 120V, the other is at 80-90V.
Swapped the feeders at the 80 amp main to ensure it wasn't the breaker. Same results.
Tested at the 80 amp breaker at main panel, both legs 120V.
When I remove the 80-90V feeder off the 80 amp main panel breaker and test it at the garage it drops from 80-90 to 0.

No breakers trip. Is this one feeder line broken underground or split open and grounded to earth?

AllanJ

Aluminum feeders broken underground is not an unusual situation.

Ground hogs or other animals may have chewed the insulation and then ground moisture will result in rapid corrosion of aluminum.

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Ninjaman

Not many underground animals around here in SC and it's clay 24" deep.

It is however in an area that I have allowed some heavy vehicles to cross perpendicular over where the wire was buried less than 8 months prior. Including dualies with loads of firewood and a school bus many times.

Could this snap the wire? And would a broken wire now in contact with earth cause the 80-90V reading without tripping the breakers?

Stubbie

Anything is possible.

Sure ... the only fault path you have is thru the earth (dirt) .. dirt provides too much resistance to current flow to allow a breaker to trip.

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Stubbie

Superrick

Yep, you have a burnout on your underground feeder. Has anyone dug or driven anything into the ground where the feeder is located. It usually takes several months to burn through. If not the cable insulation was either nicked when installed or laid on top of a rock or other sharp object. We always recommend underground wiring be in conduit to all of our customers for that reason (cheap insurance). Was this installed by a contractor? If so call them, you probably have a year warranty on the install. If you have an idea where it was dug into you can repair it with an underground splice kit. If not under warranty maybe a local contractor can do a fault locate and repair (cheaper than new feeders). Good luck!

Ninjaman

Did a little testing today. Bear with me, I had chemo yesterday and am a little wore out/tired today.

Here is what I got so far.

At the main panel 80amp breaker:
Feed wires disconnected from breaker
Breaker on
126V each leg tested to neutral
250V leg to leg

Suspect feed wire (still disconnected from breaker) to neutral 86K ohm - 300K ohm
Good feed wire (still disconnected from breaker) to neutral 5-6M Ohm

At garage panel:
Feed wires disconnected from panel but hooked to main breaker and breaker on
Good feed to neutral 124V
Suspect feed to neutral 68V
Good feed to suspect feed 190V

Suspect feed disconnected from main panel/breaker
Suspect feed to neutral no imp reading - open

So does this sound like the suspect feeder is broken underground?

dmxtothemax

Sounds like a cable burn out or loose connection !!

Try dissconnectting boths ends of both feeds,
Also dissconnect the ground wire.

Then connect the two hot feed wires and the earth wire,
together at one end only !
Then go to the other end and measure the resistance across
the earth wire and the good feed,
It should be quite low.
Now measure across the earth wire and the bad feed.
It should also be quite low, and similiar to good side..
Is it ?
If there is a significant difference,
There,s your problem !

a7ecorsair

From what you've posted your service is running at 250 v or 125 per leg. The feed to your garage is reading 190 v with one leg low. This low leg is most likely completely broken under ground somewhere. Your wire is stranded so With just on strand connected and all the rest broken you would read 125 v in the garage with all the breakers off - no load - because your meter does not load the circuit.

Ninjaman

Another good way to test wire continuity. Thanks, will give that a try.

Ninjaman

Yep, easy test.

Both feeds and neutral disconnected from panels

Both feeds and neutral bound together at the main

Tested at the other end (garage)
Good feed to neutral .2 Ohms
bad feed to neutral 500 K Ohms

She's broken.

Time to dig.

Ninjaman

Using digital meter I connected a long wire from the Neutral bar to the meter. I then used an 8" long metal screwdriver with a wire attached at the top going to the meter.

Poking all the way down where I thought it was broke I started finding 1 volt, got up to 2, then bam, 3 and 3.5 volts. Then started tapering off. Again and again I found the same spot working in all directions to make sure. Kept landing the highest voltage, 3.5V, in one spot. Marked it and will start digging soon.

It's a spot that I allow heavy vehicles to cross where the wire is buried. It's right where you can see tire tracks.

joed

Probably a sharp rock pierced the cable. They do make underground splice kits. When you find the break you can repair it and bury it again.

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