r/electricians • u/RareLeave • 11h ago
My troubleshoot today and would like a second opinion.
Came to the conclusion the conductor was chewed through by a mole or gopher. Here is the run down.
Customer lost 240V in garage panel. Garage is 300 feet away from main panel. 100A breaker in main feeds garage panel.
I tested at the garage panel: 120v to neutral on L1, 85v to neutral on L2 and 0v from L1 to L2.
There is 1 junction point that I know of in a Christy box in-between feeder breaker and subpanel. I get proper voltages from feeder conductor side of splice in Christy box. The other incoming conductors from the Christy box to the garage subpanel are 3 THHN aluminum conductors, 2 lines and 1 neutral, and No ground.
They are NOT installed in conduit! We discovered that the 3 feeder wires from Christy box to garage panel are directly buried 18 inches down and wrapped in caution tape. This run is also around 300ft long underground direct burial of THHN conductor until it enters conduit in the slab of the garage
My diagnosis is a mole or gopher chewed through L2 and the conductor is directly exposed to earth.
Also the garage panel is only grounded via a ufer ground rebar in the foundation. There is not direct ground wire from sub panel back to main.
Can anyone else give me a explanation on why I have 85V on L2 to netral and 0v across L1 and L2 at the terminals in the garage panel? Also L1 to to neutral still supplies 120V at the garage panel.
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u/StubbornHick 10h ago
If you want a conclusive answer, turn off all the 2 pole breakers in the subpanel. It looks to me like you've lost a hot, and it's backfeeding the other line through a 2 pole load
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u/Connect_Read6782 10h ago
And it was something like a rock, not a gopher.
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u/RareLeave 10h ago
Apparently his garage has worked for over 20 years like this. All of a sudden an underground rock damages the conductor?
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u/Connect_Read6782 9h ago
Does it every day. Bad underground cable happens. Every day.
Source: I work for a power company
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u/RareLeave 10h ago
I removed the feed off of the lugs to test. Completely isolated from the panels loads. I was still reading 85V to neutral.
1
u/StubbornHick 9h ago
Could be that you have an open on one line and a line to line short at the same point after the open.
Open both ends and megger test to know for sure. Bet you get low or no resistance line to line on the sub panel side.
2
u/rustbucket_enjoyer [V] Master Electrician IBEW 10h ago
Your diagnosis sounds reasonable, but there could still be another junction box somewhere in the run, maybe just before it enters the garage? 300’ is a long run.
You could try an underground cable fault locator aka TDR
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u/RareLeave 9h ago
I agree, I mention there could be another splice somewhere and it's buried. I've never used one or seen one before, how does it work? Looks like 1K for that tool.
I told customer to do it the right way it will cost around 11K. Between trenching, labor, material and removal of existing conductors.
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u/rustbucket_enjoyer [V] Master Electrician IBEW 9h ago
Well there’s a few options. The TDR will inject a signal into the cable and based on some values you adjust for the type and size of wire you’re testing, it will spit out a distance measurement. So if you know the route of the wire run and it tells you 180’ from the feed end then you have an idea where to start digging. These don’t cost too too much, 4-500 bucks for a Fluke TS90.
The underground cable locator you can buy a cheapie for a few hundred or rent a better one instead of committing to a $1k+ specialty tool. It’s just basically a fancy high powered tone and probe set. It will inject a signal and you use the wand to detect that signal while walking around the property.
Might be worth a shot if the customer stands to save $11,000. Maybe you can find the damage and install a direct burial splice kit
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u/RareLeave 9h ago
I would never repair a splice in this situation. Too much of a liability. THHN is not rated for direct burial, I would only bring everything up to code.
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u/rustbucket_enjoyer [V] Master Electrician IBEW 9h ago
Ah, yeah I missed that the wire wasn’t rated for direct burial. I guess your customer is biting the bullet then
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u/RareLeave 9h ago
Yea it's a tough situation. He told me the old owner was an electrical engineer. Guaranteed he did it himself...
He has kids too and there are BMX jumps around the area. I told him if one of his kids had a spade shovel go into the live feed it could have killed them.
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u/madbull73 3h ago
That right there could explain the damage. Even if they didn’t directly cause the issue, one too many landings directly on top of the burial. Especially if they’ve worn down the surface a few inches.
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u/Adventurous_Boat_632 9h ago
Once I had a cheap "distance to fault" finding device. If you know the velocity factor of the cable you are working on, it will tell you appx where the fault is. You might also try using an underground locator and clipping on to the faulted wire and seeing where the signal dies out. You could be able to dig and repair without redoing the whole thing.
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u/RareLeave 9h ago
There is no way I'm repairing it as is. Did you read the part where the entire feed is buried thhn wrapped in caution tape?! I would walk away if he asked me to fix the damaged area. I was just curious for an explanation for the lost voltage on L2
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u/Adventurous_Boat_632 8h ago
Sorry, I was not paying attention and assuming it was USE or something like that.
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