Electrical Help Needed
 

About 7 years ago, I installed a new wiring system for my shop, barn, and a couple of other outbuildings. All 4 of the circuits are serviced by a sub-panel, which was installed by our electrician. I did all of the other wiring myself (I've done similar projects several times over the last 20+ years). The circuits are all very simple - just a bunch of outlets with a few switched outlets for fluorescent lighting. There are no 3-way switches anywhere.

Everything worked fine upon installation, and continued to work fine until a couple of weeks ago. I went out to the barn to plug in a battery charger, and no power. Went to the box and found that the circuit breaker had tripped. That seemed odd, but I just reset it and continued on. A couple of days later, I went to turn on overhead lights, and same circuit was tripped. Strange. Reset it and continued. Last week, I encountered the same thing. This time, when I reset the circuit, I heard a "pop" coming from the barn, and the breaker tripped again. As far as I can tell, there was no load on the circuit when the breaker tripped.

So, I'm figuring that there must be a short somewhere in that circuit. A couple of questions: 1. Why would a short suddenly manifest itself after 7+ years? 2. Is there a way to track down the source without disconnecting everything? The "pop" I heard was coming from a junction box in the barn. When I removed the cover, the large wire nut (joining 6 neutral wires) was burnt to a crisp.

Any advice appreciated! Thanks for reading!

Mike

Is any of the wire separate tnn wire in conduit? I had a similar occurrence after 10 yrs in service except the breakers and GFI didn't trip, there was just no power in one leg. The wire had apparently been nicked when pulled and it took all those years of arcing to ground to finally erode the wire to the point that it would no longer conduct. The funny thing was that per code all the conduit was plastic except where the nick was, under a paved drive. At that point code called for metal conduit which facilitated the arc.

Are those Ground fault interrupters in the sub panel?

1) bad flourescent ballast
1b) condensation water+rust bubble?
1c) rodents?

guessing the real fault is somewhere downstream of that wire nut. wire nut just acted as a slow burn fuse.

All wire in the circuits is 12/2 Romex, so no separate TNN wires. All conduit is EMT above ground and PVC below. No GFI outlets anywhere in the system.

Is there a way to test for a short?

I think you already found the problem. Most likely came loose and burned up over time, causing the symptoms you had.

I did replace the fried wire nut, but that didn't solve the problem...

Not an electrician but..
There shouldn't be any junction boxes. Definitely not on a main leg.

The wiring from the house to garage sub panel should be continuous and oversized for total load.
That includes lights, compressor, and welder being used at the same time.
Feed wire and breaker should be same stats, or breaker should trip early. That's the safety link.

Then, each leg in garage should be connected individually to bus and breaker.

If it's NM/B romex it can't be used underground, even in conduit. That's considered a wet location. Romex is not rated for wet locations.

I guess I should have added that the main supply lines to the various buildings are either 6 gauge or 8 gauge individual wires running in PVC conduit. I'm fairly certain that that is the case also for the wires going from the main service entrance panel on the house to the sub panel. I didn't actually witness that installation, but it was done by a well-known electrical contractor here in the Napa Valley.

I don't think there is actually any Romex underground, but if there is, it's the gray stuff that's rated for damp locations.

The junction box I'm referring to takes the feed from the main wires, and distributes it to the outlets and light fixtures in the barn.

Good to hear that. So if I understand correctly, you're saying that the breaker tripped even with everything unplugged and switches turned off? You're absolutely sure of it?

Bad breaker ??

I'm pretty sure that's correct. I still need to unplug the overhead fluorescent lights, but they are controlled by a switch. I'll check tonight and see if anything else is plugged in down the line. If there's nothing plugged in, it would have to be a short somewhere, correct?

I didn't see that on first read, I agree that's the problem.

why are 6 whites bonded the only place that many whites should come together is at a sub-panel where they are individually bonded to the bus. Wire nutting that many 12-2 wires is asking for trouble.

I already replaced the breaker too :(

I agree that it's not a good practice, but if there is nothing turned on or plugged in, it wouldn't cause the breaker to trip.

I would agree with that. If you find other devices plugged in, check to see what the voltage is on that circuit.

Look at the connector or clamp. Sometimes they over tighten it or pull on it with the clamp tight and cut the wires. Rodens, or ....

I must run, but I will try to read through the posts and see if I can help you solve it later tonight. It sounds like an easy issue to isolate.

OK. Got home tonight and made sure everything was unplugged. Circuit still trips after about 60 seconds. I guess the next step is to start disconnecting things at the junction box in order to isolate the "bad" circuit...

I don't know about that gray stuff. We would never use it and some inspectors will not pass it. I actually don't think its code.