Direct Burial Tripping GFCI-Troubleshooting Tips

So I have 12/2 wire that is yellow (Can't read any of the markings on it) and directly buried in the ground for about a 100 foot run from a spa sub panel. That circuit is GFCI protected. Not sure if the cable is direct burial rated or not. Well whenever we would get a lot of rain it would trip the GFCI on that circuit, which made me suspicious about the integrity of that circuit. I live in Arizona so it stays mostly dry and is rarely an issue. HOWEVER, recently the GFCI is INSTANTLY tripping and its been and hot and dry for a while. I tested the GFCI circuit with this load disconnected and the GFCI is working fine. I then disconnected all the wiring at the far end of the run and capped the wiring. GFCI still trips instantly. So now I wonder if it isn't a current leak from the wire itself? Any tips on testing the integrity of the buried wire, so I can isolate this fault?

 

Yellow 12/2 cable would be regular NM cable and cannot be direct buried. It also cannot be installed outdoors and wet areas.
It will be very hard to detect current leak without purchase expensive tool unless you have a dead short or close to dead short.
With both side of wire disconnected, try measuring resistance between hot to ground, hot to neutral, and neutral to ground. All should read very high or infinite resistance. However, this doesn't mean you don't have current leak because regular multimeters are not sensitive enough.

If the run is not too long, you can try running a temporary cable to your spa. If that solves the problem, then you know it is the cable. I suggest replacing cable with UF-b or run THWN wires in a conduit. (you can run UF-b in conduit as well).

 

As mentioned..... yellow NM cable is not rated for any outdoor or damp/wet locations.
If the markings are gone from the cable then it has been outside for a while.
It will have taken on water.