New GFCI won’t reset

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I recently found a GFCI that had failed and would not reset. It is located under a sub panel on the back of my home. This particular GFCI has been in service for many years and with no load during the attempted reset I suspected a defective GFCI. I purchased a 20A TR/WR and when installed it would not reset. Fact is it chattered like crazy. I disconnected the ground from the GFCI and the chatter stopped but it would not reset. I put the new GFCI on a different circuit in my shop (different sub panel in a remote building) and it worked fine.

The sub panel on the back of the house is fed with SEU cable - from the outdoor main panel, through the attic, into EMT down a wall and along the patio and then out of the EMT, buried (I couldn’t believe it either...) for a distance of about eighteen feet, and then up another piece of EMT into the box. The date stamp on the SEU cable is 1984 which is about the time the AC unit was installed in this part of the house. Only the HVAC and GFCI are served by this sub panel. I had to replace the original panel about ten years ago because of damage due to a leaking EMT coupling on the top of the old box. It was at that time that I added the GFCI under the box and it has worked flawlessly until recently.

Checking every connection I did find a loose connection in a j-box next to the main service panel. A wire nut had been used to connect THHN to the neutral in the SEU cable. I replaced that connection with a dual-rated splice but the problem was not resolved.

I then carefully dug up the buried portion of the SEU cable and an inspection did not reveal any obvious problems.

I now started looking at voltages with the AC equipment off and the GFCI wired without the ground - just the hot and neutral. With the breaker for the GFCI off I had 242V and 121V each side to the neutral. When the breaker for the GFCI was turned on I had 242V and 115V on the leg with the GFCI to the neutral and 127V on the other leg to the neutral. Resistance readings did not reveal any faults hot to hot of hot to neutral.

I suspect that the SEU cable has been compromised and is causing the problem detected by the GFCI. Before I replace and reroute the feed to the sub panel I was wondering If anyone has any other suggestions.



Staff member
Durango, CO, 10 h 20 min without traffic from wing
wire pulling grunt
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