I'm trying to replace a two prong outlet in my garage with a GFCI.
The receptacle line is fed from a light switch on the wall, and the load runs to an outdoor(one bulb) lamp post. The general procedure is described here except I'm replacing a two prong outlet and there is only one outgoing wire(to the lamp post). Replacing receptacle with GFCI outlet which will be tapped for outdoor pole lantern
The incoming and outgoing wires are both three wires(black, white, uninsulated(bare copper) ground).
I'm feeding the load through the gfci rather than pigtailing. However, when I connect the incoming and outgoing ground to the gfci and flip the breaker the gfci trips. When I disconnect the ground the gfci lights up properly. If I connect the incoming ground only it also lights up.
Any ideas on this? Keep in mind the old outlet was not a three prong(I can't recall the exact way the ground were wired previously) so it had no grounding for anything that was connected. I read through all related topics and no one describes the exact scenario.