I'm having a problem adding a GFCI receptacle to a circuit. I'm new to this type of work so please bear with reading the step-by-step (pictures included). I determined that the outlet I wanted to setup as GFCI was on circuit "SP#19". Circuit SP#19 has three receptacles and a few basement lights (don't confuse these lights with the circuit named "DeskLights", these are separate lights). I turned SP#19 off and then tested the hot wires of my targeted receptacle with a pen-style voltage tester. Though flipping breaker SP#19 killed all appliances on each receptacle of the circuit, the voltage tester was giving a weak but definite signal that the hot wires of my target receptacle were still hot. I went back to the panel and noticed that SP#19 is linked to the breaker called "DeskLights". I turned off the breaker "DeskLights" and rechecked the hot wires at my target receptacle on SP#19 with the voltage tester. Now I couldn't get any reading from the voltage tester, as expected. As you can see in the pictures, there is a single 3-wire (red, black, white, ground) Romex in the base of the panel with red going to SP#19 and black going to "DeskLights", so it looks like this is the reason that I get a weak signal with the voltage tester unless I turn breaker DeskLights off too. At my outlet, after determining which hot wire was line and which was load I turned both breakers off (DeskLights and SP#19) and then I installed the GFCI receptacle. I turned both breakers back on and then plugged in my outlet tester. No lights on the outlet tester turned on. I checked the outlet with my pen-style voltage tester and it gave a clear indication of being hot. I tried to trip the test of the GFCI but it wouldn't work. I tried resetting the GFCI but that didn't do anything. All downstream receptacles were dead according to the appliances that were plugged into them. I didn't check the downstream receptacles with a voltage tester. I took the GFCI out and replaced it with a standard receptacle. All of circuit SP#19 is working again (all receptacles and basement lights), but I don't have GFCI protection. Can anyone help me here? Thanks.