I found a junction box under my deck that has grey cable, with each individual wire wrapped in the gray sheathing. So for example, the white wire has the white insulation around the copper wire, and grey sheathing around it... and the black wire has black insulation around the copper wire and grey sheathing around it, then the copper wire (ground) is just surrounded by the grey. But each wire is individually wrapped. I used a wire tester and determined the wires are hot, but what gauge wire would I need to use to tie into this existing box in order to not have major problems?
I am wanting to add an outdoor receptacle roughly 10 feet from the junction box I found. Adding in the receptacle is no issue for me, just want to make sure I use proper gauge wire for the pigtail.