@glyph @foone Yeah, that cord looks way too thin for 15A. You should definitely be using Romex for this sort of thing.
(Just to dissect the frog for anyone who doesn't get it: this style of cord is *extremely dangerous*, because one end has 120V on exposed prongs when the other end is plugged into an outlet. They're commonly requested by people who ran their Christmas lights wrong and only have the socket end of the light strand next to an outlet.)