[aidenn0]

Unless you're using stranded wire. It's definitely possible to use a wire-nut correctly with stranded wire, but it's much, much harder. Also (in the US at least) stranded is almost always lighting so the slightly increased contact-resistance is a non-issue.

[jacquesm]

That's true, for stranded wire wire nuts are less than ideal because the thread tends to severe the strands if you're not super careful. If you do use them always twine the threads and if you can use a crimp on sleeve of the right diameter.