In article , Jim Michaels
writes

Wire Nuts when properly installed do NOT overheat!

Aluminium wiring is common in the USA. When aluminium wiring is
combined with wire nuts, overheating and fires result.

http://www.inspect-ny.com/aluminum/ja-54.jpg
http://www.inspect-ny.com/aluminum/fr1-hotbox.jpg

It should be noted that US wiring codes require ALL splices to be in
junction boxes. If a mis-applied wire nut were to overheat and start
the insulation on fire, it would be contained and not lead to a
structure fire.

That's by no means clear, according to the above website. This sequence
of photos make it clear it's possible for fires to travel along the wire
insulation and out of the junction box, possibly spreading to the house
structu

http://www.inspect-ny.com/aluminum/pl2p13.htm