In article , Tom wrote:

I asked why do most fixtures require 90C wires? I found out many
manufactures use very thin fixture wire(18awg) which will heat up
beyond 60C wire specs with their amperage. This means anything coming
in contact with the fixture wire, under the wire nut, needs to be
protected to 90C.

18 AWG does not heat much at the .6 to 1.25 amps drawn by the fixture.
The main problem is that heat builds up in the fixture.

- Don Klipstein )