I have been noticing more situations with conductors in a box that is overcrowded starting to show signs of degragation. These are not that old, maybe 15 to 20 years. The insulation seems to be cracking in a circular ring around the conductor. This does not seem to be from scraping as one puts the wire in the box, but from drying or some other process such as that.

Has anyone else noticed this?
Do you think it is from the heat developed in the box because of overcrowding?

The other day I removed a device from the box and the wires were so crammed, that the raised lettering on the back of the device were imprinted on the insulation of the conductor.

Pierre

What you are seeing is the result of heat drying out the insulation and perhaps starting to melt the plastic. It's not hot enough yet to burn but is hot enough to make the plastic elastic and a little bit flowable.
In time the wires will fail but when is a question that will be difficult to answer.

heat from high usage, or that of dimmers, i have seen, more than from overcrowding