The tempertaure rating of the wire depends on the type of insulation, which is marked on the wire. Probably the most common type used here is THHN/THWN.
THHN/THWN has a 90 degree rating in dry applications, and 75 degree in wet.
The wire can attain those temperatures in a number of ways; generally, the ambient temperature (say, in pipe running across a rooftop) provides a significant rise; simple resistance to the amount of current passing through the wire provides the rest.
Since the temperature rating of the connections is usually lower than that of the wire, we are not able to use the wire to it's fullest potential. That is, we are limited to, say, 65 amps in a wire that otherwise might be rated for 75 amps. Since there isn't a 65 amp breaker out there, if we can't use a 60 amp breaker for the application, we would have to use a larger wire.
Other factors ... wire fill, for instance .... are also considered in 'de-rating' the wires.
While the ambient temperature can cause one section of the wire to be hotter than another, the one factor that heats the entire wire, from one end to the other, is the current carried by the wire. In the example given above, running 75 amps through that particular wire, at room temperature, can easily result in the wire inside conduit reaching 90 degrees - and that's without any connections or splices entering into the equation.
PS: I like the number 17