If you have a THHN wire rated up to 90C. You can only use either 60C or 75C for a 60C/75C breaker. What are the conditions where you could use the 75C?
I understand the temperature limit is because the internal parts circuit breaker can get this hot and it needs the conductor to dissipate the heat. But what if you install a big heatsink just outside the breaker. Then you can use 75C or even 90C already?
The idea of modifying the breaker so you could use smaller wire is wrong on a number of levels but I'll just discuss one.
But what if you install a big heatsink just outside the breaker. Then you can use 75C or even 90C already?
The electrical code requires that most of your electrical system be listed by a nationally recognized testing laboratory (NRTL) such as Underwriter's Laboratories (UL).
The electrical code requires that listed equipment must be utilized according to the manufacturer's instructions; not doing so violates the listing. If you modify a listed device, it's effectively no longer listed.
Adding a heat sink to a breaker would be a modification, and as such would not be permitted.
The 60 deg column is used for circuits under 100 amps. You can use the higher ampacity of the 90 deg table to do any derating for more than 3 current carrying or larger circuits usually can usually use the 75 deg column since this is the rating of most circuit breakers. To use the 90 degree column you usually must terminate prior to the breaker with listed lugs, then upsize the wire to the breaker. This method is used when derating is required due to the number of current carrying conductors. Heat sinks would never be allowed as the breakers are not listed for 90 deg. The problem is not breaker heat but more of wire insulation for most residential installs.
NEC article 215.2. explains how you could use the 90 deg table and exhibit 215.1 has a nice picture that makes it easier to understand. More than 3 current carrying conductors requires derating this means if you have 4 in 1 conduit now you have to start derating the wire ampacity. Larger than 100 amp circuits it is allowed to use the 75 or 90 degree collum if the breaker or termination is rated for those temps but I don't think I have seen a residential breaker rated for 90.
In order to be able to use a THHN wire at its 75°C rating, all the splices and termination-points in that stretch of wire need to be rated for at least 75°C. This is true of some styles of splicing devices (such as Polaris-type insulated mechanical connectors) and of breaker and panel lugs, but most notably not of wiring devices such as receptacles and switches.
As a practical result, this means that when working in conduit, feeder wires, along with wires to hardwired appliances, get to use the 75°C column, while general lighting and receptacle circuits are restricted to 60°C ampacities. Of course, this only applies to wires 8AWG and larger, as NEC 240.4(D) limits the breakers for 14-10AWG wires to their conventional (60°C) ratings anyway.
The 90°C column, by the way, is only conventionally used as part of applying derating factors for ambient temperature, or more commonly, conductor counts in a single conduit. While it's possible to run wire at that temperature by using a pigtail of fatter wire to interface the 75°C lug on a breaker or panel to the 90°C conductor via a 90°C rated splice, this is uncommon, and would only generally be done if conduit fill is a severely limiting factor or if existing conductors cannot be replaced, but can be safely run at the higher temperature rating.
Also note that there is no such thing as a heat sink for a breaker -- the calibration of a breaker's thermal trip depends critically on the temperature rise environment the breaker is in, and taking it outside the UL 489 testing envelope would throw that calibration off, heatsink or no heatsink.
