electrical
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?
You are required to use whatever type of wiring your local code permits for this type of connection. If you are in a residential location then it is likely NMD90; if you are in a commercial location then it will likely require BX/AC. The shielding you are referring to between the can and the box isn't really BX/AC; its just flexible metal conduit. It looks similar to BX but it's not the same thing.
If you do use BX/AC, make sure to use plastic bushings at both ends of the cable so you don't nick the conductors with the sharp BX/AC.
Usually with cans, there is a friction cable clamp you just push the NMD90 through, however if you use BX/AC then you will need to use one of the punch-outs and a proper cable clamp to secure the cable.
In the 2011 version of the National Electrical Code, there was Table 310.15(B)(7). This table was quite handy for determining the service conductor size. In the 2014 version, it looks like they've done away with this table. However, the long code trail that has replaced it, seems to basically say the same thing.
National Electrical Code 2011
Article 310 - Conductors for General Wiring
II. Installation
Typical services (in the US) are 100, 150, or 200 ampere (though 60 is common for older dwellings). Which means you'll be dealing with a possible range from 4 AWG copper to 2/0 copper (2 AWG al to 4/0 al). Which means you'll want a device that can accommodate conductors from about 0.3 in. diameter to about 0.7 in. diameter, if you want to cover the whole spectrum of possibilities.
Best Answer
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.
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.