...The issue was that thhn transferred heat somehow? Can anyone elaborate on this so I can understand?
Basically, if you load conductors to their 90 degree ampacities, terminations rated less than 90 degrees will act as a heat sink. Overtime, this will cause the connection / termination to fail. This is the purpose of 110.14 of the NEC. It is to prevent any part or component of the circuit to be exposed to temperatures greater than what they are rated for.
Let say for instance, I have a 200amp 240v service on a house. 3cc, no need to derate. My breaker have a terminal rating of 75c. Is this a code violation to use 3/0cu thhn
No. As a matter of fact you are permitted to use 2/0cu conductors per Table 310.15(B)(6).
The way you read Table 310.16 is like this:
FInd your conductor insulation type in one of the three temperature columns. Next, scroll down the table to your conductor size. The ampacity listed is the maximum load that can be applied to that size conductor with that type of insulation without exceeding its temperature.
So for example, If you were to load #6 awg TW conductors to 55 amperes, the temperature of that conductor will be 60 degrees C. 110.14 tells us that no other equipment can have a rating less than that value...
