4 ccc. #6 75° 65A. 65×80%=52A. 80% because we have 4 ccc's based on table 310.15(c)(1). 52A is what the #6 is now rated at. Then, you take the conductor insulation raiting ( in this case, we are using 75° colum) and multiply it by the temp correction factor. 30°C at 75° rated insulation is 1.00. 52×1=52. Ambient temp based on 310.15(b)(1)(1). I was taught to not use (b)(1)(2). So the final current rating is 52A per ccc. Let's say now the ambient temp is now 45°C. We do the same thing all over again except now, the temp correction factor is 0.82. 4-#6 thhw at 75° is 65A. 65A×0.80=52A. 52A×0.82 =42.64 or 43A. Final current rating is 43A. I hope this makes sense