If you study the ampacity tables you will see that in the larger sizes of wire an increase in size results in relatively little increase in ampacity. Forinstance, a 50% increase from 10 kcmil to 10.5 kcmil results in an increase in carrying capacity in amperes of less than 15%.
There is no need to parallel conductors smaller than 1/0 because the difference between the carrying capacity in these sizes is much less pronounced.
Although the NEC places the limit, it is really physics that limits paralleled conductors to 1/0 and larger.
Creighton
