Does 215.2(A)(4) apply when NMB is used for the feeder conductors?
Individual Dwelling Unit or Mobile Home Conductors. Feeder conductors for individual dwelling units or mobile homes need not be larger than service-entrance conductors. Paragraph 310.15(B)(6) shall be permitted to be used for conductor size.
Looking at 310.15(B)(6), this permits conductors to be used at ampacity levels equal to or greater than their normal ampacity as determined by tabe 310.16 (equal to in only some of the 90C cases), without any mention of temperature. In other words, the ampacity of eg. RHW and RHW-2 is exactly the same under this paragraph. The table does not mention NMB _cable_, but does mention _conductor_ type THHN.
Finally, 334.80 requires that the ampacity of type NM cable be determined using 310.15, but with current limited in accordance with the 60 degree conductor temperature rating. I note that 310.15 includes both tables 310.15(B)(6) (which does not have temperature factors) and tables 310.16-310.19 (which do have temperature factors)
A specific example: Service cable is 3-3-5 SE with XHHW copper conductors. Main breaker is 100A. Could 3-3 NM-B (3 3ga THHN copper conductors plus 8ga bare ground) be used to supply a sub-panel, protected with a 100A breaker?
If 310.15(B)(6) applies to NM-B cable with THHN conductors, then the allowed rating for 3 ga copper is 110A, and the neutral is oversized. If the 60 degree requirement needs to be met, then the allowed rating is only 85A.
Additionally, as described there are two 100A breakers in series in the same panel, which I believe is acceptable, but it strikes me as odd. If the ampacity of the feeder is the same as that of the main breaker, then it would seem that there should be some way to simply tap the panel bus, but for residential use a breaker might be the cheapest way to provide this tap.