I do not understand.
Which code rule allows that #8 to be on a 50 amp breaker?
310.16 rates 8 AWG copper 50 amps at 75 C
As long as we are not using NM and the wire, breaker and other terminals are all 75 C listed we can use a 50 amp breaker with 8 AWG copper.
NM is different regardless of the THHN inside it.
334.80 Ampacity.
The ampacity of Types NM, NMC, and NMS cable shall be determined in accordance with 310.15. The ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The 90°C (194°F) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor. The ampacity of Types NM, NMC, and NMS cable installed in cable tray shall be determined in accordance with 392.11.
