I have been doing overseas work specified to comply with the NEC. We have been converting metric wire sizes to equivalent AWG sizes and then using the ampacity tables. We check our calculations with the manufacturer's ampacity and assure we do not exceed their rating. I believe that this meets the intent of the code but the question has come up "Does this actually comply with the NEC" Is the anything in the NEC or NFPA standards that specifically allow use of metric sized wire.