Which type of conductors has the highest temperature rating?

The conductor with the highest temperature rating among the options is THHW. This designation indicates that the wire is rated for high-temperature environments, typically up to 90 degrees Celsius (194 degrees Fahrenheit) for wet and dry locations, which is higher than the ratings for the other types of conductors listed.

THHN, for example, is rated up to 90 degrees Celsius, but only for dry locations, while THW, although it can also handle up to 75 degrees Celsius in wet locations, is not designed for the higher temperature applications that THHW is capable of accommodating. NM-B conductors are rated for a maximum temperature of 90 degrees Celsius, but they are primarily used in residential wiring and applications which do not typically expose them to extreme conditions.

Therefore, THHW is specifically formulated for applications that require a higher heat resistance, making it the most suitable option when considering temperature ratings for conductors.