The use of a 75°C wire rating for THHN (Thermoplastic High Heat-Resistant Nylon) conductors when they are used in raceways is based on the National Electrical Code (NEC) requirements. The NEC specifies that the ampacity of a conductor shall be derated to the next lower standard ampacity when it is used in a raceway.The NEC ampacity tables for THHN conductors are based on a 90°C conductor operating temperature and a 75°C ambient temperature. When THHN conductors are installed in a raceway, the ambient temperature can rise above 75°C, which can cause the conductor temperature to exceed its rated 90°C. To prevent overheating, the NEC requires the ampacity of the conductor to be derated to the next lower standard ampacity.The standard ampacities for THHN conductors are based on a 75°C conductor operating temperature and a 30°C ambient temperature. By derating the ampacity of the conductor to the next lower standard ampacity, the NEC ensures that the conductor will not exceed its rated 90°C operating temperature, even if the ambient temperature rises above 75°C.The use of a 75°C wire rating for THHN conductors in raceways is a safety precaution that helps prevent overheating and potential fire hazards. Flag for review