What ambient temperature is used to find the ampacity of one 8 AWG THWN-2 copper conductor installed in free air?

The correct ambient temperature to find the ampacity of an 8 AWG THWN-2 copper conductor installed in free air is 30 degrees Celsius. This standard temperature is used because it is a common reference point established by the National Electrical Code (NEC) and testing standards in the electrical industry.

At this temperature, the ampacity ratings provided by the NEC for various types of conductors, including THWN-2, reflect the expected performance of the conductor under normal operating conditions. Conductors are rated for their ability to carry current while maintaining a safe operating temperature; thus, using a uniform reference point like 30 degrees Celsius helps ensure consistency and safety in electrical installations.

The consideration for other temperatures, while relevant in specific contexts, does not align with the standard practices for determining ampacity in free air installations. For instance, variations in temperature could lead to changes in resistance and heating characteristics of the conductor, thus affecting safety and performance, which is why 30 degrees Celsius serves as a reliable baseline.