Which statement best describes when to use the 75°C column in NEC Table 310.16 for THHN copper conductors?

Which statement best describes when to use the 75°C column in NEC Table 310.16 for THHN copper conductors?

• A. Use the 75°C column unless all components are rated for a higher temperature
• B. Always use the 60°C column
• C. Use the 90°C column for all copper conductors
• D. Use the 105°C column for high-altitude installations

Answer: (A)

When sizing ampacity for THHN copper conductors, you choose the column that matches the lowest temperature rating of all components in the circuit, including terminations and equipment. The 75°C column is used in most cases, unless every component in the circuit is rated higher, in which case you may use the higher column. In practice, THHN insulation is rated to 90°C, but if any termination or device is rated only 60°C, you must use the 60°C column. If all components are rated at 75°C or higher, you can use the 75°C column. Only if every component is rated 90°C could you use the 90°C column. There isn’t a 105°C column. So the best guidance is to use the 75°C column unless all components are rated for a higher temperature; this matches the typical rule for aligning conductor ampacity with the actual termination and equipment ratings.

When sizing ampacity for THHN copper conductors, you choose the column that matches the lowest temperature rating of all components in the circuit, including terminations and equipment. The 75°C column is used in most cases, unless every component in the circuit is rated higher, in which case you may use the higher column.

In practice, THHN insulation is rated to 90°C, but if any termination or device is rated only 60°C, you must use the 60°C column. If all components are rated at 75°C or higher, you can use the 75°C column. Only if every component is rated 90°C could you use the 90°C column. There isn’t a 105°C column.

So the best guidance is to use the 75°C column unless all components are rated for a higher temperature; this matches the typical rule for aligning conductor ampacity with the actual termination and equipment ratings.