CSLB C-10 Electrical ContractorWiring Methods and MaterialsEasy
A subpanel feeder is protected by a 60A breaker and uses copper THWN-2 conductors terminated on lugs rated 75°C. Using NEC Table 310.16, what is the minimum conductor size that may be used?
- A4 AWG (85A)
- B8 AWG (50A)
- C6 AWG (65A)
- D10 AWG (35A)
Show answer & explanationAnswer & explanation
Correct answer: C. 6 AWG (65A)
At the 75°C column, 8 AWG copper is rated only 50A, which is below the 60A breaker rating, while 6 AWG copper is rated 65A, which meets or exceeds the load. Table 310.16 ampacities must be selected based on the terminal temperature rating and equal or exceed the overcurrent device.
Why the other options are wrong
- A. 4 AWG (85A) is oversized and not the minimum required.
- B. 8 AWG (50A) is below the required 60A rating.
- D. 10 AWG is rated only 35A, far too small.
Ampacity Table Selection
Conductor size is chosen from NEC Table 310.16 using the temperature column matching the terminal rating, and the ampacity must equal or exceed the overcurrent device.
- Match conductor ampacity to breaker/terminal rating, not just load
- 75°C column is standard for most equipment terminals
- Always select the smallest size that meets or exceeds the required ampacity
Memory trick: Match the wire to the wattage, not less than the breaker's teeth.