CSLB C-10 Electrical ContractorWiring Methods and MaterialsMedium
A 6 AWG THHN copper conductor is installed to feed equipment with terminals rated for 75°C. Per NEC 110.14(C), what ampacity must be used for this conductor even though THHN insulation is rated for 90°C?
- A55 amps (60°C column)
- B65 amps (75°C column)
- C50 amps
- D75 amps (90°C column)
Show answer & explanationAnswer & explanation
Correct answer: B. 65 amps (75°C column)
NEC 110.14(C) requires conductor ampacity to be selected based on the lowest temperature rating of any connected termination, equipment, or conductor. Since the terminals are rated 75°C, the conductor must be sized using the 75°C column of Table 310.16, which gives 65A for 6 AWG copper, even though the insulation itself is rated 90°C.
Why the other options are wrong
- A. 60°C column would apply only if equipment were rated 60°C.
- C. 50A does not correspond to any standard table value for 6 AWG copper.
- D. The 90°C ampacity cannot be used because it exceeds the termination rating.
Termination Temperature Limitation
Conductor ampacity is limited by the lowest temperature rating among the conductor, termination, and equipment, per NEC 110.14(C).
- 90°C insulation doesn't guarantee 90°C ampacity use
- Terminals often rated 60°C or 75°C
- Always use the lowest rating in the circuit path
Memory trick: The weakest link (coolest terminal) sets the ampacity limit