A master electrician is installing a new 277V, 3-phase lighting circuit in a commercial office. The circuit uses 12 AWG copper conductors, each carrying 16A of continuous load. What is the minimum temperature rating of the terminal lugs and conductors required for this application, assuming no other adjustments are necessary?
- A105°C (221°F)
- B90°C (194°F)
- C75°C (167°F)
- D60°C (140°F)
Show answer & explanationAnswer & explanation
Correct answer: C. 75°C (167°F)
According to NEC 110.14(C)(1), for circuits rated 100A or less, the conductor ampacity shall be determined using the 60°C conductor column, unless the equipment terminals are listed for use with 75°C conductors. However, for continuous loads, the conductor must be sized at 125% of the continuous load (16A * 1.25 = 20A). A 12 AWG copper conductor is rated for 25A at 75°C. Since the load is continuous and 100A or less, the default rule (60°C terminals unless marked 75°C) would apply. Given the most common terminals are 75°C rated for these applications, and 12 AWG is often used at 75°C, the 75°C column is used for ampacity, requiring 75°C rated conductors and terminals. The 60°C column for 12 AWG is 20A, which means 12 AWG at 60C is barely adequate for 16A continuous (16A * 1.25 = 20A). To safely meet the continuous load requirements and common terminal ratings, 75°C is the most appropriate minimum.
Why the other options are wrong
- A. 105°C is higher than typically required for general lighting circuits and terminal ratings.
- B. While 90°C conductors can be used, the ampacity must typically be limited by the 75°C terminal rating, unless specifically rated for 90°C.
- D. While 60°C is the default for terminals <100A, 12 AWG at 60°C is 20A, which is exactly 125% of 16A. A 75°C rating provides a better margin and is common for terminals.
Conductor & Terminal Temperature Ratings
NEC 110.14(C) dictates that conductors must be sized based on the lowest temperature rating of the conductor insulation, the connected equipment terminals, or the overcurrent device, to prevent overheating.
- For circuits 100A or less, conductor ampacity often defaults to 60°C column unless terminals are listed for 75°C.
- For circuits over 100A, conductor ampacity often defaults to 75°C column unless terminals are listed for 90°C.
- Continuous loads require conductors to be sized at 125% of the load before applying temperature adjustments.
Memory trick: Terminals Set the Temp, Conductors Must Conform.