An electrician is installing a new 15-ampere branch circuit for general-purpose receptacles in a dwelling unit. Which of the following conductor types, if protected by a 15-ampere overcurrent device, would comply with NEC requirements for conductor protection without requiring adjustment factors?
- A16 AWG copper, Type UF
- B14 AWG copper, Type THHN
- C12 AWG copper, Type THWN-2
- D10 AWG aluminum, Type XHHW-2
Show answer & explanationAnswer & explanation
Correct answer: B. 14 AWG copper, Type THHN
According to NEC Table 310.16, 14 AWG copper conductors are rated for 15 amperes at 75°C (THHN's 90°C rating can be used for ampacity calculations, but the overcurrent protection is limited to 15A per 240.4(D)). This matches the 15-ampere overcurrent device. 12 AWG copper is rated for 20A, 10 AWG aluminum for 25A, and 16 AWG copper is generally not permitted for 15A branch circuits in dwelling units unless part of a listed assembly (e.g., flexible cord, fixture wire) and not a general-purpose branch circuit conductor.
Why the other options are wrong
- A. 16 AWG copper is generally limited to 10A circuits (NEC 240.4(D)) and is not permitted for 15A general-purpose branch circuits.
- C. 12 AWG copper has an ampacity of 20A (at 75°C), which is typically used for 20A circuits, not 15A. While it would be protected, it's oversized for a 15A circuit.
- D. 10 AWG aluminum has an ampacity of 25A (at 75°C), making it significantly oversized and not a typical choice for a 15A circuit.
Conductor Ampacity & Protection
Conductor ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. Overcurrent protection must be sized to protect the conductor's ampacity, often with specific limitations for smaller conductors.
- NEC Table 310.16 provides ampacities based on insulation type and temperature rating.
- NEC 240.4(D) limits overcurrent protection for small conductors (e.g., 14 AWG to 15A, 12 AWG to 20A).
- The OCPD must protect the conductor at its lowest temperature rating or the terminal rating.
Memory trick: Conductor protection is 'Amp'ly covered by 'NEC' 'Tables' and 'Limits'.