Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A technician is installing a 120V, 20A branch circuit for a bathroom receptacle in a dwelling unit. The circuit will serve only the bathroom receptacle. What is the minimum conductor size required for the ungrounded and grounded conductors, assuming THHN insulation and copper conductors?
- A10 AWG
- B12 AWG
- C8 AWG
- D14 AWG
Show answer & explanationAnswer & explanation
Correct answer: B. 12 AWG
For a 20A branch circuit, NEC Table 310.16 specifies 12 AWG copper conductors are required. While THHN insulation has a 90°C rating, the ampacity is limited to the 60°C or 75°C column unless specific conditions are met. For typical branch circuits, the 75°C column (20A for 12 AWG) is used, or the 60°C column if terminations are not rated higher. In any case, 12 AWG is the minimum for a 20A circuit.
Why the other options are wrong
- A. 10 AWG copper is rated for 30A and is larger than required for a 20A circuit.
- C. 8 AWG copper is rated for 40A and is significantly larger than required.
- D. 14 AWG copper is rated for 15A and is only suitable for 15A branch circuits.
Branch Circuit Conductor Sizing
The selection of conductor size for branch circuits based on the circuit's overcurrent protective device rating, conductor material, and insulation temperature rating.
- Sized by OCPD rating.
- NEC Table 310.16 is primary reference.
- Terminations often limit temperature rating used.
Memory trick: Branch Wires: 'OCPD's Choice' for 'Ampacity' (Table 310.16) and 'Termination Limits'!