Master Electrician Exam (NEC)Services, Feeders, and Branch CircuitsMedium
A residential dwelling unit requires a 100A, 120/240V single-phase service. The service entrance conductors are 2 AWG copper. What is the minimum size copper grounding electrode conductor (GEC) required if a ground rod is the sole grounding electrode?
- A6 AWG
- B4 AWG
- C8 AWG
- D2 AWG
Show answer & explanationAnswer & explanation
Correct answer: A. 6 AWG
According to NEC 250.66(A), when a ground rod is the sole grounding electrode, the portion of the GEC that is the sole connection to the ground rod is not required to be larger than 6 AWG copper. The service entrance conductors being 2 AWG copper would typically require a 8 AWG GEC per Table 250.66, but the ground rod limitation overrides this for the connection to the rod itself.
Why the other options are wrong
- B. 4 AWG is larger than required for a ground rod connection.
- C. 8 AWG copper would be correct if connected to a concrete-encased electrode or a water pipe, but 6 AWG is required for a ground rod connection.
- D. 2 AWG is significantly larger than required for a ground rod connection.
GEC Sizing for Ground Rods
The specific sizing requirement for the grounding electrode conductor when a ground rod is used as the electrode, often limiting the conductor size.
- Reference NEC 250.66(A).
- Maximum size typically 6 AWG copper for a ground rod connection.
- Applies even if the main GEC is larger for other electrodes.
Memory trick: Size the GEC for the 'main course' (service), but the 'dessert' (ground rod) has its own small plate.