Master Electrician Exam (NEC)General Electrical KnowledgeMedium
A residential service entrance conductor is being installed between the utility's drip loop and the service disconnect. The service is 200A, 240V single-phase. The conductors are copper with a 75°C insulation rating. What is the minimum size copper conductor required for the ungrounded service entrance conductors, according to NEC 310.12?
- A4/0 AWG
- B2/0 AWG
- C3/0 AWG
- D1/0 AWG
Show answer & explanationAnswer & explanation
Correct answer: B. 2/0 AWG
For 1-phase dwelling services, NEC Table 310.12 (formerly 310.15(B)(7) or 310.15(B)(16) before 2020) allows for a reduction in conductor size. For a 200A service, 2/0 AWG copper has an ampacity equivalent to 200A under these specific service conditions. (Refer to NEC 2020 Table 310.12 for specific ampacities for dwelling units).
Why the other options are wrong
- A. Incorrect. 4/0 AWG copper is for 250A services and larger, significantly exceeding the minimum.
- C. Incorrect. 3/0 AWG copper is suitable for 225A services and larger, exceeding the minimum for 200A.
- D. Incorrect. 1/0 AWG copper is rated for 150A (75°C column of 310.16), not enough for 200A service.
Dwelling Unit Service Conductor Sizing
The NEC allows for specific smaller conductor sizes for 1-phase dwelling unit services compared to general applications, due to diversity of loads and historical performance. Refer to NEC Table 310.12.
- Applies specifically to 1-phase dwelling services.
- Allows smaller conductors than standard ampacity tables.
- Based on historical data and diversity factors.
- Found in NEC Table 310.12.
Memory trick: HOME SERVICE: DWELL on the DIVERSITY.