A master electrician is installing a new 200A, 120/240V, single-phase service for a dwelling. The service entrance conductors are 3/0 AWG copper. What is the minimum size copper grounding electrode conductor required if the largest grounding electrode is a metal underground water pipe?
- ANo. 1/0 AWG
- BNo. 6 AWG
- CNo. 4 AWG
- DNo. 2 AWG
Show answer & explanationAnswer & explanation
Correct answer: C. No. 4 AWG
According to NEC Table 250.66, for service conductors 3/0 AWG copper, the minimum size copper grounding electrode conductor (GEC) to a concrete-encased electrode, ground rod, or plate electrode is No. 4 AWG. Since a metal underground water pipe is listed as an electrode, the same sizing applies. (Note: For services larger than 1100 kcmil copper or 1750 kcmil aluminum, 250.66(A) and (B) might apply, but for typical residential services, Table 250.66 is used.)
Why the other options are wrong
- A. No. 1/0 AWG is for services with conductors up to 600 kcmil copper.
- B. No. 6 AWG is typically for smaller services (e.g., up to 2 AWG copper service conductors).
- D. No. 2 AWG is typically for larger services (e.g., up to 350 kcmil copper service conductors).
Grounding Electrode Conductor Sizing
The size of the grounding electrode conductor is determined by the size of the largest ungrounded service-entrance conductor or equivalent area for parallel conductors, referenced in NEC Table 250.66.
- Based on the largest ungrounded service conductor size.
- Different rules for connection to specific electrode types apply (e.g., ground rods vs. concrete-encased).
- Table 250.66 is the primary reference.
Memory trick: Service conductor size dictates the GEC's might.