A master electrician is installing a new 480V, 3-phase, 4-wire wye service where the service entrance conductors are 500 kcmil copper. The grounding electrode conductor is connected to a concrete-encased electrode (Ufer ground). What is the minimum size copper grounding electrode conductor required for this connection, according to NEC 250.66(B)?
- ANo. 1/0 AWG
- BNo. 2 AWG
- CNo. 3/0 AWG
- DNo. 2/0 AWG
Show answer & explanationAnswer & explanation
Correct answer: D. No. 2/0 AWG
According to NEC Table 250.66, for service conductors 500 kcmil copper, the minimum size copper grounding electrode conductor (GEC) is No. 1/0 AWG when connected to a ground rod, plate electrode, or water pipe. However, per 250.66(B), if the GEC is connected to a concrete-encased electrode (Ufer), it is permitted to be reduced to No. 2 AWG copper or No. 1/0 AWG aluminum. This reduction is applied to the minimum size from the table. The table shows 1/0 AWG for 500 kcmil, so 2/0 AWG is incorrect. For a concrete-encased electrode, the GEC is not required to be larger than No. 2 AWG copper or No. 1/0 AWG aluminum. The question asks for the minimum size, which would be No. 2 AWG copper for a concrete-encased electrode.
Why the other options are wrong
- A. No. 1/0 AWG is the minimum size from Table 250.66 for 500 kcmil copper service conductors if connected to a ground rod, but 250.66(B) allows reduction for concrete-encased electrodes.
- B. No. 2 AWG copper is the maximum required size for a GEC to a concrete-encased electrode, regardless of service conductor size (NEC 250.66(B)). This is the correct answer as it is the minimum required size in this specific scenario.
- C. No. 3/0 AWG is larger than the maximum permitted for a concrete-encased electrode and is incorrect.
GEC Sizing Exception (Ufer)
When a grounding electrode conductor is connected to a concrete-encased electrode, it is not required to be larger than No. 2 AWG copper or No. 1/0 AWG aluminum, even if Table 250.66 would indicate a larger size.
- Found in NEC 250.66(B).
- Applies specifically to concrete-encased electrodes (Ufer grounds).
- Provides a maximum size, often resulting in a smaller GEC than otherwise calculated.
Memory trick: Ufer ground: GEC no bigger than two.