CSLB C-10 Electrical ContractorWiring Methods and MaterialsHard
A dwelling service is supplied by 400 kcmil copper THHN service-entrance conductors. Per NEC Table 250.66, what is the minimum size copper grounding electrode conductor (GEC) required?
- A4 AWG copper
- B2 AWG copper
- C2/0 AWG copper
- D1/0 AWG copper
Show answer & explanationAnswer & explanation
Correct answer: D. 1/0 AWG copper
NEC Table 250.66 specifies that for copper service-entrance conductors larger than 350 kcmil through 600 kcmil, the minimum copper grounding electrode conductor size is 1/0 AWG. Since 400 kcmil falls within this range, a 1/0 AWG copper GEC is required.
Why the other options are wrong
- A. 4 AWG applies to smaller service conductor sizes, such as 2/0-3/0 kcmil range.
- B. 2 AWG applies to service conductors over 3/0 through 350 kcmil.
- C. 2/0 AWG applies to service conductors over 600 through 1100 kcmil.
GEC Sizing per Table 250.66
The grounding electrode conductor size is based on the size of the largest ungrounded service-entrance conductor, per NEC Table 250.66.
- Table ranges increase GEC size as service conductor size increases
- 400 kcmil copper falls in the 350-600 kcmil range
- Corresponding GEC size is 1/0 AWG copper
Memory trick: Bigger service wires need a bigger ground wire—look it up in Table 250.66