A large industrial facility requires a 1000A, 480Y/277V, 3-phase, 4-wire service. The service entrance conductors are three sets of 500 kcmil copper per phase. What is the minimum size copper grounding electrode conductor (GEC) required if the grounding electrode is a metal underground water pipe electrode?
- A250 kcmil
- B3/0 AWG
- C2/0 AWG
- D300 kcmil
Show answer & explanationAnswer & explanation
Correct answer: B. 3/0 AWG
According to NEC Table 250.66, for service conductors up to and including 1100 kcmil copper (three sets of 500 kcmil is 1500 kcmil, which is greater than 1100 kcmil, so we must sum the circular mil area). Three sets of 500 kcmil is 3 x 500 = 1500 kcmil. For service conductors larger than 1100 kcmil copper, the GEC must be at least 3/0 AWG copper. This is specified in the table for a metal underground water pipe electrode, which does not have the 6 AWG limitation of a ground rod (NEC 250.66(A)).
Why the other options are wrong
- A. 250 kcmil is larger than required.
- C. 2/0 AWG copper is too small for service conductors totaling 1500 kcmil.
- D. 300 kcmil is larger than required.
GEC Sizing for Water Pipes
The sizing requirement for the grounding electrode conductor when a metal underground water pipe is used as a grounding electrode, subject to specific NEC Table 250.66 rules.
- Sized according to NEC Table 250.66.
- Based on the size of the largest ungrounded service entrance conductor or equivalent area.
- No specific maximum size limitation like for ground rods.
Memory trick: Water pipes are the 'big connection' to earth; size the GEC to the service's total 'flow'.