Journeyman Electrician Exam (NEC)Services, Feeders, and Branch CircuitsHard
A commercial building has a 400-ampere, 208Y/120V, 3-phase, 4-wire service. The main service disconnecting means is a single circuit breaker. The service entrance conductors are 500 kcmil copper, 75°C rated. What is the minimum size copper grounding electrode conductor (GEC) required to connect to a metal underground water pipe electrode?
- ANo. 2/0 AWG copper
- BNo. 1/0 AWG copper
- CNo. 1 AWG copper
- DNo. 2 AWG copper
Show answer & explanationAnswer & explanation
Correct answer: B. No. 1/0 AWG copper
According to NEC Table 250.66, for service entrance conductors of 500 kcmil copper, the minimum size copper GEC required is No. 1/0 AWG. This applies to metal underground water pipe electrodes as they are considered the primary grounding electrode.
Why the other options are wrong
- A. No. 2/0 AWG copper is larger than required for 500 kcmil service conductors.
- C. No. 1 AWG copper is for service conductors up to 400 kcmil.
- D. No. 2 AWG copper is for smaller service conductors (up to 200A or 3/0 AWG).
GEC Sizing (Water Pipe Electrode)
The minimum size of a grounding electrode conductor (GEC) for a metal underground water pipe electrode is determined by the size of the largest service-entrance conductor or equivalent ungrounded conductor, using NEC Table 250.66.
- NEC Table 250.66 is the primary reference for GEC sizing.
- The GEC size is based on the service-entrance conductor size, not the service OCPD, when conductors are larger.
- Metal underground water pipe is a common and preferred grounding electrode.
Memory trick: Service conductor size dictates the ground wire's might.