Master Electrician Exam (NEC)General Electrical KnowledgeHard
A master electrician is supervising the installation of a new 400A, 480Y/277V service for a commercial building. The service entrance conductors are 500 kcmil copper, type THWN-2. According to NEC 250.66(A), what is the minimum size required for the grounded conductor (neutral) for grounding electrode conductor connection purposes, assuming it is not smaller than required for carrying neutral current?
- A1/0 AWG copper
- B2/0 AWG copper
- C3/0 AWG copper
- D250 kcmil copper
Show answer & explanationAnswer & explanation
Correct answer: B. 2/0 AWG copper
NEC 250.66(A) states that the grounding electrode conductor (GEC) for a grounded system shall not be smaller than specified in Table 250.66. This table bases the GEC size on the largest ungrounded service-entrance conductor. For 500 kcmil copper service conductors, Table 250.66 requires a minimum 2/0 AWG copper grounding electrode conductor.
Why the other options are wrong
- A. 1/0 AWG copper is for service conductors up to 3/0 AWG (copper).
- C. 3/0 AWG copper is for service conductors up to 500 kcmil copper, but the GEC size is determined by the table as 2/0 AWG.
- D. 250 kcmil copper is for service conductors up to 1100 kcmil copper, requiring a 3/0 AWG GEC.
GEC Sizing (NEC 250.66)
The grounding electrode conductor (GEC) connects the service neutral or equipment grounding conductor to the grounding electrode system. Its minimum size is determined by the size of the largest ungrounded service-entrance conductor, per NEC Table 250.66.
- GEC sizing is based on the largest ungrounded service-entrance conductor's circular mil area.
- NEC Table 250.66 provides minimum GEC sizes for copper and aluminum service conductors.
- The GEC is typically connected to the service neutral bar and then to the grounding electrodes.
Memory trick: GEC Size: Follow the Service Conductor's Lead.