Master Electrician Exam (NEC)Grounding and BondingMedium
A master electrician is installing a new 200A, 120/240V, single-phase service for a residential dwelling. The service entrance conductors are 2/0 AWG copper. The grounding electrode system consists of a metal underground water pipe and a ground rod. What is the minimum size copper grounding electrode conductor required to connect to the metal underground water pipe electrode?
- A2 AWG
- B6 AWG
- C4 AWG
- D1/0 AWG
Show answer & explanationAnswer & explanation
Correct answer: C. 4 AWG
According to NEC Table 250.66, for 2/0 AWG copper service conductors, the minimum size copper grounding electrode conductor is 4 AWG. The connection to the metal underground water pipe must be sized based on this table, without the exceptions applied to ground rods.
Why the other options are wrong
- A. Incorrect. 2 AWG would be required for larger service conductors (e.g., 3/0 AWG or 4/0 AWG copper).
- B. Incorrect. 6 AWG is the minimum size for a ground rod electrode, but not for a metal underground water pipe with 2/0 AWG service conductors.
- D. Incorrect. 1/0 AWG would be required for even larger service conductors (e.g., 250 kcmil copper).
GEC Sizing for Water Pipe Electrode
The grounding electrode conductor (GEC) connected to a metal underground water pipe electrode must be sized according to NEC Table 250.66 based on the largest ungrounded service-entrance conductor, without the size limitations applicable to rod, pipe, or plate electrodes.
- Sized using Table 250.66.
- Based on the size of the largest ungrounded service-entrance conductor.
- Not subject to the 6 AWG copper maximum for ground rods.
- Must be continuous or spliced per NEC 250.64(C).
Memory trick: Water pipe's GEC size is like a direct current, straight from the table.