A building is served by 500 kcmil copper service conductors, which by the grounding electrode conductor table would ordinarily call for a 1/0 copper conductor. A ground ring of 2 AWG bare copper encircles the building, and the grounding electrode conductor being installed connects to nothing but that ring. What size is required for that conductor?
- A1/0 AWG copper
One-ought is what the table gives for 500 kcmil service conductors, and it would be correct for a conductor running to a water pipe or metal in-ground support structure. The ring cap overrides it for this particular run.
- B6 AWG copper
Six AWG is the cap that applies where the sole connection is to rod, pipe or plate electrodes. It is the right idea applied to the wrong electrode type.
- C4 AWG copper
Four AWG is the cap for a conductor whose sole connection is to a concrete-encased electrode. Again the concept is correct but it belongs to a different electrode.
- 2 AWG copper
Why D is correct
The ring itself is 2 AWG bare copper, and the Code says the portion of the grounding electrode conductor whose sole connection is to a ground ring need not be larger than the ring conductor. Running 1/0 to a 2 AWG ring would put a large conductor in series with a small one, which buys nothing.
What this question is testing
Tests whether you know that the grounding electrode conductor table sets a maximum as well as a minimum, that the maximum depends on which electrode the conductor terminates on, and whether you can keep the three separate caps for rings, rods and concrete-encased electrodes straight from one another.
On the job
Ground rings are common on commercial and institutional buildings, communication sites and anywhere a designer wants a low, stable ground reference. The three separate caps trip people up because they all live in the same section and all sound alike. In the field the practical consequence is money: on a large service the difference between 1/0 and 2 AWG copper around a building perimeter is significant, and knowing the cap is what lets you bid it right.
Memory technique
You never need a conductor bigger than the electrode it lands on.
Exam tip
Sole connection to a rod caps at 6 AWG, to a concrete-encased electrode at 4 AWG, to a ground ring at the ring conductor size.
Where to look it up
Article 250, the grounding electrode conductor sizing section. The caps are written as separate subsections immediately following the reference to the table, not inside the table itself.