Which description matches a concrete-encased electrode that qualifies for use in the grounding electrode system?
- At least 20 feet of bare copper conductor no smaller than 4 AWG, encased in at least 2 inches of concrete near the bottom of a footing that is in direct contact with the earth.
- BAt least 10 feet of insulated copper conductor laid into the floor slab directly above the vapor barrier and carried to the service enclosure.
Both the length and the location fail. A vapor barrier isolates the concrete from the soil, which removes the earth contact the electrode depends on, and 10 feet is half the length the section requires.
- CAt least 20 feet of bare copper conductor no smaller than 8 AWG laid on top of the finished footing before the foundation walls are formed and poured.
The conductor is two sizes too small and it is not encased in anything. Lying on a cured footing provides none of the 2 inches of concrete cover the definition calls for, so the error path gets the length right and misses the rest.
- DAny length of reinforcing steel tied together in the foundation, provided each bar carries a nonconductive epoxy coating that protects it from corrosion for the life of the building.
A nonconductive coating stops the steel from making electrical contact with the concrete around it, so epoxy-coated bar cannot serve as the electrode. The Code also states a minimum bar size and a minimum length rather than accepting any amount.
Why A is correct
It hits every element: minimum length, minimum conductor size, bare conductor, minimum concrete cover, and a footing in direct earth contact. Miss any one of those and the electrode does not qualify. Reinforcing steel of the stated diameter may be substituted for the copper conductor.
What this question is testing
Whether you can recognize a qualifying concrete-encased electrode from a physical description and spot the details that disqualify it, particularly loss of direct earth contact, an undersized conductor, and coatings that break the electrical path between steel and concrete. Jobsite timing is part of the same knowledge.
On the job
This electrode is usually the best performer on the site because a footing presents an enormous surface to the soil. Coordinate with the concrete sub before the pour: either clamp a listed connector to the bottom rebar mat and stub a conductor up out of the form, or lay twenty feet of bare 4 AWG in the footing and bring the tail up. Rebar sections may be tied together in the usual way to reach the 20 feet. After the pour it is gone.
Memory technique
Twenty feet in the footing, two inches of cover, no plastic underneath.
Exam tip
For this electrode, memorize twenty feet, 4 AWG bare copper or half-inch rebar, two inches of concrete, footing on dirt.
Where to look it up
It is one of the numbered electrode types in the electrode list of Article 250 Part III.