Master Electrician Exam (NEC)General Electrical KnowledgeMedium
A master electrician is designing a grounding electrode system for a new commercial building. Which of the following is NOT generally permitted as a grounding electrode by NEC Article 250?
- AA 1/2-inch (12.7 mm) rebar encased in 2 inches (50.8 mm) of concrete and in direct contact with earth.
- BMetal underground water pipe in direct contact with earth for 10 ft (3.0 m) or more.
- CThe metal frame of the building or structure, where effectively grounded.
- DAn aluminum plate electrode not less than 0.06 inch (1.5 mm) thick and with a surface area of at least 2 sq ft (0.186 sq m).
Show answer & explanationAnswer & explanation
Correct answer: D. An aluminum plate electrode not less than 0.06 inch (1.5 mm) thick and with a surface area of at least 2 sq ft (0.186 sq m).
NEC 250.52(A) lists acceptable grounding electrodes. While metal underground water pipes, building steel, and concrete-encased electrodes (Ufer grounds) are permitted, aluminum electrodes are specifically prohibited by 250.52(A)(8) due to their susceptibility to corrosion in direct contact with the earth.
Why the other options are wrong
- A. This is a permitted grounding electrode (Ufer ground) per NEC 250.52(A)(3).
- B. This is a permitted grounding electrode per NEC 250.52(A)(1).
- C. This is a permitted grounding electrode per NEC 250.52(A)(2).
NEC Grounding Electrodes
NEC Article 250 specifies various types of acceptable grounding electrodes and their installation requirements to ensure a safe and effective path for fault current to the earth.
- Common electrodes include metal underground water pipe, building steel, concrete-encased electrodes (Ufer), ground rings, and rod/pipe electrodes.
- Specific dimensions and installation depths are required for certain electrodes.
- Aluminum electrodes are prohibited where in direct contact with the earth.
Memory trick: Grounding: Earth's Safe Path, No Aluminum.