What counts as a grounding electrode
The NEC lists what may serve as a grounding electrode, including metal water pipe, concrete-encased (Ufer) electrodes, ground rings, rods and plates, each with minimum lengths and sizes. Gas piping and aluminum may never be used.
What the code requires
Metal underground water pipe counts if at least 10 ft is in direct contact with the earth and the pipe is electrically continuous; a bonded metal well casing counts too. A metal in-ground support structure counts with 10 ft or more of vertical contact with the earth.
A concrete-encased electrode, often called a Ufer, is at least 20 ft of bare, galvanized or conductively coated rebar of ½ in. diameter or larger (one length, or pieces tied or welded together), or at least 20 ft of bare 4 AWG or larger copper. It must be encased in at least 2 in. of concrete in a footing or foundation that touches the earth. If several are present, only one has to be bonded.
A ground ring is at least 20 ft of bare 2 AWG or larger copper encircling the building. Rods and pipes must be at least 8 ft long. Pipe must be at least ¾ trade size, galvanized or metal-coated if steel. Rods must be at least 5/8 in. in diameter unless listed. Plates need at least 2 sq ft exposed to soil and must be at least ¼ in. thick for iron or steel, or at least 0.06 in. for nonferrous metal. Other listed electrodes also qualify, as do other local metal underground systems or structures, such as piping systems, underground tanks and metal well casings that are not bonded to a metal water pipe.
Metal underground gas piping, aluminum, and the pool structures and rebar described in 680.26(B)(1) and (B)(2) may never be used as grounding electrodes.
On the job
- Concrete separated from the earth by insulation or a vapor barrier is not in direct contact with the soil, so that footing does not qualify as a Ufer electrode.
- Gas pipe treated as the ground is a correction. Gas piping is never a grounding electrode, although it still has to be bonded under 250.104(B).
- Rods shorter than 8 ft, or unlisted rods thinner than 5/8 in., will not pass.
Related sections
- NEC 250.50: Bonding all grounding electrodes together
- NEC 250.53: Ground rod installation and the 25-ohm rule
- NEC 250.66: Sizing the grounding electrode conductor
- NEC 250.68(C)(3)
- NEC 250.104(B)
- NEC 680.26: Equipotential bonding for pools
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