Section 250.53 of the National Electrical Code provides grounding electrode installation rules that apply to grounding electrodes that must be installed and are not typically inherent in construction. The grounding electrodes addressed in this rule include ground rings and rod, pipe and plate electrodes. These three electrodes must be spaced at least 6 feet from any other electrode. A requirement to supplement a metal underground water pipe electrode with an additional electrode is also included in this section.
The details and descriptions of rod, pipe and plate grounding electrodes are covered in 250.52(A). Rod and pipe electrodes are described in 250.52(A)(5) and are required to be at least 8 feet in length. Rod and pipe electrodes must consist of pipe or conduit used as grounding electrodes and must not be smaller than metric designator 21 (trade size ¾). If the grounding electrode is made of steel, it must have an outer surface that is galvanized or otherwise metal-coated for corrosion protection. A driven pipe electrode, as covered in this section, is not the metal underground water piping system grounding electrode described in 250.52(A)(2). Stainless-steel grounding electrodes and copper- or zinc-coated steel electrodes must be at least 5/8-inch in diameter, unless they are listed by a qualified electrical testing laboratory.
These are grounding electrodes that must be installed if necessary. If installers choose to use rod, pipe or plate electrodes because there are no electrodes present for use, a single electrode is required to be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(8). The supplemental electrode is permitted to be connected to the rod, pipe or plate electrode; the grounding electrode conductor; the grounded service-entrance conductor; a nonflexible grounded service raceway; or any grounded service enclosure.
There is an exception that permits using a single rod, pipe or plate grounding electrode when the single electrode has a resistance to earth of 25 ohms or less. Demonstrating the resistance to ground of a single electrode is possible by specialized earth electrode testing methods. The requirement for installing two electrodes reflects how installers typically handle this requirement in the field. The NEC has been improved to require installation of two electrodes as the first priority, rather than installing the second when the 25-ohm resistance is exceeded. If a single electrode meets the 25-ohm requirement, there is no need for any additional electrodes to be installed.
The resistivity of a single grounding electrode such as a rod, pipe or plate is determined by testing, although the NEC does not directly state that testing is required. The 25-ohm resistance value is a variable, and resistance in the connection to ground will increase or decrease—it’s a moving target. The overall resistance is the connection between the electrode and the soil itself. The resistivity has a lot to do with the connection to the earth. Resistivity values differ among locations and seasonally. In some areas of the world, the soil is rich with mineral content and stays relatively moist throughout the year, keeping the resistivity values low.
Rod and pipe electrodes must be embedded below permanent moisture level when possible. This provides the most effective electrode performance. Frost line depths can vary geographically and can present connection problems between the earth and the electrode. Where the earth freezes, the connection between the earth and electrode is affected. Rod, pipe or plate electrodes must also be clean and free of nonconductive coatings such as paint, enamel or other substances that affect the connection between the earth and the electrode.
Electrode contact with the earth
Rod and pipe electrodes must be driven into the earth to establish the best contact. When a grounding rod or pipe electrode is installed, it is required to be in direct contact with the earth for a distance of no less than 8 feet. Sometimes there is difficulty driving an 8- or 10-foot ground rod or pipe electrode. The solution is not to cut off the exposed portion of the rod. This results in less than the required NEC minimums for electrode contact with the earth.
The NEC recognizes this challenge and provides alternatives when the electrode cannot be driven because of hitting bedrock. The first step in this hierarchy of alternatives is to install the electrode at an angle not more than 45 degrees from vertical. Often this solves ground rod installation problems.
If it does not, a third option is laying the rod or pipe electrode in a trench not less than 30 inches deep. Installers should be aware that laying the rod or pipe electrode in a trench is only a last resort, as indicated by the hierarchy arrangement of 250.53(G).