Grounding Electrode Conductors
In this article we will discuss the Grounding Electrode and Grounding Electrode Conductor(s) of an electrical system. We will look at how to properly size grounding electrode conductors, as well as discuss what types of grounding electrodes can be used per NEC article 250.
What is a Grounding Electrode and Grounding Electrode Conductor?
NEC Article 100 definitions:
Grounding Electrode: A conducting object through which a direct connection to earth is established.
Grounding Electrode Conductor: A conductor used to connect the system-grounded conductor or the equipment to a grounding electrode, or to a point on the grounding electrode system.
Essentially, the grounding electrode is a conducting object that connects the electrical system to earth so there is an effective ground path. The conductors used to connect the grounding electrode to the electrical equipment and/or grounded conductor (neutral) is called the grounding electrode.
Types of Grounding electrodes
Let’s discuss the different types of grounding electrodes. NEC 250.52(A) Lists all the different types of grounding electrodes recognized. If any of the grounding electrodes mentioned below are present within each structure or building, they must be connected or bonded together to form a single grounding system as per NEC 250.50. There is an exception for existing buildings or structures with inaccessible concrete encased electrodes not being required to be connected to the grounding electrode system.
Grounding Electrode Types
- Metal Underground water pipes
- Metal in ground support structure(s)
- Concrete-encased Electrode
- Ground Ring
- Rods and Pipe, Electrodes.
- Other Listed Electrodes
- Plate Electrodes
- Other local Metal Underground Systems or structures.
Grounding Electrode Installation Requirements
NEC 250.53 covers the installation requirements of various types of Grounding electrodes. Let’s have a look at some of the requirements.
1. A rod, pipe and electrodes shall meet the following requirements
a. Embedded below moisture level. Must be free of coatings such as paint/enamel/epoxy.
b. A single rod, pipe or plate electrode shall be supplemented by an additional electrode to any type specified in NEC 250.52(A). It shall be permitted to be bonded to any one of the following
i. Rod, pipe, or plate electrode
ii. Grounding electrode conductor
iii. Grounded service-entrance conductor
iv. Nonflexible grounded service raceway
v. Any grounded service enclosure
c. Exception: If a single rod, pipe, or plate electrode has a resistance to earth of 25 ohms or less, the supplemental electrode shall not be required.
d. Multiple rods, pipes, or plate electrodes must be spaced at least 6 feet apart.
The takeaway from this section is that in most instances you will need to install at least two ground rods, pipes or plates unless of course 25 ohms or less can be measured or verified. Measuring soil resistivity can be challenging and tedious, so it is recommended to install an additional ground electrode. Also note that the additional grounding electrode can be bonded a number of ways as mentioned. Most commonly, the grounding electrodes are bonded together with a bonding jumper for ease of installation.
- Multiple rods, pipes or plate electrodes must be spaced at least six feet apart.
- Rod or pipe electrodes must be driven to have at least eight feet of contact with the soil. It may be driven at a 45-degree angle where rock bottom is encountered. It also may be buried in a trench of at least 30 inches deep, if rock bottom is encountered at a 45-degree angle. The upper end of the electrode must be flush with or below ground level, unless the aboveground end and the grounding electrode conductor attachment are protected against physical damage.
The rod or pipe electrodes must be a minimum of six feet apart. Rods may only be installed at an angle if it is not possible to drive the rod eight feet vertically. Burying the rod horizontally 30 inches below grade is only permitted if driving the rods vertically or at angle is not possible due to rock bottom.
Also of note, mechanical type ground clamps must be listed for direct earth burial. Ground clamps or rods above grade must be protected from physical damage.
- Plate electrodes shall be installed not less than 30 inches below the surface of the earth.
- Electrodes of different types shall be installed at least six feet apart from each other.
- Metal underground water pipe electrodes shall not rely on mater meters of filter devices for continuity of grounding path. Metal underground pipe shall be supplemented by an additional electrode type as mentioned in NEC 250.52(A).
Sizing Grounding Electrode Conductors
We use Table 250.66 to size grounding electrodes.
Note 1 mentions the proper way to size grounding electrode conductors for multiple sets. The way this is done is by taking the sum of the equivalent area of one phase/line conductor in each set. For example, if we had five sets of three 500kcmil copper conductors, we would add up the area of one conductor of each set. 5 x 500kcmil = 2000kcmil. We would use 3/0 copper grounding electrode conductor based on Table 250.66.
Raceways and Enclosures Used for Grounding Electrode Conductors
NEC 250.64(E) requires that all ferrous metal raceways and enclosures used for grounding electrode conductors shall be electrically continuous from the point of attachment and be bonded at each end of the raceway or enclosure to the grounding electrode or electrode conductor. This rule does not apply to nonferrous metal raceways and enclosures.