Understanding Our Electrical World: 8 Items that Form the Grounding Electrode System
NFPA 70®, National Electrical Code® (NEC®) has many areas of interest that keep the technical staff at NFPA on their toes. One of the areas that always seems to get a lot of questions through NFPA’s Technical Questions Service, available to members and AHJs, has to do with the electrical system grounding. Questions range from how to size the various grounding conductors and bonding jumpers to what can be used to connect the system to earth. Before we jump into figuring out how big the wire needs to be for the grounding electrode conductor, it is critical that we understand exactly how we will be connecting our electrical system to the ground and why.
First, we need to understand a few terms that are used within the NEC when it comes to grounding and bonding so that we can fully understand the intent of what is required. When we hear the term “grounded electrical system,” what does that even mean? Well, since the NEC defines “ground” as the earth and “grounded” as being connected to ground, or a conductive object that extends the ground connection, to have a grounded system means that you have an electrical system that is connected to the earth. Other terms that we must familiarize ourselves with is grounding electrode and the grounding electrode system. Basically, a grounding electrode is a conductive object that establishes a direct connection to the earth or ground. The important part is that a grounding electrode has direct contact with ground. There are lots of conductive objects within a structure, however, not all of them establish a direct connection to ground. This is where the grounding electrode system begins to take shape.
The NEC contains a list of items that are permitted to be used as grounding electrodes and requires that if any are present, they must be used to form the grounding electrode system. There are 8 items that are listed in 250.52 as allowable grounding electrodes, here is the list:
- Metal Underground Water Pipe
- Concrete-encased Electrode
- Metal In-ground Support Structure
- Ground Ring
- Rod and Pipe Electrodes
- Plate Electrodes
- Other Listed Electrodes
- Other local underground metal systems or structures
Any of these electrodes that are present in the building or structure, must be bonded together to form the grounding electrode system. There are some qualifying conditions for each item on the list that we will address shortly, but it is important to note that the first three on the list are components of the building itself and the rest are what is sometime referred to as “made electrodes.” In other words, a building will either have the first three or it will not, but 4-8 are items that the installer will put into the ground to establish the grounding electrode system. Let’s take a look at each of the specific items on the list:
- Metal Underground Water Pipe
The metal underground water pipe electrode is often called the “water bond” by many in the field. For a metal underground water pipe to qualify as an electrode, we need to have at least 10 feet in direct contact with Earth. This also needs to be electrically continuous or made electrically continuous to the point of attachment for the grounding electrode conductor or bonding jumper.
- Metal In-Ground Support Structure
The metal in-ground support structure electrode is often referred to as “building steel” but it is important to note that not all steel frames of a building will qualify as this type of electrode. To qualify as a grounding electrode, there needs to be direct contact with the ground or concrete-encasement that has direct contact with the ground. Steel building frames are often bolted down to bolts that are embedded into the concrete foundation and have no physical contact with the Earth itself. For the metal frame of a building to qualify as an electrode, there must be a minimum of 10 feet vertically in contact with the ground, with or without concrete-encasement. If there are numerous metal pilings that meet this qualification, only one needs to be connected to the grounding electrode system. However, there would be nothing to prevent the use of multiple metal in-ground electrodes as part of the building grounding electrode system.
- Concrete-Encased Electrode
The concrete-encased electrode is an electrode that uses a building’s concrete structural components to establish a connection with the Earth. Often referred to as the Ufer ground, this method is very effective in making the connection to Earth. There are two different methods for establishing this electrode. This electrode can be either a minimum of a #4 AWG bare copper conductor or it can be unencapsulated reinforcing steel rods with a minimum diameter of ½ inch. Either method must be a minimum of 20 feet in length and encased within a minimum of 2 inches of concrete that is in direct contact with the Earth. When this electrode consists of reinforcing steel, it is allowed to join multiple shorter sections of rods together through the usual methods but the final assembled length must meet or exceed 20 feet. Again, in buildings with multiple electrodes available, it is permitted to simply use a single electrode in the overall system.
- Ground Ring Electrode
A ground ring electrode is a grounding electrode that completely encircles the building or structure. This consists of a bare copper conductor that is a minimum of a #2 AWG conductor and must be a minimum of 20 feet in length. This type of electrode must be installed and is not a part of the building or structure like the first three electrodes.
- Rod or Pipe Electrodes
Rod and pipe electrodes are another type of electrode that can be installed to build a more robust grounding electrode system or when the building or structure does not contain a component that qualifies as an electrode, such as when the water supply to a home is in PVC and the footings are not in direct contact with Earth. These electrodes must be a minimum of 8 feet long and in contact with the Earth and a minimum of trade size ¾ inch when consisting of pipe or conduit and 5/8 when a rod type electrode. Smaller diameter ground rods can be used when they are listed as grounding electrodes. If corrosive materials, such as steel are used, they must be galvanized or have other measures taken for corrosion protection.
- Plate Electrodes
A grounding connection can also be established through the use of a conductive plate. The plate must expose a minimum of 2 square feet of surface area to contact with Earth. This could be mean that a grounding plate can measure 12 inches by 12 inches since there are 2 sides to the plate in contact with the Earth. For plates made from uncoated iron or steel, the minimum thickness of the plate is ¼ inch to account for corrosion of the plate over time. Non-ferrous metal plates are permitted to have a thickness of just 1.5 millimeters.
- Other Electrodes
Other electrodes are permitted to be used and 250.52 lists two categories that fall under the term “other”. If an electrode of a type not previously mentioned is listed by a nationally recognized testing laboratory as being a grounding electrode, the AHJ can permit the use of such an electrode. There are also other local underground metal structures and systems that are permitted to be used such as piping systems, metal well casings not bonded to a metal water line, and underground tanks. However, keep in mind that there are certain systems not permitted to be used as grounding electrodes, such as metal underground gas lines and the equipotential bonding grid required for in-ground pools. The AHJ must make the determination if such an object meets the requirements for a grounding electrode.
We must also talk about how these electrodes will be installed in order to form the grounding electrode system. Like stated earlier, metal underground water pipe, metal in-ground support structure, and concrete-encased electrodes are typically either a part of the building and therefore required to be used or they are not present and one of the other installed or “made” electrodes must be used. There is one exception to the general rule that if an electrode exists it must be used and that is for existing buildings. It is not the intent of the NEC to require that the concrete footing be disturbed to expose the reinforcing steel within and connect to it. The exception allows an installer the ability to not use an existing concrete-encased electrode if it would require disturbing the concrete.
Rod, pipe, plate, and metal underground water pipe electrodes all require the use of a supplemental grounding electrode. It is important to understand what can be used as a supplemental electrode as well. For instance, a ground rod can be used to supplement a metal underground water pipe however, a metal underground water pipe is not permitted to supplement a ground rod. Yet, 250.53(A) still requires rod, pipe, and plate electrodes to have a supplemental grounding electrode. This means that we are often installing a second ground rod or plate to supplement the ground rod which was installed to supplement the metal underground water pipe. This is because the metal underground water pipe has the potential to be changed out by the water utility for PVC and the homeowner is not often aware of the fact that this would place them only with a single ground rod afterwards. However, metal in-ground support structures, concrete-encased electrodes, and ground rings are all not required to be supplemented and therefore might be a viable option instead.
We also have the requirements for physically installing each electrode. In addition to needing to be in contact with the ground, there are specific requirements such as burial depth that we must follow. Rod and pipe electrodes must have a minimum of 8 feet in contact with the Earth and be installed vertically, unless bedrock is encountered at less than an 8 foot depth. In this case the electrode can be installed at an angle or horizontal if need be. In the event that a rod must be laid flat, it must be buried at a depth of 30 inches. This is a common burial depth for most “made” electrodes. Plate and ground ring electrodes must also be installed at a minimum depth of 30 inches.
Lastly, there is the connections of the grounding electrode conductors and bonding jumpers to consider as well. Just like with most every connection in the electrical world, we need any mechanical connections to remain accessible after installation. With a few exceptions for those listed for concrete-encasement or direct burial. Keep in mind that since these accessible locations are no longer in contact with the Earth, there are sections in the NEC granting permission to use items like the first 5 feet of interior metal water pipe, building steel, or exposed reinforcing steel to extend the connection to the electrode as well.
Understanding exactly how our electrical systems connect to ground helps us better achieve the goal set forth in 250.4 of grounding a system in a manner that limits the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation. Which in turn will ultimately help achieve the purpose statement of the NEC itself and that is the practical safeguarding of persons and property from the hazards that arise from the use of electricity. Being able to properly apply these concepts leads us all down a path towards protecting the world from the dangers present when electricity enters our world. At NFPA we can’t do it alone and we need your help to accomplish our mission of saving lives! Remember, it’s a big world, let’s protect it together!
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Important Notice: This correspondence is neither intended, nor should it be relied upon, to provide professional consultation or services.