This month, we continue to develop a better understanding of Article 250 of NFPA 70, National Electrical Code. Previously, we took examined the reasons why grounding and bonding are so critical to how we install a safe electrical system. We looked at which systems must be grounded or are permitted to be ungrounded. Now that we know the “why” and “what,” it is time to examine the “how” grounding and bonding is performed. Where do we start? With Article 250, which has a lot of “how-to”.  So let’s look at the  method for grounding and bonding at the service.

Where do we start?

Section 250.24 contains the requirements for grounding service-supplied AC systems. Remember, being grounded means being connected to the earth or a conductive object that extends the connection to earth. This is where we will start. 

How do we connect the system to the earth? Section 250.24 states that a grounded service must have a grounding electrode conductor (GEC) connected to the service grounded conductor at the service equipment. This connection can be made at any accessible point from the end of the overhead or underground service conductors to the terminal where the grounded service conductor connects to the service equipment.

Note that, if the utility owns the overhead or underground conductors, they are referred to as a service drop for overhead or a service lateral for underground. There are different rules based on ownership of these conductors, but regardless, they are the conductors coming from the utility transformer. 

However, if the main bonding jumper is a wire or busbar that connects the equipment grounding conductor (EGC) to the grounded conductor, the GEC can be connected to the EGC terminal instead. More to come on the main bonding jumper.

As you can see, the GEC is a critical component to the equation. But exactly how does it make the connection to the earth? Section 250.24(E) states that the GEC is used to connect the EGC, the service enclosure(s)and, if used, the system grounded conductor to the grounding electrode system required later in Article 250. This grounding electrode system is made up of conductive objects in direct contact with the earth as spelled out in Part III of Article 250.

Let’s take a look at what types of electrodes are required to make up this system. Part III starts at Section 250.50 with a general rule that says if any of the electrodes listed in Section 250.52(A)(1)–(A)(7) are present, they must be used to create this system.

Underground water pipe

The electrodes listed in 250.52 cover several different conductive objects that might be present at a building, and a few the installer can create. Obviously, if the building gives us an effective way to connect to earth, we should start there. These are the electrodes listed in 250.52(A)(1)–(A)(3). 

The first one is the metal underground water pipe. This only counts as an electrode if the metal pipe has at least 10 feet in direct contact with earth and is electrically continuous to the point where the GEC connects to the pipe. If there are joints, unions or metering devices in between the 10 feet in contact with earth and the connection point, the installer must make this electrically continuous by using bonding jumpers around these breaks in the pipe. 

This electrode has become less common over the years, especially in residential settings, with the popularity of plastic water supply lines. We’ll discuss how this type needs to be supplemented by another electrode later.

Metal underground support

The next type of electrode is the metal underground support structure. Many buildings have metal beams driven into the earth that create an excellent connection. 

However, there are also conditions for this electrode. Just like with the water pipe, a minimum of 10 feet of the underground support structure must be in contact with the earth. But there is good news, too. If the building contains several of these metal beams driven into the earth, it is permissible to only connect to one. Also, unlike the water pipe, these structures are not in any danger of being replaced by plastic anytime soon, so they need no supplemental electrode.

Ufer ground

The last type of electrode that comes with the building is the concrete-encased electrode, more commonly called the Ufer ground after the consultant that developed this method for grounding bomb storage vaults in the desert during World War II. 

This electrode consists of either 20 feet of copper conductor or No. 4 unencapsulated rebar installed in the foundation or footing directly in contact with the earth.

The electrode must be encased in a minimum of 2 inches of concrete and can be in either horizontal or vertical concrete structures underground. However, if a moisture barrier is laid down before the footing is poured or the vertical structure is contained in a sonnet tube, this does not qualify as being in direct contact with the earth, and this electrode will not work. If multiple concrete-encased electrodes are present, it only needs to be employed in the grounding electrode system.

‘If needed’ options

The rest of the options in 250.52 are electrodes that can be installed if needed. These include rod and pipe electrodes with at least 8 feet in contact with earth, ground rings surrounding the building, plate electrodes of a minimum of 2 square feet in contact with the earth and other listed devices installed for creating a grounding electrode.

When required, these types of electrodes must be incorporated into the grounding electrode system. 

Other local underground metal piping and structures are also permitted to be used in the grounding electrode system but are not required. However, this cannot include anything already connected to the metal underground water pipe electrode, metal underground gas piping, anything made from aluminum or the equipotential bonding conductors of swimming pools.

Installed or “made” electrodes are often used as a supplemental electrode to the metal underground water pipe electrode. Or, in the case of the ground ring, it can be installed as the sole grounding electrode should the building or structure not contain any other electrodes. Metal underground water pipe, driven rod or pipe electrodes and ground plate electrodes require a supplemental one. The supplemental electrode can be any of the ones specified in 250.52, except for the metal underground water pipe due to the possibility of it being replaced with plastic.

The combination of these electrodes and their connection to form a system and connect to the service through a properly sized GEC are what connects the system to the earth to meet the performance requirements discussed in the first part of this series for grounding electrical systems. 

Remember, this is done to establish a stable voltage-to-ground under normal conditions and limit the voltage imposed on the system due to lightning strikes, line surges and unintentional contact with other higher-­voltage lines. Of course, this only works if this system is properly sized. 

Next month’s article will pick up on how to size the conductors and jumpers that form this system. 

As always, be safe, and always remember to test before you touch.

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