Welcome back to this series on developing a better understanding of Article 250 in NFPA 70, National Electrical Code. This series has taken a look at some of the most important parts of a building’s electrical grounding and bonding system and has sought to answer some questions. This article will explore requirements for one of the most critical components for safety in the grounding and bonding system: the equipment grounding conductor.

Editor's Note: read part 8 to catch up, or start with part 1 of this series.

What is an EGC?

We need to start by asking what the function of the equipment grounding conductor (EGC) is in the system. The NEC defines this in Article 100 as: “A conductive path(s) that is part of an effective ground-fault current path and connects normally noncurrent-carrying metal parts of equipment together and to the system grounded conductor or to the grounding electrode conductor, or both.”

The EGC connects everything together to prevent differences in potential on conductive surfaces to minimize risk of shock and stabilize the voltage to ground during normal system operation.

This definition really spells out the role an EGC plays in an electrical system. It starts by connecting all noncurrent-carrying metal parts. This is more of a bonding function than anything. This means that all the metal parts connected to the EGC will have the same electrical potential and thus prevent the build up of any different potential voltages that could shock a building occupant.

After the EGC connects all noncurrent carrying metal parts of equipment together, it then connects to the system grounded conductor, the grounding electrode conductor, or both. This part of the definition spells out that the EGC is also connected to the ground reference of the system. If you remember way back in part 1 of this series, we discussed that one of the reasons why we connect electrical systems to the earth is to stabilize the voltage to ground throughout the system. The EGC helps accomplish this by connecting all surfaces together and then connecting that to ground through the grounding electrode conductor. This is where the EGC gets its name as it connects equipment to ground or earth.

The last important part of the definition is that the EGC is part of an effective ground-fault current path. Section 250.4(A)(5) requires that electrical equipment and other conductive materials likely to become energized must be installed in a manner that creates a low-impedance path that facilitates the automatic operation of the overcurrent device providing circuit protection or the ground detector device for impedance grounded systems. I’ll discuss this in later articles. The EGC is this low-­impedance path until it makes the connection to the grounded or grounding electrode conductor.

The bigger picture emerges

The bigger picture is now starting to take shape. The EGC connects everything together to prevent differences in potential on conductive surfaces to minimize risk of shock and stabilize the voltage to ground during normal system operation. In the event  there is a problem, it helps the automatic protective device operate by establishing a low-impedance path back to the source. If we take any of these functions away, the risk of injury from the electrical system goes up considerably.

However, taking the EGC from concept to reality is quite another task. Notice how the definition calls the EGC a “conductive path” and not specifically a conductor? 

This is not a typo or an accident, as the EGC is permitted to take many different forms when installed. The two most common types of EGC materials used are wire or busbar  and nonflexible metal raceway. There are also several other kinds of conductive paths that can be used as the EGC or as a part of it.

Permitted types of EGCs

We can find a list of 14 permitted EGC types in Section 250.118. Some have no restrictions on using them as an EGC, such as a separate conductor or rigid metal conduit, while others come with stipulations.

However, one thing that you might notice about all permitted types of EGCs is that they are either run with the circuit conductors or are totally or partially enclosing the circuit conductors. 

This is important because the location where a fault occurs isn’t always where we expect it to happen. 

For instance, if a nonmetallic sheathed cable feeds a receptacle outlet device, there is certainly the threat of a faulty piece of equipment being connected through the receptacle device. What about when a drywall screw punctures the cable within a wall? If there were no EGC run with the circuit conductors, the protective device might not sense a spike in current and open the circuit.

As mentioned, there are several permitted types of EGCs with conditions that govern their use as an EGC. Take flexible metal conduit (FMC), for example. To use FMC as an EGC, both ends of the FMC must terminate in listed fittings, the circuit can’t be protected by an overcurrent device that has a rating greater than 20 amperes, the FMC can’t be larger than trade size 1¼ inches and the total combined length of FMC in the effective ground-fault current path must not exceed 6 feet. 

If the FMC is made of stainless steel or is used for flexibility after the installation to minimize the impact of equipment vibration, or because the equipment requires movement, then a wire-type EGC must be used or a bonding jumper must be installed that effectively bypasses the FMC portion of the EGC.

The structural metal frame of a building is not suitable to form a low-impedance path for facilitation of the overcurrent protective device. Also, the grounding electrode conductor is not sized based on facilitation of the overcurrent protection and is not permitted to be used for an EGC.

Certain cable types can also have the metal jacket or metal sheath used as an EGC. However, these cables typically must be specifically listed to provide the EGC’s needed functionality. 

The most common types of cables used in this application are Type AC cable or special variations of Type MC that provide a continuous bonding strip just inside the corrugated metal sheath that bonds it all together.

There are a few other listed items that can be used as an EGC as well, such as cable trays, the metal frameworks of cablebus, other listed metal raceways that are electrically continuous, auxiliary gutters and surface-mounted raceways if they are listed for grounding.

What can’t be used

Finally, it is also important to know that there are specific items not permitted to be used as the EGC. The structural metal frame of a building is one. 

While it might be acceptable to connect portions of the grounding electrode system, it is not suitable to form a low-impedance path for facilitation of the overcurrent protective device. 

The other item not permitted to be used for an EGC is the grounding electrode conductor since the GEC is not sized based on facilitation of the overcurrent protection. However, there is an exception that allows the GEC to be used as an EGC if it meets the requirements for a GEC and an EGC.

Understanding why the EGC is required and what can be used to accomplish the purpose of an EGC is critical to installing safe electrical systems. 

Next month’s article will dive into what specific equipment needs to be connected to the EGC and how we can identify these critical safety components. 

Until next time, stay safe and remember to always test before you touch!