Welcome to part 6 in the series on understanding Article 250 of the National Electrical Code (NEC). If you have been following along, this column has been all about understanding why and when we connect an electrical system to the earth. We’ve also focused on how the NEC guides us on this journey to make the connection. 

Having established what systems we need to ground and how the grounding electrode system is to be installed to meet the performance requirement of Section 250.4, we shift focus to the second half of Article 250’s title. Let’s examine the concept of bonding.

Bonding is to electrical systems what a parachute is to skydiving. Could you go without it? I guess so! But you would be a lot safer if it was included. If we think all the way back to our discussion on Section 250.4, normally non-current-carrying conductive material needs to be connected in a manner that establishes an effective ground-fault current (EGFC) path. This pathway must be a low-impedance circuit to facilitate the operation of the overcurrent protective device (OCPD). It is the OCPD operation function of the EGFC path that makes bonding the parachute of the electrical system.

Bonding components

Because bonding is a critical safety feature of every electrical system, the methods used for this purpose must be sufficient to safely carry the maximum amount of fault current that could be imposed on the bonding path. We start by identifying all bonding components that create the EGFC path. 

Some components of the grounding and bonding system play a dual role, as they are a part of the EGFC path and make the connection of system conductors or equipment to the grounding electrode system. However, being a part of the EGFC path will typically dictate how these items are sized.

Starting with the service equipment, there are two bonding jumpers that play a critical role. The first is the main bonding jumper, which brings together the grounded circuit conductor and the equipment grounding conductor (EGC) or the supply-side bonding jumper, or both, at the service. For grounded systems, this is the final bridge that ground-fault current must cross on the journey back to the source because it connects to the service grounded conductor. Without this connection, or if it is not low impedance, there is no facilitation of the OCPD operation.

The supply-side bonding jumper is also critical because it connects conductive material on the line side of the service OCPD. Any faults that occur ahead of the service equipment travel along this path to find their way back to the source. However, because this bonding jumper connects on the line side of the service OCPD, it does not operate the main OCPD. If there are protective relays or OCPDs ahead of the supply connection, the supply-side bonding jumper might facilitate their operation. The supply-side bonding jumper is also something used in separately derived systems.

Speaking of separately derived systems, there the system bonding jumper is a critical bonding connection that must be made. Like the main bonding jumper, this connects the grounded system conductor and supply-­side bonding jumper or EGC, or both, at a separately derived system. What happens at a separately derived system is that the circuit conductor path is either broken or doesn’t exist to an upstream source.

Because of this new system being created, a fault on the secondary side would simply appear to the primary side as another load and may or may not spike the primary current high enough to open the OCPD. Therefore, we need a new pathway for fault current to return to the source. This helps the OCPDs on the secondary side of the separately derived system to clear the fault.

EGC

The next part of the bonding system to discuss is the EGC. The role of this conductor is often confused or misunderstood due to the name. In fact, several folks have even submitted proposals to change the name to the equipment bonding conductor due to the important role in the bonding system that the EGC plays. 

The NEC defines the EGC as a conductive path part of the EGFC path, and it connects normally non-current-­carrying metal parts of equipment together and to the system grounded conductor or the grounding electrode conductor (GEC), or both. In most cases, it is both. However, as you can see in the definition, the primary function of the EGC is connecting things together, which is the whole concept of bonding.

Bonding jumpers

There are also several lesser-known bonding components to discuss. However, some of these are part of the EGFC path, and some are completely different. For instance, an equipment bonding jumper is a bonding jumper that connects two sections of the EGC and is therefore part of the EGFC path, whereas the bonding jumper required around the water meter is just connecting the water pipe pieces together and is not intended to be a part of the EGFC path. Depending on its intended function in the bonding system, the rules for each might be different.

Identifying bonding jumpers’ purposes is often the first step in understanding how to determine sizing and installation practices. Follow the fault current path back to the transformer or other source and determine the role of the bonding jumper. Once we know what it is doing, we can start to find requirements in Article 250 to properly bond electrical systems. 

It is also important to understand that many bonding jumpers can be formed from something other than a wire-type conductor. It is not uncommon to see a screw or a busbar that is the main or system bonding jumper or a busbar that carries the supply-­side bonding jumper connection to a location where it can connect to the system grounding conductor.

Just like with the GEC system, we will start our journey in Section 250.24 for grounding of service-supplied AC systems. However, we won’t stay there long, as 250.24 requires a main bonding jumper to be installed, but then sends us right into 250.28 for guidance on what the main bonding jumper can be made from, how to connect it and how to size it. This section is also applicable to system bonding jumpers, since they essentially perform the same function but are found in separately derived systems instead of services.

Next month’s article will examine the process in Article 250 for main and system bonding jumpers. It will cover how and where they are to be installed and, most importantly, how to size them. Understanding this critical point will ensure that we don’t install a system without a parachute.

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

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