Welcome back to Part 7 of this series of columns on establishing a better understanding of Article 250, Grounding and Bonding, in the National Electrical Code. This article takes an in-depth look at the main and system bonding jumper and the role it plays in grounding and bonding, how we size it up and install it. To start our journey on these two critical components of the grounding and bonding system, we return to Section 250.24.

Section 250.24 contains requirements for grounding of service-supplied AC systems. DC systems will be covered later. Because this section deals with service-­supplied systems, this is where the requirements are for installing the main bonding jumper. Section 250.24(C) requires that for a grounded system, an unspliced main bonding jumper must connect the equipment grounding conductor(s) and service enclosure to the grounded conductor within the service equipment enclosure. 

For a grounded system, an unspliced main bonding jumper must connect the equipment grounding conductor(s) and service enclosure to the grounded conductor within the service equipment enclosure. This connection must not happen anywhere downstream from the service equipment.

This connection must not happen anywhere downstream from the service equipment as it could create an alternate path for normal neutral current. It is important to note that there is also a requirement to run a grounded conductor from the transformer to the service equipment for all grounded services. Even if there are no line-to-neutral loads on the service, the grounded conductor must be run to complete the effective ground-fault current path we discussed in the last article.

System bonding jumper requirements, found in Section 250.30(A)(1), are slightly different. This section still requires that the system bonding jumper be unspliced and connect the equipment grounding conductor(s) and the equipment enclosure to the grounded conductor for the same reason as the main bonding jumper in the service. However, the system bonding jumper can be installed at any point between the source of a separately derived system and the first disconnecting means or overcurrent protective device. This difference is due to the ownership of the equipment. With the main bonding jumper, the facility only owns the service equipment, whereas the facility typically owns the source of a separately derived system and the first disconnecting means.

Sections 250.24(C) and 250.30(A)(1) send us to Section 250.28 to learn information such as what material these jumpers can be made of, how they are permitted to be connected and what size wire for wire-type main and system bonding jumpers to use.

What material can be used?

Let’s start with what can serve as a main or system bonding jumper. Section 250.28(A) allows us to use wire, bus, a screw or other similar conductor in this critical role. The most common types used are wire, screw and busbar. They must also be made from copper, aluminum, copper-clad aluminum or other corrosion-resistant material. For screws that serve as the main or system bonding jumpers, the screw must have a green finish and be visible after installation. Main and system bonding jumpers can be attached using any of the methods specified in Section 250.8 for attaching grounding and bonding equipment.

How are they connected?

Now let’s look at the last part of Section 250.28, which deals with sizing of the main and system bonding jumpers. Here we are once again directed to a different part of Article 250. To determine the size of these jumpers, we must now go to Section 250.102, specifically to Table 250.102(C). This table should look somewhat familiar if you remember Table 250.66 for sizing the grounding electrode conductor.

Because bonding jumpers are installed in a manner that bridges the gap from the equipment grounding conductor system to the ultimate fault current return path to the source, the main bonding jumper and system bonding jumper are sized based on how much fault current the service or system can supply. The NEC uses the size of the ungrounded conductors as the basis for this. As the size of the ungrounded conductors gets larger, the main and system bonding jumpers must also increase in size to handle the potentially larger amount of fault current.

Table 250.102(C) contains four columns split into two groups. The first group is for the size of the ungrounded conductor supplying the system, and the second is for the size of the grounded conductor or bonding jumper. Each group is then broken up by conductor material, and there is a column for copper conductors and one for conductors made from aluminum or copper-clad aluminum. The nice thing is that the way this table is set up makes it easy to determine sizes when mixing and matching material. For instance, if the ungrounded conductors are copper, you can easily determine the size of an aluminum main or system bonding jumper.

What size?

Since we are dealing with fault current path conductors, there is no max size in this table. As the ungrounded conductors get larger, or if several conductors are installed in parallel for the ungrounded conductors, the fault current capabilities continue to increase and so must the bonding jumpers’ current-­carrying capability. For parallel conductors, we must determine the total equivalent area and the bonding jumper size from that number. So, if five sets of 500-kcmil copper conductors are paralleled to supply the service, the ungrounded conductor size used in making this determination would be 2,500 kcmil.

However, the table only lists specific bonding jumper sizes for ungrounded conductor sizes up to 1,100-kcmil copper and 1,750-kcmil aluminum or copper-clad aluminum. For sizes over these limits, the table says to see notes 1 and 2. 

Note 1 deals with determining the percentage of the ungrounded conductor size the bonding jumper needs to be. That is 12.5% of the ungrounded conductor size. So, to use our example of five sets of 500-kcmil copper conductors, we would need to have a bonding jumper that is at least 312.5 kcmil, or in standard conductor sizes, we would need at least a 350-kcmil copper bonding jumper.

But what if the ungrounded conductor and bonding jumper are different materials? This is where Note 2 comes in and directs us to determine the needed size of the other material to achieve the equivalent ampacity of an ungrounded conductor. 

Using the same example, let’s say copper is used for the ungrounded conductors but the bonding jumper is aluminum. First, we need to determine the ampacity of the parallel 500-kcmil conductors. Table 310.16 sets this at 380A per 500-kcmil conductor. Now we need to determine what size aluminum conductors need to be used to achieve the same 1,900A of ampacity. Seven sets of 500-kcmil aluminum brings us to just over the 1,900A mark. This gives us an equivalent area of 3,500 kcmil, and 12.5% of that works out to be 437.5 kcmil, or a 500-kcmil aluminum bonding jumper.

The main and system bonding jumpers are the critical point in the grounding and bonding system that ensure the safety of those around electrical equipment. Understanding how to size and install them is critical to understanding how to create a safe grounded electrical system meeting Section 250.4. 

October’s article will cover another critical piece in the fault current path: the grounded conductor. Until then, stay safe and remember to always test before you touch!