This article continues our little walk through NFPA 70, National Electrical Code, as we break down key areas that always seem to drive many questions. This article will pick up where we left off on the grounding electrode system. Last month, we covered the electrodes and how they make up a system, and as promised, we are back at it to walk through how to properly size the conductors and bonding jumpers that tie the system together.

The first thing to understand is that there are grounding and bonding conductors meant to carry fault current, and others are only meant to electrically join components together. For instance, because the earth is not considered an effective ground-fault current path, the grounding electrode conductor (GEC) isn’t typically going to need to be large enough for fault-level currents. However, the main and system bonding jumpers that connect the grounded conductor to the equipment grounding conductor are part of the effective ground-fault current path and will need to be sized appropriately for the potential fault current in the system. But more on these requirements after we get a handle on this GEC conversation.

Section 250.24

Since we are talking here about the grounding electrode system for a service-supplied AC system, we will head back to Section 250.24. Here we find guidance on the GEC, but in typical NEC fashion, it sends us to another area of Article 250. Specifically, 250.24(E) sends us to Section 250.66 for guidance on how to size the grounding electrode conductor. 

However, we must also acknowledge that 250.24(E) references all of Part III in Article 250, and there are also installation requirements to be aware of in addition to the sizing requirement. But the installation of the GEC and bonding jumpers is a topic for another day.

Let’s look at how we size grounding electrode conductors. This happens in Section 250.66 and is based on the size of the ungrounded conductors supplying the service/system. If the ungrounded conductors are in a parallel installation, we need to add up the cross-sectional areas of the individual parallel conductors. 

For instance, if we had four parallel 500 kcmil conductors, the size we would use is 2,000 kcmil. Once we know this size and the conductor material, we can select the proper size for the GEC from Table 250.66. Bonding jumpers connecting different electrodes must also follow the same sizing rules. However, remember that the GEC is not intended to be a low-impedance fault current path.

Table 250.66 gives ranges that our ungrounded conductor sizes will fall into. The smallest GEC we are permitted to use is a No. 8 AWG copper or No. 6 AWG aluminum (Al) or copper-clad aluminum (CCA), and this applies to ungrounded conductors equal to or less than No. 2 AWG copper or 1/0 AWG Al or CCA. As the size of the ungrounded conductors increases, so does the required GEC size. However, because of the GEC’s function, the table does not require anything larger than a 3/0 AWG copper or 250 kcmil Al or CCA grounding electrode conductor. Even if the parallel ungrounded conductors had an equivalent area of 6,000 kcmil, the GEC would still not be required to be larger than what is specified in the table.

Electrodes and GEC sizes

There are also a few specific sizes mentioned for certain electrodes, which are based on either limitations of the electrode or because the electrode contains a conductor that is not required to be larger than a certain size. It makes sense that if an electrode is formed from a certain size conductor, the GEC required to connect to it would not need to be larger than the conductor that makes it up. Also, the specific electrode requirements found outside of Table 250.66 do not apply if a bonding jumper is extended from the electrode in question to another that would require a larger electrode, such as a metal underground water pipe. This would require the size specified in Table 250.66.

First, for rod, pipe or plate type electrodes, 260.66(A) does not require the GEC to be larger than a 6 AWG copper or 4 AWG Al or CCA. The reasoning here is that the rod, pipe and plate electrodes have limitations that really don’t necessitate anything larger than that for the GEC. However, this is not the NEC saying that there is a bias against these electrodes. They still meet the requirements for grounding electrodes and the performance requirement in 250.4 that we discussed when we kicked this series off.

Concrete-encased electrodes are another type with a specific GEC size. Section 250.66(B) does not require larger than a No. 4 AWG copper conductor since the electrode is not required to be larger than a No. 4 AWG copper conductor, and it can even be the unencapsulated reinforcing rods connected using typical tie wires. Notice that it references only copper as a GEC material, since concrete will corrode other GEC materials. More on this next time. 

Finally, Section 250.66(C) is a specific rule for GECs connecting to ground rings. Remember that a ground ring is required to be a minimum of a No. 2 AWG bare copper conductor. The ground ring GEC is therefore not required to be larger than the conductor used for the ground ring electrode. 

However, this does not limit the GEC to a No. 2 AWG copper if the ground ring is a larger conductor, nor does it require a No. 2 AWG if Table 250.66 specifies a smaller GEC than No. 2 AWG. 

For example, let’s say the required size in Table 250.66 is a 3/0 AWG copper. If the ground ring is a No. 2 AWG, then the GEC is not required to be larger than that. However, if the engineer specified a No. 2/0 AWG ground ring, the GEC would be required to be a No. 2/0 AWG. 

Likewise, if the ground ring is a No. 2 AWG copper conductor, but Table 250.66 only requires a No. 6 AWG copper based on the size of the ungrounded conductors, then it only requires a No. 6. This is also the same for concrete-encased electrodes. 

Understanding how these critical connectors of the grounding electrode system are sized is key to ensuring compliance with Section 250.4. Without a properly sized GEC, the whole grounding system shifts away from its original intention to limit the voltage imposed by lightning, line surges or unintentional contact with higher-voltage lines, which will stabilize the voltage to earth during normal operation. 

However, getting the size right is only part of the equation. The GEC must be installed in a manner that ensures it will perform this critical function. Next month, we will dive deep into GEC installation requirements and how to make the final connection to the electrode. 

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

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