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How to Size an EGC: Connecting up to Code, part 11

By Derek Vigstol | Jan 15, 2025
How to Size an EGC: Connecting up to Code, part 11
Welcome back to the series on examining the critical safety features of electrical system grounding and bonding. I can’t think of a better way to kick off 2025 than with a continued in-depth look at the star of the effective ground-fault current path. 

Welcome back to the series on examining the critical safety features of electrical system grounding and bonding. I can’t think of a better way to kick off 2025 than with a continued in-depth look at the star of the effective ground-fault current path. Having already covered what can be used as an equipment grounding conductor (EGC), how to make EGC connections and what needs to be connected to the EGC, this article will dive into one of the most critical aspects: how to size the EGC to perform its intended function.

Editor's Note: read part 10 to catch up, or start with part 1 of this series.

With so many options for what can be used as an EGC, it is important to note that the only type of EGC with sizing requirements in Article 250 is the wire type. We will focus on this but consider the length and whether this can still function as intended when using a raceway, cable tray or cable armor as an EGC. Remember that one of the main functions of the EGC is to facilitate the operation of the overcurrent protective device (OCPD). If the raceway, cable armor or cable tray is long, the internal resistance of the metal can impede the effectiveness of this ground-fault current path in performing this function. See NECA’s “Standard for Good Workmanship in Electrical Construction” for further guidance on using raceways as an EGC.

Wire-type EGC

The size of a wire-type EGC is detailed in Section 250.122. The general rule for wire-type EGCs is that they need to meet the sizes specified in Table 250.122. However, generally they are not required to be larger than the circuit conductors supplying the equipment, but wire-type EGCs are permitted to consist of sectionalized conductors in multiconductor cables. When this is the case, the combined cross-sectional area of the sectioned EGC must add up to the equivalent area of the size required by Table 250.122.

If the general rule is all that applies to the EGC, the process is relatively straightforward. Table 250.122 uses the size of the overcurrent protective device to set the minimum size of the EGC, since the automatic operation of the OCPD is what we are after. 

You will notice that there are two columns for wire size based on the EGC material. The first column is for EGCs made of copper and the second is for conductors made of aluminum or copper-clad aluminum. Simply select which type of conductor material will be used and find the OCPD size that corresponds to the OCPD that is protecting the circuit conductors. Notice, though, that the OCPD column does not include every standard size. The heading of this column specifies that it is the “rating or setting, not exceeding” of the device ahead of the circuit.

For example, if the OCPD ahead of the circuit is rated at 80A, there is no such entry in the table. However, there is a 60A and a 100A rating specified. Therefore, to properly use the table, we will select the EGC size based on the values associated with a 100A rating, since 80A exceeds 60. Next, we need to determine the conductor’s material. If using a copper conductor, Table 250.122 specifies No. 8 AWG copper EGC. If we chose to use aluminum or copper-clad aluminum, the required size would be No. 6 AWG.

Remember that when using the table, the final EGC size must still meet the requirements of Section 250.4. It might be necessary to have a qualified person verify that the selected EGC is going to meet the requirements laid out in Section 250.4. Conditions such as voltage drop, length of raceway or segmented EGCs in multiconductor cable are just a few examples of when the effectiveness of the EGC path comes into question. It doesn’t happen all the time, but there are some instances where the EGC must be different than the sizes specified in Table 250.122.

Subsection requirements

Subsections (B) through (G) list several conditions that have the potential to require an EGC size that is different than specified in the table. The requirement in 250.122(B) states that if the ungrounded conductors are increased in size, the EGC’s size must increase proportionately based on the increase in circular-mil area. 

However, there are two instances that don’t apply here. If the ungrounded conductors are increased in size due to ampacity adjustments such as temperature correction factors or having more than three current-carrying conductors in the raceway or cable, the EGC’s ability to facilitate the operation of the OCPD is not affected by these factors.

The most common reason for increasing the ungrounded conductor size outside of ampacity issues is due to voltage drop. Remember that even though circuit conductors are low resistance, they are not resistance-free. Therefore, as the conductor gets longer, the internal resistance increases. This is also true for the EGC and, since resistance limits current flow, an increase in resistance in the EGC could lead to a scenario that impedes the operation of the OCPD.

Since 250.122(B) refers to the circular-mil area of the ungrounded conductor, the first thing we need to determine is how much the ungrounded conductors increased in size. 

For example, a feeder to an outbuilding on my property is about 350 feet long. This feeder is supplied by a 100A OCPD and feeds the 100A panelboard in my shop. Because of the length, I decided to increase the conductor size from a minimum of a No. 3 AWG copper to a No. 2 AWG copper conductor due to voltage drop.

Chapter 9, Table 8 shows that by doing this, I increased my ungrounded cross-­sectional area from 52,620 circular mils to 66,360. This is an increase of 26% (66,360 ÷ 52,620 = 1.26) for the ungrounded conductors. I must also increase my EGC by at least 26%. Since the circuit breaker is rated for 100A, Table 250.122 requires a minimum  No. 8 AWG copper EGC, which has an area of 16,510 circular mils. Increasing this by 26% (16,510 × 1.26), puts me at 20,821 circular mils. In this case, increasing by a single AWG size to a No. 6 AWG is more than adequate at 26,240 circular mils.

That example is relatively straightforward and simple, but it doesn’t always work out that way. This requirement can get complicated when there are other factors involved, such as when conductors are run in parallel and are increased in size. Parallel installations need to have their own explanation, so we will dive deep into this next time.

The importance of having a properly sized EGC cannot be understated. Remember, the function of the EGC is to serve as the effective ground-fault current path required in 250.4 and to facilitate the automatic operation of the OCPD.

Join us next time as we wrap up our discussion centered around Section 250.122 and properly sizing the EGC. Until then, stay safe, and always remember to test before you touch!

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About The Author

Vigstol is an electrical safety consultant for E-Hazard, a provider of electrical safety consulting and training services. He is also the co-host of E-Hazard’s electrical safety podcast “Plugged Into Safety.” For more information, check out www.e-hazard.com.

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