How to Size GECs with NEC Table 250.66: Step-by-Step

As a certified CE instructor and master electrician, I’ve seen firsthand how misinterpreting grounding and bonding rules can lead to failed inspections and unsafe installations. This guide is written for licensed professionals to clarify one of the most fundamental tasks: correctly sizing the Grounding Electrode Conductor (GEC).

Your Answer-First Guide to Sizing with NEC Table 250.66

Sizing a Grounding Electrode Conductor (GEC) is done by using NEC Table 250.66, a critical component of Article 250 of the National Electrical Code. The process requires you to first determine the size of the largest ungrounded service-entrance conductors. If the service conductors are run in parallel, you must calculate their total circular mil (or kcmil) area to find the equivalent size of a single conductor. With this size determined, you reference the first column of Table 250.66 to find the row corresponding to your service conductors. The adjacent columns then provide the minimum required grounding electrode conductor sizing for either copper or aluminum conductors. This table is the primary tool for this task, distinct from NEC Table 250.122, which is used for Equipment Grounding Conductors (EGCs). Proper GEC sizing is foundational to creating a safe and compliant electrical system.

The Foundation of Safety: Understanding Grounding vs. Bonding

Before diving into the specifics of ground conductor sizing, it’s essential to reinforce the distinction between grounding vs bonding. Grounding connects a system to the earth, creating a reference point and a path for lightning and other high-voltage events. Bonding connects all metallic parts of an electrical system that are not meant to carry current, ensuring they are at the same electrical potential. Both are governed by NEC Article 250 (often searched simply as nec 250) and work together to create a safe path for fault current to facilitate the operation of overcurrent protection devices. The Grounding Electrode Conductor (GEC) is a key player in this system, as it provides the physical connection to the grounding electrode system.

The Role of the Grounding Electrode Conductor (GEC)

The electrode grounding conductor is the wire that connects the system’s grounded conductor at the service equipment to one or more grounding electrodes (e.g., ground rods, Ufer grounds). Its sole purpose is to connect the electrical system to the earth. It is crucial not to confuse this with other conductors:

• Grounded Conductor: This is the conductor in a system that is intentionally grounded, typically the neutral. We refer to these as grounded conductors.
• Equipment Grounding Conductor (EGC electrical): This conductor provides a ground-fault current path by connecting the non-current-carrying metal parts of equipment back to the system’s grounded conductor, the GEC, or both. Its sizing is determined by NEC Table 250.122.

The connection between the GEC, the grounded conductor, and the equipment grounding conductor bus is made via a main bonding jumper (or a system bonding jumper in a separately derived system). Many professionals searching for information on the neutral-to-ground connection sometimes use shorthand like “neutral obj” when looking for these bonding rules.

Step-by-Step Guide to Sizing GECs with NEC Table 250.66

Proper grounding electrode conductor sizing is not a guessing game; it’s a precise process mandated by the NEC. Follow these steps to accurately determine the correct grounding electrode conductor size using what is often called the NEC grounding table.

1. Identify the Service-Entrance Conductors: The first step is to determine the size of your largest ungrounded service-entrance conductors. This is the “hot” or “phase” conductor. The size is typically given in AWG or kcmil.
2. Calculate the Equivalent Area for Parallel Conductors: If your service uses parallel conductors in multiple raceways, you cannot use the size of a single conductor. Per NEC 250.66, you must find the equivalent area for parallel conductors. For example, if you have two parallel runs of 3/0 AWG copper, you must find the total circular mil area (2 x 167.8 kcmil = 335.6 kcmil) and then find the single conductor size with at least that area (in this case, 350 kcmil). This is the size you use for the table. This calculation is a key part of both sizing grounded conductor and ungrounded conductor installations.
3. Reference the NEC Grounding Chart: With your conductor size determined, turn to NEC Table 250.66, “Grounding Electrode Conductor for Alternating-Current Systems.” Locate your service-entrance conductor size in the left two columns of the table.
4. Select the GEC Size: Once you’ve found the correct row, move to the right to find the required GEC size. The table provides options for both copper versus aluminum GEC applications. Select the appropriate conductor material and size.

Note that this primary method of sizing applies to the GEC itself. The rules for sizing a supply side bonding jumper are found in 250.102(C)(1) and largely mirror this table, but have different upper limits.

Critical GEC Sizing Exceptions in NEC 250.66(A) and 250.66(B)

While Table 250.66 is the primary rule, licensed electricians must know the GEC sizing exceptions. These can save material and labor but must be applied correctly.

NEC 250.66(A): Connections to Rod, Pipe, or Plate Electrodes

If your GEC connects *solely* to one or more rod, pipe, or plate electrodes as allowed in 250.52(A)(5) or (A)(7), the GEC is not required to be larger than 6 AWG copper or 4 AWG aluminum. However, if that same GEC also connects to a ground ring or concrete-encased electrode, this exception does not apply, and you must revert to Table 250.66 sizing. If multiple electrodes are present, you may need a continuous grounding conductor or jumpers between them.

NEC 250.66(B): Connections to a Concrete-Encased Electrode

When connecting to a concrete-encased electrode (Ufer ground) as described in 250.52(A)(3), the portion of the GEC that is the sole connection to that electrode is not required to be larger than 4 AWG copper. This recognizes the vast surface area and effectiveness of the Ufer ground.

Other Important Considerations in NEC Article 250

Mastering GEC sizing requires understanding its context within the broader article 250 of the national electrical code.

• Separately Derived System Grounding: The sizing rules for GECs in a separately derived system grounding application (like a transformer) also use the 250.66 nec table as a basis. However, the specific requirements are detailed in NEC 250.30. For a deeper dive, learn more about how GEC rules affect transformer installations.
• EGC vs. GEC Sizing: A common mistake is confusing GEC and EGC sizing. To reiterate, GECs are sized with nec table 250.66 based on service conductors. Equipment Grounding Conductors (EGCs) are sized with nec table 250.122 based on the rating of the overcurrent protection device.
• GEC Connections: The integrity of your GEC is only as good as its connections. These terminations must be made with irreversible means or listed connectors. Understanding how GEC connections are handled in the NEC is just as important as sizing.
• Staying Current: Proper service disconnect grounding and GEC sizing are subject to updates with each code cycle. It’s vital to stay informed on how GEC sizing rules change in the NEC.

Never mis-size a conductor again. Enroll in our NEC calculations courses to master these critical skills and ensure every installation is safe and compliant.

Primary Sources

• NFPA 70, National Electrical Code (NEC)

Frequently Asked Questions (FAQ)

What is the primary difference in sizing a GEC with nec table 250.66 versus an EGC with nec table 250.122?

The primary difference is their reference point. A GEC is sized using NEC Table 250.66 based on the size of the service-entrance conductors. An EGC is sized using NEC Table 250.122 based on the amperage rating of the circuit’s overcurrent protection device (e.g., the circuit breaker or fuse).

How do I determine the grounding electrode conductor size when my service conductors are run in parallel?

You must calculate the total circular mil area of the parallel conductors per phase. Sum the kcmil area of all parallel conductors for one phase, then find the single conductor size in NEC Chapter 9, Table 8 that has an equivalent or greater area. You use this calculated equivalent size to find the required grounding electrode conductor size in Table 250.66.

Does the 250.66 nec table apply to separately derived system grounding?

Yes, the sizing principles of the 250.66 nec table are used for separately derived system grounding. However, the specific rules for application are located in NEC 250.30. The GEC is sized based on the total area of the derived phase conductors, using Table 250.66 for the conductor-to-GEC size relationship.

When can I use the GEC sizing exceptions in NEC 250.66(A) and NEC 250.66(B)?

You can use these GEC sizing exceptions only when connecting to specific electrodes. Use NEC 250.66(A) (max 6 AWG CU / 4 AWG AL) only when the GEC’s sole connection is to a ground rod, pipe, or plate electrode. Use NEC 250.66(B) (max 4 AWG CU) only when the GEC’s sole connection is to a concrete-encased electrode. If the GEC connects to any other electrode type (like a ground ring or the metal frame of a building), you must use the full size required by Table 250.66.