A Guide to NEC 2023 Grounding and Bonding: Sizing Equipment Grounding Conductors (EGCs) per 250.122

ALT Text for Featured Image: A certified electrician inspecting the equipment grounding conductor connection inside a commercial electrical panel, referencing the NEC 2023 codebook.

Properly sizing an Equipment Grounding Conductor (EGC) is a fundamental skill for every electrician, but it’s a topic with crucial nuances that are often overlooked. As a master electrician and CE instructor, I see firsthand how misinterpretations of NEC 250.122 can lead to non-compliant and unsafe installations. This guide will provide a comprehensive, code-based breakdown of how to correctly size EGCs, ensuring your work meets the rigorous standards of NEC 2023 grounding and bonding.

The EGC’s Critical Role in Electrical Safety

Before we dive into the tables and calculations, let’s establish a core concept: the purpose of the Equipment Grounding Conductor. Unlike a neutral conductor that carries current under normal operating conditions, the EGC is an emergency-use-only conductor. Its sole job is to provide a safe, low-impedance path from the metal frame of equipment back to the source, typically the service panel or transformer.

In the event of a ground fault—where a “hot” conductor accidentally touches a conductive part of the equipment—the EGC directs this massive surge of current back to the overcurrent device. This creates a short circuit, causing the breaker to trip or the fuse to blow almost instantaneously, de-energizing the circuit and preventing electric shock. This entire system is what the NEC defines as an “Effective Ground-Fault Current Path.” Without a properly sized EGC, the ground-fault current path impedance could be too high, preventing the OCPD from tripping and leaving the equipment dangerously energized.

The Foundation: Sizing EGCs with NEC Table 250.122

The primary tool for sizing most EGCs is found in Article 250 of the NEC. Table 250.122 provides a straightforward method based on a single factor: the rating of the OCPD.

Step-by-Step EGC Sizing Using Table 250.122

Follow this simple process for basic EGC sizing:

1. Identify the Overcurrent Protective Device (OCPD): Find the circuit breaker or fuse that protects the circuit in question.
2. Determine the OCPD Rating: Note the ampere rating of the OCPD (e.g., 20A, 60A, 200A).
3. Consult Table 250.122: Locate the OCPD rating in the left-hand column, “Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment…” If your exact OCPD rating isn’t listed, the code requires you to use the next higher rating shown in the table.
4. Select the EGC Size: Read across to the right-hand columns to find the minimum required size for your wire-type EGC, in either copper or aluminum/copper-clad aluminum.

Example: A branch circuit is protected by an 80-ampere circuit breaker. According to Table 250.122, an 80A OCPD falls between the 60A and 100A rows. We must use the next higher rating, which is 100A. This requires a minimum #8 AWG copper or #6 AWG aluminum EGC.

Key Adjustments and Complex Scenarios in NEC 250.122

While Table 250.122 is the foundation, real-world installations often require adjustments. The following sections of NEC 250.122 address these common complexities.

Increase in Size: When Ungrounded Conductors are Upsized (NEC 250.122(B))

This is perhaps the most critical—and most frequently missed—adjustment. NEC 250.122(B) Increase in Size states that if you increase the size of your ungrounded conductors (for reasons like correcting for voltage drop on a long run), you must also increase the size of your EGC proportionally. The EGC must be increased by the same percentage as the ungrounded conductors.

Calculation Example:

• A 200A circuit requires a minimum of 3/0 AWG copper ungrounded conductors (per Table 310.16). Per Table 250.122, the minimum EGC is #6 AWG copper.
• Due to a long run, you upsize the ungrounded conductors to 250 kcmil copper to mitigate voltage drop.
• First, find the circular mil area of both conductors from NEC Chapter 9, Table 8:
  • Original (3/0 AWG): 167,800 cmil
  • New (250 kcmil): 250,000 cmil
• Calculate the ratio of increase: 250,000 cmil / 167,800 cmil = 1.49
• Now, find the circular mil area of the original EGC (#6 AWG): 26,240 cmil.
• Apply the ratio to the EGC: 26,240 cmil * 1.49 = 39,100 cmil.
• Finally, look back at Chapter 9, Table 8 to find the conductor with at least this circular mil area. The next standard size is #4 AWG copper (41,740 cmil). Your new EGC must be at least #4 AWG copper.

Multiple Circuits in a Raceway (NEC 250.122(C))

When multiple circuits are installed in a single raceway or cable, you are permitted to run a single, common EGC. If you do, that single EGC must be sized based on the largest OCPD protecting any circuit in the raceway. For example, if a conduit contains circuits protected by 15A, 20A, and 30A breakers, the single EGC must be sized for the 30A breaker (#10 AWG copper).

Parallel Conductor EGC Sizing and Motor Circuits

For parallel installations, NEC 250.122(F) provides options. Typically, a full-sized EGC, based on the circuit OCPD rating, must be run in each parallel raceway. For motor circuit grounding, the EGC is sized based on the rating of the motor branch-circuit short-circuit and ground-fault protective device, not the motor’s full-load amperage (FLA).

Raceway as EGC and Other Approved Types

It’s important to remember that a green insulated copper wire isn’t your only option. NEC 250.118 lists several approved types of EGCs. Most notably, a properly installed metal raceway as EGC (like RMC, IMC, or EMT) is permitted. When using a raceway system as the EGC, the integrity of every coupling and fitting is paramount to ensuring a continuous, low-impedance path.

The EGC is a load-side conductor, performing a different function than a grounding electrode conductor (GEC), which connects the system to the grounding electrode (e.g., a ground rod). These two are often confused, but their sizing and connection rules are entirely different. To clarify the GEC side of the system, you can review our guide on how grounding electrode conductor connections are handled in the 2023 NEC.

The EGC’s Role in the Complete Grounding and Bonding System

A properly sized EGC is a linchpin in the overall NEC 2023 grounding and bonding scheme. It works in concert with the system bonding jumper and the supply-side bonding jumper to clear ground faults. Its performance is critical for achieving a sufficient Short-Circuit Current Rating (SCCR) and for the proper operation of systems with Ground-Fault Protection of Equipment (GFPE).

The effectiveness of the entire safety system relies on carefully managing impedance. For more advanced grounding topics, you can explore how the 2023 NEC updates impedance grounding system requirements for specialized installations. Mastering these core concepts is what separates a good electrician from a great one. Staying current with your knowledge is key to safe, compliant, and professional installations. Learn more with ExpertCE’s NEC 2023 lessons.

Key Takeaways for Sizing EGCs

• The EGC is always sized based on the OCPD rating, not the connected load.
• Table 250.122 is your starting point for determining the minimum EGC size.
• Always remember to proportionally increase the EGC size per 250.122(B) if the ungrounded conductors are upsized.
• For multiple circuits with a single EGC, the EGC is sized for the largest OCPD in the group.
• An EGC’s primary purpose is creating a low-impedance effective ground-fault current path.
• The mechanical integrity of EGC connections (lugs, screws, fittings) is just as important as the wire size.

Frequently Asked Questions (FAQ) about EGC Sizing

Q1: What is the primary rule for sizing an Equipment Grounding Conductor (EGC) according to NEC 2023 grounding and bonding standards?

A: The primary rule, found in NEC 250.122, is that the EGC must be sized based on the ampere rating of the overcurrent protective device (OCPD) protecting the circuit. The minimum sizes are listed in Table 250.122.

Q2: When do I need to increase the size of an EGC beyond what’s listed in Table 250.122?

A: According to NEC 250.122(B), you must increase the EGC size proportionally whenever the ungrounded (phase) conductors are increased in size. This is common on long circuits where conductors are upsized to compensate for voltage drop.

Q3: Can I use a metal raceway as an EGC instead of a wire?

A: Yes. NEC 250.118 lists several approved types of EGCs, including metal raceways like Electrical Metallic Tubing (EMT), Rigid Metal Conduit (RMC), and Intermediate Metal Conduit (IMC). When using a raceway as an EGC, you must ensure all connections and couplings are tight to maintain a continuous and effective ground-fault current path.

Q4: How do I size an EGC for multiple circuits in the same conduit?

A: Per NEC 250.122(C), if you run multiple circuits in the same raceway and use a single EGC for all of them, that EGC must be sized according to the largest OCPD protecting any circuit within that raceway.

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