Sizing EGCs with NEC Table 250.122: A Practical Guide

Quick Summary: Sizing Your EGC with NEC Table 250.122

Properly sizing an Equipment Grounding Conductor (EGC) is a critical safety task for any licensed electrician. The primary tool for this is NEC Table 250.122, which dictates the minimum size EGC required for a circuit. The fundamental principle is that the relationship between the OCPD rating and EGC size is direct; the table sizes the EGC based on the amperage of the circuit’s overcurrent protective device (OCPD), not the connected load. This ensures the EGC can handle enough fault current to trip the breaker or blow the fuse, clearing the fault quickly. Correct equipment grounding conductor sizing is non-negotiable for establishing an effective ground-fault path, which is the cornerstone of protecting personnel and property from electrical hazards. Misinterpreting this table or confusing it with other tables can have dangerous consequences, making mastery of its application essential for code-compliant and safe installations.

The Cornerstone of Safety: Understanding NEC Article 250

For any professional in the electrical trade, article 250 of the National Electrical Code is the definitive guide to grounding and bonding. While often used interchangeably in casual conversation, understanding the difference between grounding vs bonding is paramount. Grounding is the act of connecting an electrical system to the earth itself, providing a path for lightning and dissipating static charges. Bonding, however, is the act of connecting all metallic, non-current-carrying parts of an electrical system together to create a continuous, low-impedance path. The EGC is primarily a bonding conductor, designed to create this path back to the source to clear a ground fault. This distinction is crucial for preventing what the code calls objectionable current on conductive paths where it shouldn’t be.

EGC vs. GEC: A Critical Distinction for Proper Sizing

A common point of confusion within nec 250 is the difference between an Equipment Grounding Conductor (EGC) and a Grounding Electrode Conductor (GEC). An EGC is the “green wire” or raceway that protects equipment; it creates the effective ground-fault path from a faulted piece of equipment back to the source. Its size is determined by NEC Table 250.122. In contrast, the grounding electrode conductor (GEC) connects the system’s grounded conductor at the service to the grounding electrode system (e.g., ground rods). The grounding electrode conductor size is determined by a different table, NEC Table 250.66. Using the 250.66 NEC table for equipment grounding conductor sizing is a serious code violation. Likewise, sizing grounded conductors (the neutral) follows separate rules entirely, and this process should not be confused with ground conductor sizing for equipment.

How to Use NEC Table 250.122 for Minimum Size EGC

At its core, using the nec grounding chart found in Table 250.122 is a straightforward process based on a single input: the rating of the OCPD. The logic is that the EGC must be robust enough to withstand the immense current of a ground fault until the OCPD can interrupt the circuit. Any changes to the code can impact how we apply these rules, and it’s vital to stay current on how equipment grounding conductor sizing and splicing rules change in the NEC.

Here is the step-by-step process for determining the minimum size EGC:

1. Identify the OCPD Rating: Determine the ampere rating of the fuse or circuit breaker protecting the circuit conductors. This is your key input value.
2. Locate the Rating in the Table: Find this ampere rating in the left-hand column of NEC Table 250.122, titled “Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes).”
3. Find the Corresponding EGC Size: Move horizontally to the right to find the required conductor size in the appropriate column for either Copper or Aluminum/Copper-Clad Aluminum. This is your minimum required size.
4. Check for Exceptions and Conditions: Always review the subsections of 250.122 to see if any modifications are necessary, such as for voltage drop or parallel runs. These are the critical 250.122 exceptions that can’t be ignored.

Key Considerations and Modifications in EGC Sizing

While the table provides the baseline, several common scenarios require adjustments to the minimum size. These are outlined in the subsections of 250.122 and are just as important as the table itself.

Upsizing UnGrounded Conductors? You Must Upsize the EGC – NEC 250.122(B)

One of the most important modifications involves voltage drop. If you increase the size of your ungrounded (phase) conductors for upsizing EGC for long runs to combat voltage drop, you must also increase the size of your EGC proportionally. According to NEC 250.122(B) voltage drop rules, the EGC must be increased in circular mil area in the same proportion as the increase in the ungrounded conductors. This ensures the EGC remains an effective, low-impedance path over that longer distance.

Sizing EGC for Parallel Conductors – NEC 250.122(F)

When dealing with parallel conductor installations, the rules for sizing EGC for parallel conductors in 250.122(F) apply. For circuits run in multiple raceways or cables, a full-sized EGC must be installed in each individual raceway or cable. The size of each of these EGCs is based on the rating of the circuit’s overcurrent protective device. Referencing a reliable ground wire size chart NEC, like Table 250.122, is essential here.

Special Cases and Related Components

The code also provides guidance for less common situations. For instance, EGC sizing for motor circuits is addressed in 250.122(D) and must account for the motor’s specific OCPD. Furthermore, the principles of EGC sizing are closely related to equipment bonding jumper sizing and ensuring the integrity of the system bonding jumper and supply side bonding jumper. These components all work together to ensure a fault can be cleared by providing a path with a low enough impedance to handle the high short-circuit current rating of the system. The requirements for the egc electrical path become even more specific in certain environments, such as those detailed in the code for equipment grounding in healthcare spaces.

Beyond the Wire: Other Types of EGCs

It’s important to remember that the EGC doesn’t have to be a wire. NEC 250.118 lists several approved types of EGCs. Using a raceway as equipment grounding conductor—such as Electrical Metallic Tubing (EMT), Rigid Metal Conduit (RMC), or Intermediate Metal Conduit (IMC)—is a common and acceptable practice. However, when using a raceway, all fittings and connections must be made up wrench-tight to ensure electrical continuity. A loose connection can compromise the entire effective ground-fault path. In hazardous locations, these rules are even stricter to ensure safety, which is why it’s important to understand the grounding and bonding requirements in Class I locations.

Key Takeaways for EGC Sizing

• The primary NEC grounding table for EGCs is NEC Table 250.122, which sizes the conductor based on the OCPD rating, not the circuit load.
• Do not confuse grounding vs bonding; an EGC is a bonding conductor that creates the critical fault-clearing path.
• Never use NEC Table 250.66 for equipment grounding conductor sizing. That table is strictly for the grounding electrode conductor size.
• Always check for conditions requiring an upsized EGC, especially per 250.122(B) when ungrounded conductors are increased for voltage drop.
• The goal is always to install a low-impedance, continuous grounding conductor path to ensure the OCPD can operate effectively during a fault.

Feeling confident about EGCs is one thing, but mastering the entire NEC Article 250 is another. Master conductor sizing with our advanced NEC code courses.

Primary Sources

This article is based on the requirements found in the National Fire Protection Association’s NFPA 70, National Electrical Code (NEC). Always consult the latest edition of the NEC adopted by your local jurisdiction for official requirements.

Frequently Asked Questions (FAQs)

What is the primary purpose of NEC Table 250.122?

The primary purpose of NEC Table 250.122 is to specify the minimum size EGC (Equipment Grounding Conductor) required for a circuit. The size is determined by the ampere rating of the overcurrent protection device (OCPD) protecting that circuit, ensuring the EGC can carry sufficient fault current to trip the device.

When do I need to size an EGC larger than the minimum size EGC listed in the table?

You must increase the EGC size, as per NEC 250.122(B) voltage drop rules, whenever the ungrounded circuit conductors are increased in size to compensate for voltage drop on long runs. The EGC must be increased in circular mil area proportionally to the increase of the phase conductors.

Can I use NEC Table 250.66 for equipment grounding conductor sizing?

No. This is a critical mistake to avoid. NEC Table 250.66 is used exclusively for determining the size of the Grounding Electrode Conductor (GEC). For all equipment grounding conductor sizing, you must use NEC Table 250.122, which is the correct nec grounding table for this application.

How does OCPD rating and EGC size relate to each other in NEC Table 250.122?

The relationship between the OCPD rating and EGC size is the foundation of NEC Table 250.122. A higher OCPD amperage rating requires a larger EGC. This ensures the EGC has a low enough impedance and sufficient thermal capacity to carry the high current of a ground fault without failing before the OCPD can clear the fault.

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