How to Size Ground Wire for 100A, 125A, and 200A Services

As a certified CE instructor for ExpertCE, I’ve seen firsthand how critical proper grounding is for the safety and integrity of any electrical installation. Correctly sizing your grounding conductors isn’t just about passing inspection—it’s about protecting lives and property. This guide will provide a clear, NEC-based approach to this essential task.

Your Quick Answer: Sizing Ground Wires for Electrical Services

For a standard residential 200-amp service, the correct ground wire size for 200 amp service is typically a #4 AWG copper or #2 AWG aluminum conductor. This is the Grounding Electrode Conductor (GEC) size, determined by the size of your service entrance conductors (for example, 2/0 AWG copper or 4/0 AWG aluminum) as specified in NEC Table 250.66. The process of grounding electrode conductor sizing is based on the service-entrance conductor size, not on the main breaker trip rating.

Understanding the Fundamentals: GEC vs. EGC

Before diving into sizing tables, it’s crucial to distinguish between the two primary types of grounding conductors you’ll encounter. Confusing them is a common and dangerous mistake.

• Grounding Electrode Conductor (GEC): This is the conductor that connects the system grounded conductor (the neutral) and the equipment grounding conductor at the service to a grounding electrode (like a ground rod or Ufer ground). Its job is to connect the electrical system to the earth. Its size is determined by NEC Table 250.66.
• Equipment Grounding Conductor (EGC): This conductor runs with the circuit conductors and connects the non-current-carrying metal parts of equipment, raceways, and enclosures to the system’s grounded conductor at the service. The EGC provides a low-impedance path for fault current to flow, which facilitates the operation of an overcurrent protective device (OCPD). The correct equipment grounding conductor size is found in NEC Table 250.122.

Sizing the Grounding Electrode Conductor (GEC) for 100A, 125A, and 200A Services

The core of sizing GEC for a 200A panel—or any service—revolves around NEC Table 250.66. The most important rule to remember is that the GEC is sized based on the size of the largest ungrounded service entrance conductor, not strictly on the breaker amperage.

Step-by-Step Guide for Sizing GEC using NEC Table 250.66

Follow this process to accurately determine the GEC size for your service.

1. Identify the Service Entrance Conductors: Determine the size and material (copper or aluminum) of your ungrounded service entrance conductors. For example, a common 200A service application uses 2/0 AWG copper or 4/0 AWG aluminum.
2. Find the Equivalent Conductor Size: If you are using parallel conductors, add their circular-mil areas together to get a single equivalent size (Table 250.66 and its notes explain this requirement).
3. Consult NEC Table 250.66: Locate your service entrance conductor size or equivalent area in the table. The corresponding size listed for copper or for aluminum is the minimum required GEC.

Example Sizing:

• 100A Service: The required GEC depends on the size of the service conductors installed. In many common residential installations the resulting GEC is #8 AWG copper, but you must verify the actual service-entrance conductor size and use Table 250.66 to be certain.
• 200A Service: If the service-entrance conductors are 2/0 AWG copper (a common choice for 200 A), NEC Table 250.66 specifies a #4 AWG copper GEC; for equivalent aluminum sizes the table likewise provides the appropriate aluminum conductor size. This is the correct AWG for 200 amp ground when those service conductors are used.

Specific GEC Sizing for Certain Electrodes

The NEC provides special rules for certain electrodes that can affect your final conductor configuration:

• Ground Rod Wire Size: When the sole connection to a rod, pipe, or plate electrode is the only grounding electrode conductor run to that electrode, the conductor portion used solely for that connection is permitted to be smaller (for example, a #6 AWG copper or #4 AWG aluminum) per the specific exceptions in the code.
• Concrete-Encased Electrode (Ufer): The portion of the GEC used solely to connect to a concrete-encased electrode is permitted to be #4 AWG copper in many common applications. The code also explains allowable alternative connections for equipment with integral grounding bus or listed combinations.
• Water-Pipe Connections: A GEC connected to a metal underground water pipe or to interior metal water piping must follow the grounding conductor sizing rules in the NEC, and the methods of making these connections are important. For a detailed look at proper termination methods, you can review how grounding electrode conductor connections are handled in the 2023 NEC.

Sizing the Equipment Grounding Conductor (EGC) using NEC Table 250.122

Unlike the GEC, the EGC is sized based on the rating of the fuse or circuit breaker protecting that circuit. This is where NEC Table 250.122 applies for branch circuits and feeders.

Finding the Right Wire Size for a 125 Amp Subpanel

Let’s say you are installing a subpanel protected by a 125 A breaker in the main service panel. To determine the required EGC to run with your feeder conductors:

1. Consult NEC Table 250.122.
2. Find the rating of the overcurrent device protecting that feeder—the table is organized by overcurrent-device rating (use the exact rating shown or the next higher standard rating in the table).
3. The corresponding EGC size in the table is the minimum required (for many typical 125 A feeder protections this results in a #6 AWG copper or #4 AWG aluminum equipment grounding conductor).

Key Considerations for 200 Amp Service Grounding Requirements

Properly installing a 200-amp service involves more than just the GEC. Keep these critical points in mind to ensure a safe and compliant installation.

• Main Bonding Jumper Size: The main bonding jumper (MBJ) that connects the grounded conductor (neutral) to the equipment grounding conductor at the service is sized based on the size of the ungrounded service conductors using NEC Table 250.102 (see Article 250 guidance), not by the GEC table.
• Copper vs. Aluminum: NEC tables include both copper and aluminum (or copper-clad aluminum) sizes. If you choose aluminum, use the aluminum column sizes and follow proper termination practices and materials for aluminum conductors.
• Physical Protection: Grounding electrode conductors must be protected from physical damage. Note that a grounding electrode conductor of #6 AWG or larger may be run exposed where it is not subject to physical damage; if it is exposed to physical damage it must be protected in a suitable raceway (see NEC grounding conductor installation rules).
• Code Updates: The NEC is updated on a three-year cycle. Be aware of how recent changes might affect your installation; for a concise review of the 2023 updates affecting GEC sizing and related provisions you may find this ExpertCE lesson helpful.

Mastering these grounding and bonding rules is a hallmark of a professional electrician. Ensure all your installations are safely grounded with our NEC courses.

Sizing Conductors for Larger Services

The same principle applies for larger services: the GEC is determined from the largest ungrounded service-entrance conductor(s) using the exact ranges in NEC Table 250.66. For very large conductors (including parallel sets), use the table and its notes to determine the required GEC—do not assume a particular single gauge without consulting the table for your conductor size or parallel conductor equivalent area.

Frequently Asked Questions (FAQ)

What is the correct ground wire size for 200 amp service?

The most common ground wire size for 200 amp service (the Grounding Electrode Conductor) is #4 AWG copper or #2 AWG aluminum when the service-entrance conductors are 2/0 copper or 4/0 aluminum, respectively. Always confirm by checking the actual service-entrance conductor size and consulting NEC Table 250.66.

How do I determine the 100 amp ground wire size?

The 100 amp service ground wire size is determined by the size of the installed service entrance conductors and then selecting the GEC from NEC Table 250.66. In many common residential setups this results in a #8 AWG copper GEC, but verify for your particular conductor sizes.

Is grounding electrode conductor sizing different from equipment grounding conductor size?

Yes. The grounding electrode conductor sizing (GEC) is based on the largest ungrounded service-entrance conductors per NEC Table 250.66. The equipment grounding conductor size (EGC) is based on the rating of the overcurrent protective device (breaker or fuse) protecting the circuit per NEC Table 250.122.

Notes on sources checked

I cross-checked the requirements above with the NEC grounding and conductor sizing rules and installation guidance. The NEC text and tables are the authoritative references for sizing both grounding electrode conductors and equipment grounding conductors; for practical examples and classroom detail the ExpertCE lessons linked above are useful complements to the NEC text.