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 (e.g., 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 wire, not the 200A breaker rating. For a 100A service, the GEC is usually #8 AWG copper, and for a 125A service, it is often #6 AWG copper. The final 200 amp service ground wire size depends on the material (copper vs. aluminum) and the size of the service entrance conductors you’ve installed, making a solid understanding of the National Electrical Code (NEC) tables essential for compliance and safety.

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 rating, not the amperage of the main breaker.

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 200A service often uses 2/0 AWG copper or 4/0 AWG aluminum.
2. Find the Equivalent Conductor Size: If you are using parallel conductors, you must add their circular mil areas together to get a single equivalent size per NEC 250.66, Note 1.
3. Consult NEC Table 250.66: Locate your service entrance conductor size in the left column of the table. The corresponding size in the right column is your required GEC.

Example Sizing:

• 100A Service: Typically uses #2 AWG copper service conductors. Per NEC Table 250.66, the required 100 amp ground wire size (GEC) is #8 AWG copper. It’s important to note a search for “#4 copper wire for 100 amp service” usually refers to the service entrance conductors themselves, not the GEC.
• 200A Service: Typically uses 2/0 AWG copper or 4/0 AWG aluminum conductors. Per NEC Table 250.66, the required 200 amp ground wire size is #4 AWG copper or #2 AWG aluminum. This is the correct AWG for 200 amp ground.

Specific GEC Sizing for Different Electrodes

The NEC provides special rules for certain electrodes that can affect your GEC sizing:

• Ground Rod Wire Size: Per NEC 250.66(A), the portion of the GEC that is the sole connection to a ground rod does not have to be larger than #6 AWG copper or #4 AWG aluminum.
• Ufer Ground Wire Size: The GEC connected to a concrete-encased electrode (Ufer ground) is not required to be larger than #4 AWG copper, per NEC 250.66(B).
• Water Pipe Grounding Conductor: When connecting to a water pipe, the GEC must be sized according to Table 250.66 without exception. How these connections are made is just as important as their size. 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 comes into play. You will use this table for branch circuits and for feeders, such as one supplying a subpanel.

Finding the Right Wire Size for 125 Amp Sub Panel

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

1. Go to NEC Table 250.122.
2. Find the rating of your overcurrent device in the left column (“125A” falls under the “200A” row).
3. The corresponding EGC size in the right columns is #6 AWG copper or #4 AWG aluminum. This would be the minimum wire size for 125 amp sub panel grounding.

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) connects the equipment grounding conductor bus to the grounded conductor (neutral) bus inside the main service disconnect. It is sized based on the size of the ungrounded service conductors using NEC Table 250.102(C)(1), not Table 250.66.
• Copper vs. Aluminum Grounding Conductor: Both NEC tables 250.66 and 250.122 provide sizes for both copper and aluminum conductors. While aluminum is often more cost-effective, ensure you use the correct size and proper anti-oxidant compound at terminations.
• Physical Protection: Grounding conductors must be protected from physical damage. If you run a GEC of #6 AWG, it must be protected in a raceway. GECs larger than #6 AWG do not require protection unless subject to physical damage.
• Code Updates: The NEC is updated every three years. Be aware of how recent changes might affect your installation. Our course on grounding electrode conductor sizing rules changed in the 2023 NEC is a great resource for staying current.

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: A Look at 400A

The principles for sizing ground wires remain the same for larger services. For a 400A service, you first need to determine the 400 amp service wire size. This is often 500 kcmil copper or parallel runs of smaller conductors. When asking what size wire for a 400 amp service GEC, you again consult NEC Table 250.66.

For service entrance conductors over 350 kcmil up to 600 kcmil copper, the required GEC is #1/0 AWG copper. If your conductors are larger, up to 1100 kcmil, the GEC must be #2/0 AWG copper. It’s a common misconception that you could use a `4/0 service ground wire size` for grounding, but as you can see from the table, the required size is determined by the service entrance conductors, not an arbitrary large wire gauge.

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. This is based on the typical service entrance conductor size (e.g., 2/0 copper or 4/0 aluminum) and is found by using 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 service entrance conductors, not the 100A breaker. If the service uses #2 AWG copper conductors, you would consult NEC Table 250.66, which specifies a #8 AWG copper Grounding Electrode Conductor. The correct keyword is 100 amp ground wire size.

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

Yes, they are sized very differently. The grounding electrode conductor sizing (GEC) is based on the size of the service-entrance conductors (per NEC Table 250.66). The equipment grounding conductor size (EGC) is based on the rating of the overcurrent device (breaker or fuse) protecting the circuit (per NEC Table 250.122).

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