Ground Wire Size Calculator: How to Size a Grounding Conductor

A ground wire size calculator helps determine the minimum size of an equipment grounding conductor (EGC) for a US electrical circuit. For circuits covered by NEC Table 250.122 the EGC is primarily selected according to the rating or setting of the overcurrent protective device such as a circuit breaker or fuse. The conductor material and any required proportional increase must also be considered.

This guide explains how to determine ground wire size, how to read the NEC grounding wire size chart when the grounding conductor must be increased and why a 300 amp circuit does not automatically use the same grounding conductor as its phase conductors.

Key Takeaways

A ground wire size calculation is different from ordinary load conductor sizing.

Ground size is tied to overcurrent protection

For equipment grounding conductors covered by NEC Table 250.122 the breaker or fuse rating is the primary starting point rather than simply the circuit load or phase conductor ampacity.

Copper and aluminum have different sizes

The minimum EGC size varies according to whether the grounding conductor is copper or aluminum/copper clad aluminum.

Upsized circuit conductors can affect the EGC

When ungrounded conductors are increased in size under the conditions addressed by NEC 250.122(B) the wire type equipment grounding conductor must also be increased proportionally.

EGC and GEC are not the same thing

An equipment grounding conductor and a grounding electrode conductor perform different functions and are sized under different NEC provisions.

What Is a Ground Wire Size Calculator?

A ground wire size calculator is a tool used to determine an appropriate equipment grounding conductor size for an electrical circuit. In a typical US application, the calculation starts with the circuit's overcurrent protective device and the conductor material.

The applicable NEC requirements are then used to identify the minimum EGC size. The result is normally given in AWG or kcmil for US electrical work. A ground wire sizing calculator should therefore be treated differently from a general wire size calculator.

A general wire calculator may focus on load current, ampacity and voltage drop while an EGC calculator focuses on the requirements that apply to the grounding conductor.

How Does Ground Wire Sizing Work?

For equipment grounding conductors NEC Table 250.122 provides the basic minimum size based on the rating or setting of the circuit's overcurrent protective device. The basic sizing path is:

Overcurrent device rating → NEC Table 250.122 → conductor material → minimum EGC size

This table provides the starting point but the final EGC size can be affected by requirements such as proportional upsizing when the ungrounded circuit conductors are increased.

NEC Ground Wire Size Chart

The following simplified reference shows common equipment grounding conductor sizes from NEC Table 250.122.

These are reference values from the NEC Table 250.122 framework. The applicable NEC edition adopted by the authority having jurisdiction (AHJ) should always be verified before using the table for an actual installation.

How to Calculate Ground Wire Size?

Use the following process when determining the size of an equipment grounding conductor.

Step 1: Identify the Overcurrent Device

Start with the circuit breaker or fuse protecting the circuit. For Table 250.122 EGC sizing this rating is the key input. Do not simply use the load's running current or the ampacity printed for the ungrounded conductors.

Step 2: Identify the Grounding Conductor Material

Determine whether the equipment grounding conductor is copper or aluminum/copper clad aluminum. The NEC table provides different minimum sizes for these materials.

Step 3: Find the Applicable Table 250.122 Row

Locate the overcurrent device rating in Table 250.122 and read across to the applicable conductor material.

Step 4: Check Whether the Circuit Conductors Were Upsized

If the ungrounded conductors are larger than the minimum size for the circuit, determine whether NEC 250.122(B) requires the equipment grounding conductor to be increased proportionally.

This step is especially important on long circuits where the phase conductors may have been increased to reduce voltage drop.

Step 5: Check the Complete Installation

The final grounding conductor selection should be verified against the applicable NEC provisions, equipment requirements, conductor configuration and local code adoption. A calculator can simplify the sizing process but it should not replace the complete electrical design or code review.

What Size Ground Wire Is Required for a 300 Amp Circuit?

For a 300 amp overcurrent device NEC Table 250.122 gives a minimum equipment grounding conductor size of:

The important point is that this answer is based on the 300 amp overcurrent device, not simply the size of the load or phase conductors.

• 4 AWG copper
• 2 AWG aluminum or copper clad aluminum

What Happens When You Upsize the Circuit Conductors?

One of the most important parts of grounding wire sizing is NEC 250.122(B). If wire type ungrounded circuit conductors are increased in size beyond the minimum required size under the conditions addressed by the provision, the equipment grounding conductor must generally be increased proportionately based on circular mil area.

This means you should not automatically assume that the EGC remains at the Table 250.122 minimum simply because the breaker size has not changed.

How Do You Size the Grounding Cable?

Grounding cable sizing follows the applicable equipment grounding conductor requirements rather than simply matching the size of the ungrounded circuit conductors.

Start with the overcurrent device rating and conductor material then check the applicable Table 250.122 requirements. If the circuit conductors have been increased in size also check whether proportional EGC upsizing is required.

Parallel conductors, raceways and other installation arrangements may introduce additional requirements so the final selection should be verified against the applicable NEC provisions.

Ground Wire Size vs Circuit Wire Size

The equipment grounding conductor does not necessarily have to be the same size as the ungrounded circuit conductors. For example Table 250.122 may specify a smaller EGC than the phase conductors for a particular overcurrent device.

However this does not mean the grounding conductor can always remain smaller when the circuit conductors are deliberately increased. The proportional sizing requirement of 250.122(B) can change the result.

This distinction is important because the grounding conductor and the current carrying circuit conductors serve different purposes.

Copper vs Aluminum Grounding Wire

Copper and aluminum grounding conductors have different minimum sizes in Table 250.122. Copper is commonly selected where its physical properties, connection requirements and installation characteristics make it suitable. Aluminum can also be used where permitted and properly installed.

The table therefore provides separate columns rather than treating every grounding conductor material as equivalent. Always verify conductor compatibility, termination requirements and applicable installation restrictions before selecting aluminum or copper clad aluminum.

Equipment Grounding Conductor vs Grounding Electrode Conductor

The phrase ground wire can refer to different conductors which is why identifying the type of grounding conductor is essential.

Equipment Grounding Conductor (EGC)

The EGC provides a grounding path for equipment and normally runs with the circuit conductors. Its sizing is addressed by provisions including NEC 250.122.

Grounding Electrode Conductor (GEC)

A GEC connects the grounding electrode system to the electrical system at the applicable point. It is governed by different NEC provisions so its size cannot simply be taken from Table 250.122.

Why the Difference Matters

A search for grounding wire size does not automatically tell you which conductor you need to size. Before using a calculator determine whether you are sizing:

an equipment grounding conductor, a grounding electrode conductor or another grounding/bonding conductor.

What Information Does a Ground Wire Size Calculator Need?

A ground wire size calculator may require several inputs depending on the circuit and the calculation method being used. Typical inputs include the following:

Overcurrent device rating

Enter the breaker or fuse rating because it is the primary Table 250.122 input for an equipment grounding conductor.

Conductor material

Select copper or aluminum/copper clad aluminum because the applicable minimum EGC size can differ by material.

Circuit conductor size

The actual ungrounded conductor size may be needed when checking proportional EGC upsizing.

Minimum required conductor size

A proportional calculation may require comparison between the minimum required conductor size and the actual conductor installed.

Installation arrangement

Parallel conductors, multiple raceways and other configurations may require additional NEC checks beyond a basic table lookup.

Ground Wire Sizing for Long Runs

Long circuits may require larger ungrounded conductors to control voltage drop. When the circuit conductors are increased beyond the minimum required size check NEC 250.122(B) to determine whether the equipment grounding conductor must also be increased proportionally.

Our Wire & Cable Voltage Drop Calculator can help evaluate voltage loss when circuit length is a concern. However voltage drop sizing and equipment grounding conductor sizing are separate calculations and should be checked independently.

Common Ground Wire Sizing Mistakes

Sizing the Ground From Load Current Alone

The EGC should not automatically be selected by matching the load current to a general wire ampacity chart. For Table 250.122 applications the overcurrent device rating is the primary table input.

Assuming the Ground Must Match the Hot Wires

The EGC does not automatically have to be the same size as the ungrounded conductors. Its minimum size comes from the applicable grounding requirements with additional checks for conductor upsizing.

Ignoring 250.122(B)

Using the Table 250.122 minimum without checking whether the ungrounded conductors were upsized can result in an incorrect EGC selection. Check NEC 250.122(B) whenever the circuit conductors are increased.

Confusing EGC and GEC

An EGC and GEC are different conductors with different sizing requirements. Do not use Table 250.122 as a universal sizing chart for every grounding application.

Using an Old Ground Wire Size Chart

NEC editions can change and local jurisdictions may adopt different editions or amendments. For actual electrical work, verify the NEC edition adopted by the AHJ instead of relying on an old chart.

Ground Wire Size Calculator vs Wire Size Calculator

A general wire size calculator and a ground wire size calculator answer different questions. A general wire size calculator may evaluate:

A ground wire size calculator focuses on the requirements for the equipment grounding conductor including the applicable overcurrent protection and Table 250.122.

• Load current
• Conductor ampacity
• Wire material
• Temperature
• Installation conditions
• Voltage drop

When Should You Use a Ground Wire Size Calculator?

A grounding wire sizing calculator can be useful when:

For actual installation work the calculated result should still be checked against the adopted NEC edition, equipment requirements and local authority requirements.

• Designing a new branch circuit
• Checking a feeder
• Selecting a grounding conductor for a larger overcurrent device
• Reviewing a long circuit where conductors were upsized
• Comparing copper and aluminum grounding conductors
• Checking a preliminary conductor schedule
• Understanding the NEC Table 250.122 starting point

Conclusion

A ground wire size calculator helps determine the appropriate equipment grounding conductor size by applying the requirements that govern the circuit. For many US applications NEC Table 250.122 provides the starting point based on the rating or setting of the circuit's overcurrent protective device.

The process becomes more involved when ungrounded conductors have been increased in size because NEC 250.122(B) can require a proportional increase in the wire type equipment grounding conductor.

FAQs

How do I determine ground wire size?

For an equipment grounding conductor covered by NEC Table 250.122 start with the circuit's overcurrent protective device rating or setting and select the applicable copper or aluminum size. Then check for proportional upsizing and other applicable NEC requirements.

How do I size the grounding cable?

First identify whether you are sizing an equipment grounding conductor or another type of grounding conductor. For an EGC use the applicable overcurrent device rating and Table 250.122 then check conductor material, upsizing and installation requirements.

How do I calculate earthing cable size?

In US electrical work earthing cable is generally described using grounding terminology. If you mean an equipment grounding conductor Table 250.122 is a key sizing reference a grounding electrode conductor is sized under different NEC requirements.

What size ground wire is required for a 300 amp circuit?

For a 300 amp overcurrent device, Table 250.122 gives 4 AWG copper or 2 AWG aluminum/copper clad aluminum as the minimum equipment grounding conductor size. If the circuit conductors are increased in size under 250.122(B) the EGC may also need to be increased proportionally.

Is the ground wire the same size as the hot wires?

Not necessarily. The equipment grounding conductor has its own sizing requirements and may be smaller than the ungrounded circuit conductors. However conductor upsizing can trigger proportional EGC increases.

Does wire length affect ground wire size?

Circuit length does not directly determine EGC size in the same way it affects voltage drop. However a long run may require larger ungrounded conductors for voltage drop, which can then trigger proportional EGC upsizing under NEC 250.122(B).

Can I use a ground wire size chart instead of a calculator?

Yes a Table 250.122 reference chart can provide the minimum starting size for an equipment grounding conductor. A calculator can be more useful when proportional upsizing or other circuit details need to be considered.

What is the difference between a grounding conductor and a ground wire?

Ground wire is a general term that may refer to different grounding conductors. In US electrical work an equipment grounding conductor and a grounding electrode conductor have different functions and sizing requirements.

Does a 200 amp circuit need a 6 AWG ground?

Under the Table 250.122 starting values a 200 amp overcurrent device corresponds to 6 AWG copper or 4 AWG aluminum/copper clad aluminum for the equipment grounding conductor. Additional requirements can affect the final selection.