Grounding Cable Size as Per NEC: Complete Sizing Guide, Tables and Examples

Choosing the correct grounding conductor is essential for electrical safety, fault clearing, and code compliance. The National Electrical Code (NEC) provides specific rules for equipment grounding conductors, grounding electrode conductors, and bonding conductors. These conductors do not always use the same sizing method.

This guide explains Grounding Cable Size as Per NEC, including NEC Table 250.122, Table 250.66, conductor upsizing, copper and aluminum options, and practical sizing examples.

NEC Grounding Cable Size Chart

For most branch circuits and feeders, the equipment grounding conductor (EGC) is selected from NEC Table 250.122. The table uses the rating of the overcurrent protective device rather than the ampacity of the circuit conductor.

Always verify the applicable NEC edition and installation conditions before final design. The table above is intended as a practical reference, not a substitute for the complete NEC requirements.

If you need a broader reference covering conductor selection, metric sizes, copper and aluminum conductors, use our earth cable size selection chart for quick comparison and project planning.

How NEC Determines the Equipment Grounding Conductor Size

The most important rule for an equipment grounding conductor is NEC 250.122. The required EGC size is generally determined by the rating of the circuit’s overcurrent protective device.

For example:

• A 20-amp breaker generally requires a minimum 12 AWG copper EGC.
• A 60-amp breaker generally requires a minimum 10 AWG copper EGC.
• A 100-amp breaker generally requires a minimum 8 AWG copper EGC.
• A 200-amp breaker generally requires a minimum 6 AWG copper EGC.
• A 400-amp breaker generally requires a minimum 3 AWG copper EGC.

This is why Grounding Cable Size as Per NEC should not be selected simply by matching the grounding conductor to the phase conductor.

The grounding conductor provides a low-impedance path for fault current. During a line-to-ground fault, sufficient current must flow through this path to operate the overcurrent protective device quickly.

Equipment Grounding Conductor vs Grounding Electrode Conductor

These two conductors are often confused, but they perform different functions.

The EGC is part of the fault-clearing path, while the GEC connects the electrical system to electrodes such as ground rods, concrete-encased electrodes, building steel, or a metal water pipe.

Grounding Electrode Conductor Sizing

Grounding electrode conductors are sized differently from equipment grounding conductors. NEC 250.66 generally bases the GEC size on the largest ungrounded service-entrance or feeder conductors.

A simplified copper GEC reference is shown below.

There are important exceptions and limitations depending on the type of grounding electrode.

For example, NEC 250.66(A) limits the grounding electrode conductor connected to a rod, pipe, or plate electrode to no larger than 6 AWG copper or 4 AWG aluminum when the electrode is the type covered by that provision.

Grounding Cable Size as Per NEC for a 100 Amp Circuit

Consider a 100-amp feeder protected by a 100-amp circuit breaker.

According to NEC Table 250.122, the minimum copper equipment grounding conductor is 8 AWG copper.

Therefore:

The phase conductor size does not automatically determine the EGC size. The overcurrent device rating is the starting point for selecting the equipment grounding conductor.

NEC 250.122(B) and Grounding Conductor Upsizing

One of the most important details in Grounding Cable Size as Per NEC calculations is conductor upsizing.

When ungrounded circuit conductors are increased in size from the minimum required size for reasons such as voltage drop, NEC 250.122(B) can require the equipment grounding conductor to be increased proportionally.

For example, assume a circuit requires 10 AWG copper phase conductors based on ampacity, but the conductors are increased to 6 AWG copper to reduce voltage drop.

You should not automatically keep the original EGC size without checking 250.122(B).

A proportional calculation can be made using the circular-mil area of the conductors:

EGC New Size = Original EGC Area × (New Ungrounded Conductor Area ÷ Original Ungrounded Conductor Area)

The resulting area is then converted to an available standard conductor size.

This requirement is especially important on long feeders, motors, solar installations, EV charging circuits, and other systems where voltage-drop considerations lead to larger phase conductors.

Grounding Cable Size as Per NEC for Motors

Motor circuits require additional attention because conductor sizing and overcurrent protection are not always based on the same value.

For a motor installation, the equipment grounding conductor is generally selected according to the rating of the branch-circuit short-circuit and ground-fault protective device, following NEC 250.122 and the applicable motor provisions.

For motor feeders, always check the complete requirements of NEC Article 430 together with Article 250.

Grounding Cable Size for Solar PV Systems

Solar photovoltaic systems can contain both DC and AC grounding requirements. Equipment grounding conductors may be required for PV modules, mounting structures, inverters, disconnects, raceways, and other exposed metal parts.

The correct conductor size depends on the circuit, overcurrent protection, installation method, and applicable NEC requirements, including Article 690.

For a solar installation, do not size every grounding conductor using one universal value. DC PV circuits, inverter output circuits, and grounding electrode connections may have different requirements.

For a detailed project calculation, our earthing conductor size guide can help compare conductor sizing methods and common grounding applications.

Copper vs Aluminum Grounding Conductors

Copper is widely used for grounding because it has excellent conductivity and is available in many conductor sizes. Aluminum conductors can also be used where permitted, but installation conditions and NEC requirements must be considered.

The conductor material should be selected along with the correct termination, environmental rating, and compatibility with equipment terminals.

Grounding Cable Size Calculation Example

Suppose a feeder is protected by a 200-amp breaker and uses copper conductors.

From NEC Table 250.122:

200 A → 6 AWG copper EGC

Now assume the phase conductors are increased substantially because of voltage drop. The EGC must be reviewed under NEC 250.122(B) to determine whether proportional upsizing is required.

The calculation should follow these steps:

1. Determine the circuit overcurrent protective device.
2. Find the minimum EGC from Table 250.122.
3. Determine the minimum required ungrounded conductor size.
4. Compare that size with the actual installed conductor.
5. If the phase conductors were increased, apply the proportional EGC rule.
6. Select the next standard conductor size where required.
7. Verify terminal, raceway, temperature, and installation requirements.

Use our earthing cable size calculation resource when you need a faster way to work through conductor sizing for different electrical systems.

Common Mistakes in Grounding Conductor Sizing

Several mistakes appear repeatedly in electrical installations.

Matching the EGC to the phase conductor

The EGC is not normally required to be the same size as the phase conductor. Table 250.122 provides its minimum size based on the overcurrent protective device.

Ignoring conductor upsizing

Increasing phase conductor size for voltage drop can trigger additional EGC sizing requirements under NEC 250.122(B).

Confusing GEC with EGC

A grounding electrode conductor and equipment grounding conductor have different purposes and sizing rules.

Using a generic grounding chart

A chart is useful for quick reference, but the complete NEC requirements should be checked for special installations.

Ignoring conductor material

Copper and aluminum have different minimum sizes and installation considerations.

Quick Grounding Conductor Selection Checklist

Before approving a grounding conductor size, verify:

• Circuit breaker or fuse rating
• Copper or aluminum conductor material
• Minimum phase conductor size
• Actual installed phase conductor size
• Voltage-drop conductor upsizing
• NEC Table 250.122 requirements
• NEC 250.122(B) proportional upsizing
• Grounding electrode type
• NEC 250.66 requirements for GEC
• Raceway and cable installation method
• Terminal and equipment listing requirements
• Applicable special rules for motors, PV systems, EV chargers, or other equipment

Frequently Asked Questions

Related Tools

• Earth Cable Size Calculator

Final Takeaway

Correct Grounding Cable Size as Per NEC selection depends on understanding which grounding conductor is being sized. For equipment grounding conductors, NEC Table 250.122 is the key starting point, while grounding electrode conductors are generally sized under Table 250.66.

The most commonly overlooked issue is conductor upsizing. When larger phase conductors are installed for voltage drop or other reasons, NEC 250.122(B) may require proportional EGC upsizing. Always verify the current NEC edition and applicable installation-specific requirements before finalizing an electrical design.

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