Lesson 40 – Grounding and Bonding Calculations

What Conductor Are You Sizing?

Several conductors can appear in the same service, feeder, or separately derived system. Their names can sound similar, but their functions and sizing rules are different.

Using 250.66

For an alternating-current grounding electrode conductor, the basic sizing method begins with NEC 250.66 and Table 250.66. The table relates the grounding electrode conductor size to the size of the largest ungrounded service-entrance conductor or the equivalent area of parallel ungrounded conductors.

For separately derived systems, the applicable Article 250 provisions use the derived ungrounded conductors as the basis for determining the required grounding electrode conductor size.

The Basic GEC Sizing Relationship

Table 250.66 is organized around the size of the largest ungrounded conductor or equivalent area of parallel conductors. As the ungrounded conductor size increases, the minimum grounding electrode conductor size also increases according to the table.

The GEC does not simply match the service conductor size. It is selected from the table after identifying the material and size of the largest applicable ungrounded conductor.

The 6 AWG Copper Limitation

Where the grounding electrode conductor or bonding jumper connects only to qualifying rod, pipe, or plate electrodes and does not continue to another type of electrode requiring a larger conductor, the NEC does not require the conductor to be larger than 6 AWG copper under the applicable 250.66 provision.

This limitation is frequently tested because a large service can produce a much larger Table 250.66 result, yet the conductor to the rod electrode itself can still have a lower maximum required size under the electrode-specific rule.

Concrete-Encased Electrode Sizing

Article 250 also limits the required size of the grounding electrode conductor or bonding jumper where it connects only to a qualifying concrete-encased electrode and does not extend to other electrodes requiring a larger conductor.

Under the applicable 250.66 provision, the conductor is not required to be larger than 4 AWG copper for that electrode connection. :contentReference[oaicite:1]{index=1}

Ground-Ring Connections

Ground-ring electrodes have a separate limitation under 250.66. Where the GEC or bonding jumper connects to a qualifying ground ring and does not extend to another electrode requiring a larger conductor, it is not required to be larger than the conductor used for the ground ring.

Again, this demonstrates why the electrode type must be identified after using the general Table 250.66 sizing method.

Equivalent Area for Parallel Conductors

When service-entrance conductors are installed in parallel, grounding electrode conductor sizing can require determining the equivalent circular-mil area of the parallel ungrounded conductors rather than treating a single conductor in one raceway as though it represented the complete service.

For examination problems, carefully identify how many parallel conductors of each phase are involved and use the applicable equivalent-area rule before entering Table 250.66.

Using 250.122

Wire-type equipment grounding conductors are generally sized using NEC 250.122 and Table 250.122. Unlike Table 250.66, which begins with the size of an ungrounded conductor, Table 250.122 generally begins with the rating or setting of the overcurrent protective device protecting the circuit.

This difference is one of the most important Article 250 distinctions to recognize on the exam.

Basic EGC Sizing Procedure

Example — Equipment Grounding Conductor

Suppose a circuit is protected by a 100-ampere overcurrent device and the question asks for the minimum wire-type copper equipment grounding conductor. The correct workflow is to identify the 100-ampere protective device and then use Table 250.122 to find the minimum conductor size.

The key examination point is not memorizing one value from the table. It is recognizing that the OCPD rating controls the initial table lookup.

When Ungrounded Conductors Are Increased in Size

Article 250 contains additional rules when ungrounded conductors are increased in size. Depending on the applicable NEC edition and reason for the increase, a wire-type equipment grounding conductor can also require adjustment.

Do not assume that the original Table 250.122 value automatically remains sufficient after substantially increasing the circuit conductors. Read the applicable 250.122 provision carefully and apply the rule for the edition used on the examination. :contentReference[oaicite:2]{index=2}

One EGC Serving Multiple Circuits

A single equipment grounding conductor can be permitted to serve multiple circuits installed together in the same raceway, cable, trench, or cable tray where the applicable requirements are met.

Under the general rule, that EGC is sized from Table 250.122 using the largest overcurrent protective device protecting the circuit conductors installed together. :contentReference[oaicite:3]{index=3}

Equipment Grounding Conductors for Motors

Motor circuits deserve special attention because the branch-circuit short-circuit and ground-fault protective device can be much larger than the motor running current. Article 250.122 contains specific motor provisions coordinated with Article 430.

For the general motor rule, the EGC is sized using the applicable branch-circuit short-circuit and ground-fault protective device as directed by 250.122. Special provisions apply to certain instantaneous-trip circuit breaker and motor short-circuit protector applications. :contentReference[oaicite:4]{index=4}

EGCs with Parallel Circuit Conductors

Circuits using parallel ungrounded conductors require careful application of the equipment grounding conductor rules. Article 250.122 contains provisions addressing equipment grounding conductors installed with parallel circuit conductors.

Do not simply divide one calculated EGC size among the raceways unless the applicable rule specifically permits the arrangement. Identify the wiring configuration and apply the parallel-conductor provision directly.

Sizing the Main Bonding Jumper

The main bonding jumper is not an equipment grounding conductor and should not automatically be sized from Table 250.122. For a service, Article 250 directs main bonding jumper sizing to the applicable bonding-jumper requirements associated with the size of the service conductors.

For examination questions, first confirm that the conductor being requested is actually the main bonding jumper and then follow the Article 250 cross-reference to the applicable sizing table or calculation.

Sizing Supply-Side Bonding Jumpers

Supply-side bonding jumpers provide electrical continuity and a fault-current path on the supply side of service equipment or in other specific installations. Their sizing generally relates to the associated ungrounded conductors rather than the downstream overcurrent-device method used for ordinary equipment grounding conductors.

Article 250.102 contains important bonding-jumper sizing requirements. Larger conductor arrangements and parallel conductors can require calculation using circular-mil area rather than a simple single-conductor lookup.

Using Circular-Mil Area

Some grounding and bonding calculations require working with conductor areas rather than simply reading an AWG size from a table. This commonly occurs when large ungrounded conductors or parallel conductor sets exceed the ranges handled by a direct table row.

The procedure generally requires determining the total circular-mil area of the applicable ungrounded conductors, applying the percentage specified by the applicable Code rule, and then selecting a conductor with at least the required area.

Finding Equivalent Area

If a phase consists of multiple parallel conductors, determine the area of one conductor and multiply by the number of conductors in parallel for that phase. Use that combined area when the applicable grounding or bonding rule calls for the equivalent area of parallel ungrounded conductors.

Do not add all three phase conductors together. The Code calculation is based on the applicable conductor or equivalent area for one phase or line, as directed by the specific rule.

Copper vs. Aluminum

Grounding and bonding tables can contain separate values for copper and aluminum or copper-clad aluminum conductors. Always identify the material of both the conductors used as the sizing basis and the grounding or bonding conductor being selected.

A common examination error is entering the correct row but reading the wrong material column.

Why GEC Sizing Can Produce Two Different Numbers

A grounding electrode conductor problem can produce an initial Table 250.66 size and then a smaller maximum-required size under an electrode-specific provision. Both numbers can be mathematically correct at different stages of the calculation.

The final answer depends on where the conductor terminates and whether it continues on to other grounding electrodes that require a larger conductor.

Grounding and Bonding Calculation Errors

Seven Steps for Grounding and Bonding Calculations

Use Article 250 as a Navigation System

Grounding and bonding problems reward Code navigation more than memorization. Learn where the major conductor-sizing rules are located and use the section cross-references to move from a conductor definition or installation requirement to the correct sizing provision.

Grounding and Bonding Calculations