Article 250 structure and key definitions

On a master exam, grounding questions are almost never hard because the rule is obscure. They are hard because Article 250 is long, its parts look alike, and its defined terms sound alike. Dana Whitfield timed herself finding twenty grounding rules and averaged two minutes and forty seconds per lookup, nearly double her time for other articles. Most of the delay came from starting in the wrong part of the article, or from confusing a grounding electrode conductor with an equipment grounding conductor and following the wrong table. This lesson gives you the map of Article 250 and the vocabulary to use it quickly.

As always, the lesson describes the structure and the meaning of each term in our own words. It does not reproduce the code's definitions or text. Open your own copy of the 2023 NEC to Article 250 and to the definitions in Article 100, and follow along. If your state has adopted the 2026 edition, the structure is largely the same, but the parts covering systems over 1,000 volts have moved out of Article 250 into the new Article 270, and some wording in the general requirements has changed.

The parts of Article 250

Article 250 is divided into parts, each with a theme. Learning the theme of each part is the fastest way to cut your lookup time in half, because once you know a question is about bonding, you go straight to Part V and skip the hundred sections that cannot hold the answer. Here are the parts of the 2023 edition and what each one covers, in plain words.

Three parts carry most of the exam weight. Part II answers questions about where and how the system's grounded conductor connects to ground, including services, transformers and separate buildings. Part III answers questions about electrodes and the conductor that runs to them. Part VI answers questions about equipment grounding conductors and what they must be sized at. Part V, on bonding, comes a close fourth. If you tab only four places in Article 250, tab 250.24, 250.52, 250.66 and 250.122, then add 250.30, 250.32, 250.102 and 250.104 as you go.

The conductors that sound alike

The single largest source of grounding mistakes is confusing the conductors. Their names share the same words in different orders, and each one is sized by a different table. Learn each by its job and its two ends.

The grounded conductor is a circuit conductor that is intentionally grounded. In most systems it is the neutral, the white or gray conductor that carries the unbalanced current of line-to-neutral loads. It is a current-carrying conductor in normal operation. It connects to ground at one place, the service, through the main bonding jumper, or at a separately derived system through the system bonding jumper. It is sized like any other circuit conductor, from the load, with the neutral rules of 220.61 and the minimum sizes in 250.24(C) for the service.

The equipment grounding conductor is the conductive path that connects the normally non-current-carrying metal parts of equipment together and to the system grounded conductor, the grounding electrode conductor, or both. In normal operation it carries no current. During a ground fault it carries the fault current back to the source. Its ends are the equipment at one end and the service or the separately derived system's bonding point at the other. It can be a wire, green, green with yellow stripes, or bare, or it can be a qualifying metal raceway or cable armor listed in 250.118. It is sized by Table 250.122 from the rating of the overcurrent device ahead of the circuit.

The grounding electrode conductor is the conductor that connects the system grounded conductor, or the equipment, to a grounding electrode or to a point on the grounding electrode system. Its ends are the service or separately derived system at one end and the electrode at the other. It is sized by Table 250.66 from the size of the largest ungrounded service or derived conductor, with special limits for rods, concrete-encased electrodes and ground rings.

The bonding jumpers

The bonding jumpers are short conductors with specific jobs, and the exam tests whether you can tell them apart. The main bonding jumper connects the grounded conductor and the equipment grounding conductor at the service. The system bonding jumper does the same thing at a separately derived system, such as a transformer, either at the source or at the first disconnecting means. The supply-side bonding jumper is a conductor on the supply side of a service or of a separately derived system's overcurrent device that keeps metal parts electrically connected, such as the conductor that bonds a transformer enclosure to the first disconnect enclosure. A load-side bonding jumper is the equivalent on the load side of an overcurrent device. And the equipment bonding jumper connects two or more parts of the equipment grounding conductor, such as the jumper around a flexible section of raceway.

Each has a sizing rule. Main and system bonding jumpers and supply-side bonding jumpers are sized from Table 250.102(C)(1), based on the largest ungrounded conductor they serve, with a 12.5% rule for very large services. Load-side bonding jumpers are sized from Table 250.122, based on the overcurrent device. You will practice these in Lessons 3, 6 and 8.

Other terms you must know

Several other defined terms appear in grounding questions. A grounding electrode is a conducting object through which a direct connection to earth is established, such as a rod, a concrete-encased electrode or a metal water pipe in contact with the earth. The grounding electrode system is all the electrodes present at a building bonded together. A separately derived system is an electrical source, other than a service, that has no direct connection from its circuit conductors to the supply conductors of another system, except through grounding and bonding connections. Most transformers that serve premises wiring are separately derived, and so are many generators that switch the neutral.

The intersystem bonding termination is a device that provides a means for connecting bonding conductors for communications systems to the grounding electrode system. The ground-fault current path is any electrically conductive path from the point of a ground fault through normally non-current-carrying conductors, equipment or the earth to the source. Its sibling, the effective ground-fault current path, is the intentionally constructed, low-impedance path you met in Lesson 1. Mark the difference: the ground-fault current path can include the earth, while the effective path cannot.

Finally, two adjectives matter. Solidly grounded means connected to ground without inserting any resistor or impedance device. Impedance grounded, the subject of Lesson 10, means connected through a resistor or reactor. Most premises systems are solidly grounded. High-impedance grounded neutral systems under 250.36 are the main exception on exams.

Section numbering habits

Article 250 follows the code's usual numbering, with sections numbered in steps that leave room for new material, and lettered and numbered subdivisions inside them. A reference such as 250.30(A)(1) means Article 250, section 30, subdivision (A), list item (1). Exam questions sometimes give only a section number and ask what it covers, so knowing the neighborhoods helps: the 20s are system grounding at services and other systems, the 50s and 60s are electrodes and their conductors, the 90s and 100s are bonding, the 110s and 120s are equipment grounding and its conductors, and the 130s and 140s are methods. Learning the neighborhoods is faster than learning every section.

That two-step routine, deciding the part and then the table, is the habit this course builds. In the worked example, a candidate who saw the word transformer and flipped to Article 450 would waste three minutes. A candidate who saw a grounding electrode conductor but sized it by the transformer's primary overcurrent device, using Table 250.122, would get a wrong answer that is very likely among the choices.

A lookup drill you can edit

The listing below is a tiny lookup helper that maps the conductor named in a question to the table that sizes it. It is not a substitute for the code, but writing it, or reading it, reinforces the pattern. Add the jumpers you meet in later lessons.

SIZED_BY = {
    "grounding electrode conductor": "Table 250.66 (largest ungrounded service or derived conductor)",
    "equipment grounding conductor": "Table 250.122 (rating of the overcurrent device)",
    "main bonding jumper": "Table 250.102(C)(1) through 250.28(D)",
    "system bonding jumper": "Table 250.102(C)(1) through 250.30",
    "supply-side bonding jumper": "Table 250.102(C)(1)",
    "load-side bonding jumper": "Table 250.122",
}

for name in ["equipment grounding conductor", "main bonding jumper", "grounding electrode conductor"]:
    print(f"{name}: {SIZED_BY[name]}")

equipment grounding conductor: Table 250.122 (rating of the overcurrent device)
main bonding jumper: Table 250.102(C)(1) through 250.28(D)
grounding electrode conductor: Table 250.66 (largest ungrounded service or derived conductor)

How the exam uses definitions

Definition questions on master exams usually take one of two forms. Some ask directly what a term means or which conductor fits a description, such as the conductor that connects the system to the grounding electrode. Others embed a definition in a scenario: a generator whose transfer switch does not switch the neutral is not a separately derived system, so it does not get a system bonding jumper at the generator. To answer the second type, you must know not just the name of each term but the facts that make something fit it.

Priya Natarajan, Brightwater's estimator, helped Dana build flash cards with the job on the front and the term on the back, rather than the other way round. Within a week Dana could name the conductor from its job in under three seconds, and her grounding lookup time fell to about ninety seconds. The habit pays off again in Lesson 7, where the exam asks you to size an equipment grounding conductor and offers a grounding electrode conductor size as the most attractive wrong answer.