Grounding vs Bonding: The NEC 250 Question Most Electricians Get Wrong on Exam Day
If you're studying for a journeyman or master electrician exam, Article 250 is where you need to spend serious time. Grounding and bonding questions show up on every version of the exam, and the distinction between the two is one of the most frequently missed concepts on the test.
The problem isn't that the material is impossible — it's that the NEC uses "grounding" and "bonding" in very specific ways that don't match how most people use those words in the field. Getting the definitions straight, knowing which sections to look up, and understanding why the code separates these concepts is what separates passing from failing on this topic.
This guide covers the NEC 2023 requirements you're most likely to see on the exam, with exact section citations so you can tab your codebook accordingly.
What Is Grounding?
Grounding means connecting an electrical system or equipment to the earth. The NEC defines "grounded" in Article 100 as "connected to ground or to a conductive body that extends the ground connection."
The purpose of system grounding is spelled out in NEC 250.4(A)(1): electrical systems that are grounded shall be connected to earth in a manner that will:
- Limit voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines
- Stabilize the voltage to earth during normal operation
This is a critical distinction for the exam: grounding is about the earth connection. It's about voltage stabilization and limiting dangerous voltages from external events. Grounding does not provide the path that clears faults — that's bonding's job.
The physical connection to earth is made through the grounding electrode system (covered in NEC 250.50) via a grounding electrode conductor (sized per Table 250.66).
Key Exam Point
When the exam asks "what is the purpose of grounding an electrical system?" — the answer is voltage stabilization and limiting voltage from lightning and surges. It is not to clear ground faults. This trips up a lot of test takers.
What Is Bonding?
Bonding means connecting metallic parts together to establish electrical continuity and conductivity. Article 100 defines "bonding" as "connected to establish electrical continuity and conductivity."
NEC 250.4(A)(3) states that normally non-current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.
NEC 250.4(A)(5) drives the point home: electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device.
The general bonding requirement is stated in NEC 250.90: bonding shall be applied where necessary to ensure electrical continuity and the capacity to conduct safely any fault current likely to be imposed.
In plain terms: bonding creates the path that allows fault current to flow back to the source so the breaker or fuse can trip. Without proper bonding, a ground fault may not generate enough current to open the overcurrent device — leaving energized metal parts that can kill.
Grounding vs. Bonding: Side-by-Side Comparison
This table summarizes the key differences. Expect the exam to test your understanding of these distinctions:
| Grounding | Bonding | |
|---|---|---|
| NEC Definition | Connected to ground or a body that extends the ground connection (Article 100) | Connected to establish electrical continuity and conductivity (Article 100) |
| Primary Purpose | Limit voltage from lightning/surges; stabilize voltage to earth | Create low-impedance fault current path to trip overcurrent devices |
| Key NEC Section | 250.4(A)(1) | 250.4(A)(3) and 250.4(A)(5) |
| What It Connects To | Earth, via grounding electrodes | Metal parts to each other and to the supply source |
| Clears Faults? | No — earth is a poor conductor of fault current | Yes — provides the path that operates the overcurrent device |
| Physical Components | Grounding electrode conductor, grounding electrodes | Bonding jumpers, equipment grounding conductors |
| Sizing Reference | Table 250.66 (grounding electrode conductor) | Table 250.122 (EGC), 250.28(D) (main bonding jumper) |
The One-Sentence Exam Answer
Grounding connects the system to earth to stabilize voltage. Bonding connects metal parts together to provide a fault-clearing path. They work together but serve different purposes.
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Try Free Questions →High-Frequency Exam Topics in Article 250
These are the areas within Article 250 that appear most often on journeyman and master electrician exams. Tab these sections in your codebook.
Main Bonding Jumper (NEC 250.28)
The main bonding jumper connects the grounded conductor (neutral) to the equipment grounding conductor and the service enclosure at the service. It is the critical link that allows fault current to return to the source.
Sizing — Per NEC 250.28(D), wire-type main bonding jumpers are sized using Table 250.66. For service conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the jumper must be not less than 12.5% of the area of the largest phase conductor.
Exam questions on this topic typically give you a service conductor size and ask you to size the main bonding jumper using Table 250.66.
Grounding Electrode System (NEC 250.50 and 250.52)
NEC 250.50 requires that all grounding electrodes described in 250.52(A)(1) through (A)(7) that are present at a building or structure shall be bonded together to form the grounding electrode system.
The key electrodes listed in NEC 250.52(A) include:
- (A)(1) — Metal underground water pipe in direct contact with the earth for 10 feet or more (must be electrically continuous)
- (A)(2) — Metal in-ground support structures (building steel)
- (A)(3) — Concrete-encased electrode (often called a "Ufer ground") — minimum 20 feet of bare copper conductor not smaller than 4 AWG, or minimum 20 feet of steel reinforcing bar not less than 1/2 inch diameter, encased in at least 2 inches of concrete in direct contact with earth
- (A)(4) — Ground ring
- (A)(5) — Rod and pipe electrodes (minimum 8 feet in length for ground rods)
- (A)(7) — Plate electrodes
The grounding electrode conductor that connects the system to the grounding electrode system is sized per Table 250.66, based on the size of the largest ungrounded service conductor.
Equipment Grounding Conductors (NEC 250.118, 250.119, 250.122)
NEC 250.118 lists the types of conductors and raceways permitted to serve as equipment grounding conductors (EGCs). These include copper or aluminum conductors, rigid metal conduit, intermediate metal conduit, electrical metallic tubing, and others.
NEC 250.119 covers identification — EGCs are identified by a continuous green color, green with yellow stripes, or bare. Conductors 4 AWG and larger can be identified by marking at each end with green tape or paint at the time of installation.
Table 250.122 is the go-to reference for sizing EGCs. The EGC is sized based on the rating of the overcurrent device (breaker or fuse) ahead of the circuit — not based on the conductor size.
Common exam reference points from Table 250.122:
| OCPD Rating (Amps) | Copper EGC (AWG) | Aluminum EGC (AWG) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 200 | 6 | 4 |
NEC 250.122(B) — When ungrounded conductors are increased in size for voltage drop or any other reason, the equipment grounding conductor must be proportionally increased in size.
Bonding at Services (NEC 250.92)
NEC 250.92(B) lists the methods required for bonding at the service — this is a higher standard than bonding downstream. Standard locknuts and bushings are not sufficient for bonding at the service. Approved methods include bonding bushings, bonding-type locknuts, grounding wedges, and other listed devices.
This is a favorite exam question: asking whether a standard locknut is an acceptable bonding method at the service (answer: no).
Bonding of Metal Piping and Structural Steel (NEC 250.104)
NEC 250.104(A) requires the interior metal water piping system to be bonded. 250.104(B) covers bonding of other metal piping systems that may become energized. 250.104(C) addresses bonding of exposed structural metal that is interconnected to form a building frame.
How to Study Article 250 for the Exam
Article 250 is one of the longest articles in the NEC, and it's one of the most heavily tested. Here's how to approach it:
1. Learn the definitions first. Open Article 100 and read every definition related to grounding and bonding. These definitions are tested directly and are the key to understanding the rest of Article 250.
2. Understand the "why" in 250.4. Section 250.4 lays out the performance requirements — why we ground and why we bond. Once you understand the purpose behind each requirement, the specific rules in the rest of the article make more sense.
3. Tab Tables 250.66 and 250.122. You will look these up on the exam. Tab them now and practice using them with different conductor sizes and OCPD ratings.
4. Know 250.50 and 250.52. The grounding electrode system is tested heavily. Know which electrodes exist, when they're required, and that all electrodes present must be bonded together.
5. Practice timed lookups. On exam day, you'll have roughly 3 minutes per question. If an Article 250 question requires a table lookup, you should be able to find the answer in under 90 seconds.
Grounding and bonding questions appear on nearly every journeyman and master electrician exam. GetLicenseReady has 1,600+ NEC 2023-aligned practice questions in exam mode — 100 questions, timed. The first 25 are completely free at getlicenseready.com — no account or credit card required.
Conclusion
Grounding connects the electrical system to earth. Bonding connects metal parts to each other to clear faults. They're related but serve fundamentally different purposes — and the NEC treats them separately throughout Article 250.
For exam day: when you see a question about grounding or bonding, start by asking yourself — is this question about the earth connection, or about the fault current path? That single mental check will help you eliminate wrong answers fast.
Tab 250.4, Table 250.66, Table 250.122, and 250.50–250.52. Those are the pages you'll flip to most. Know them cold.
Frequently Asked Questions
What is the difference between grounding and bonding in the NEC?
Grounding connects the electrical system or equipment to the earth (NEC 250.4(A)(1)), primarily to limit voltage from lightning and surges. Bonding connects metallic parts together to create an electrically continuous path that can safely carry fault current and facilitate overcurrent device operation (NEC 250.4(A)(3)). Grounding is about the earth connection; bonding is about connecting metal parts to each other.
What NEC article covers grounding and bonding?
NEC Article 250 covers all grounding and bonding requirements. It is organized into 10 parts, from general requirements (Part I) through specific rules for grounding electrode systems (Part III), bonding (Part V), and equipment grounding conductors (Part VI). Article 100 contains the definitions of grounding, bonding, and related terms.
How is the main bonding jumper sized?
Per NEC 250.28(D), the main bonding jumper is sized using Table 250.66 when it is a wire-type jumper. For service conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the bonding jumper must be not less than 12.5% of the area of the largest phase conductor.
What are the required grounding electrodes in the NEC?
NEC 250.52(A) lists the permitted grounding electrodes, including metal underground water pipes (250.52(A)(1)), metal in-ground support structures (250.52(A)(2)), concrete-encased electrodes (250.52(A)(3)), ground rings (250.52(A)(4)), rod and pipe electrodes (250.52(A)(5)), and plate electrodes (250.52(A)(7)). NEC 250.50 requires all electrodes present at a building to be bonded together to form the grounding electrode system.
How do I size an equipment grounding conductor?
Equipment grounding conductors (EGCs) are sized using NEC Table 250.122, based on the rating of the overcurrent device ahead of the circuit. For example, a 20-amp circuit requires a minimum 12 AWG copper EGC. If the ungrounded conductors are increased in size for voltage drop, the EGC must be proportionally increased per NEC 250.122(B).
Related Guides
- NEC 314.16 Box Fill Calculations for the Electrician Exam — Another heavily-tested NEC calculation topic with five counting categories and worked examples
- GFCI and AFCI Requirements: NEC 210.8 and 210.12 Explained — Circuit protection requirements that pair directly with grounding and bonding knowledge
- California Electrician License Requirements — One of the first states to adopt NEC 2023, with active grounding and bonding enforcement
- Texas Electrician License Requirements — High-volume commercial and industrial market where Article 250 mastery is essential
- Compare Electrician License Requirements by State — See which states test NEC 2023 and their exam formats
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