Electrician ExamStudy Topic
Grounding and Bonding for the Electrician Exam
Study grounding and bonding for your electrician licensing exam. Covers NEC Article 250, GEC sizing, EGC sizing, main bonding jumpers, and common mistakes.
Topic Overview
Grounding and bonding are two related but distinct concepts covered primarily in NEC Article 250. Grounding refers to connecting electrical systems and equipment to the earth, creating a reference point for the system voltage and providing a path for fault current to clear overcurrent devices. Bonding refers to connecting conductive parts together to ensure they share the same electrical potential, reducing the risk of voltage differences that could cause a shock hazard or equipment damage.
The grounding electrode system (Article 250, Part III) defines what must be used as grounding electrodes, including metal underground water pipes, building steel, concrete-encased electrodes (Ufer grounds), ground rings, and driven ground rods. When multiple electrodes are available, they must be bonded together to form a single system. Ground rods must be at least 8 feet long and driven to full depth unless rocky soil prevents it, in which case they may be driven at an angle or laid in a trench.
The grounding electrode conductor (GEC) connects the grounded system conductor (usually the neutral) to the grounding electrode system. The GEC size is determined by the size of the largest service entrance conductors per NEC Table 250.66. The GEC must be continuous and may not be spliced except in specific permitted ways (such as irreversible compression connectors or listed splicing methods).
Equipment grounding conductors (EGC) provide the fault current path back to the source so overcurrent devices clear on a fault. They run with the circuit conductors. EGC sizing is based on the rating of the overcurrent device protecting the circuit, per NEC Table 250.122. EGCs may be a wire, a metal raceway, cable armor, or other listed means. A key exam point: an EGC must provide a low-impedance path capable of carrying fault current until the overcurrent device opens.
Bonding requirements are found throughout Article 250. Service equipment enclosures, metal raceways, metal water piping, and structural metal framing must all be bonded together and connected to the grounded system. The main bonding jumper (MBJ) connects the neutral (grounded conductor) to the equipment grounding path at the service, creating the only intentional connection between the grounded and grounding systems at the service. System bonding jumpers serve the same role at separately derived systems. This distinction is a frequent source of exam questions.
- Confusing bonding and grounding: grounding connects to earth, bonding connects conductive parts to each other to equalize potential.
- Sizing the grounding electrode conductor by the service ampere rating instead of using NEC Table 250.66 (which is based on service conductor size).
- Sizing the equipment grounding conductor by the load current instead of by the overcurrent device rating per NEC Table 250.122.
- Installing the main bonding jumper at a sub-panel instead of only at the service equipment, causing a neutral-to-ground connection at the wrong point.
- Forgetting that multiple grounding electrodes must be bonded together to form a single grounding electrode system.
- Assuming a ground rod alone is always sufficient; NEC requires a supplemental electrode if a single ground rod does not achieve the required resistance.
Checkpoint Quiz
Test your understanding of Grounding and Bonding
These questions are for study practice only and are not official exam questions.
1. What is the primary purpose of grounding an electrical system?
2. What is the difference between grounding and bonding?
3. Which conductor connects the grounded circuit conductor (neutral) to the grounding electrode at the service?
4. Which of the following is a recognized grounding electrode type?
5. In a residential installation, the equipment grounding conductor (EGC) in a branch circuit is normally identified by which color or marking?
6. A metal water piping system is being used as a grounding electrode. Where must the bonding connection to this electrode be made?
7. Bonding is required on metal gas piping in a building primarily to:
8. What is the function of the grounding electrode conductor (GEC)?
9. In a separately derived system (such as a transformer secondary), where is the system bonding jumper installed?
10. Why is the neutral conductor NOT bonded to the equipment ground bar in a subpanel (remote distribution panel)?
Frequently asked questions
What is the difference between grounding and bonding?
Grounding connects electrical systems and equipment to the earth, establishing a voltage reference and providing a fault current path. Bonding connects conductive parts together to equalize their electrical potential and prevent shock hazards from voltage differences between conductive surfaces.
What NEC article covers grounding and bonding?
NEC Article 250 covers grounding and bonding in detail. It addresses grounding electrode systems, grounding electrode conductors, equipment grounding conductors, bonding requirements, and main and system bonding jumpers.
How do I size a grounding electrode conductor?
The grounding electrode conductor (GEC) is sized based on the size of the service entrance conductors, using NEC Table 250.66. It is not sized by the service ampere rating directly. Table 250.66 gives the minimum GEC size for various service conductor sizes.
Where does the main bonding jumper go?
The main bonding jumper connects the grounded conductor (neutral) to the equipment grounding path inside the service equipment enclosure. It must be installed only at the service, not at downstream panels. Installing it in a sub-panel creates a hazardous parallel neutral-ground current path.
What is a concrete-encased electrode (Ufer ground)?
A concrete-encased electrode is a grounding electrode made from at least 20 feet of bare copper wire (No. 4 AWG or larger) or steel rebar (at least 1/2 inch diameter) encased in concrete in contact with the earth. It is one of the most effective grounding electrodes available and is required to be used when present.
