Bonding vs Grounding: What the 2023 NEC Actually Requires

About the Author: John Carter is a Master Electrician and a certified CE instructor with over 25 years of experience in commercial and industrial electrical systems. He specializes in demystifying NEC Articles for licensed professionals.

Answering First: Grounding vs. Bonding in the 2023 NEC

In the context of the NEC 2023, grounding and bonding are distinct concepts with different safety functions, both governed primarily by NEC Article 250. Grounding is the act of connecting an electrical system or equipment to the earth itself, typically via a Grounding Electrode System. Its primary purpose is to stabilize system voltage during normal operation and to protect against external voltage surges like lightning. Bonding, conversely, is the permanent joining of metallic parts to form an electrically conductive path. Its critical safety function is to create a low-impedance Effective Ground-Fault Current Path. This path ensures that a fault current is large enough to quickly operate an Overcurrent Protective Device (OCPD) like a circuit breaker or fuse, de-energizing the circuit and preventing electric shock or fire. While often confused, grounding connects to the earth for voltage stabilization, while bonding connects components together to facilitate fault clearing.

The Core Difference: Grounding for Stability, Bonding for Safety

As a professional electrician, you know that the terms “grounding” and “bonding” are not interchangeable. A misunderstanding here isn’t just a semantic error; it can lead to unsafe installations that fail inspection and, more importantly, fail to protect people and property. The National Electrical Code (NEC) is explicit in its definitions and requirements, and the 2023 edition continues to refine these critical safety concepts.

Think of it this way:

• Grounding is the system’s “anchor” to the earth. It gives the electrical system a common point of reference.
• Bonding is the “safety net” that electrically connects all metallic components that could become energized during a fault, ensuring they all rise to the same potential and providing a path for fault current to flow.

Understanding Grounding in the 2023 NEC

Grounding, as defined in NEC Article 250, is about connecting to the earth. The earth itself is a massive, electrically conductive body, making it the ultimate reference point. The primary purposes of grounding an electrical system are:

1. Limiting voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines.
2. Stabilizing the voltage to earth during normal operation.

Notice what’s missing from that list: clearing a fault. The earth has a relatively high impedance and cannot be relied upon to carry enough current to trip a standard OCPD in a timely manner. This is one of the most common and dangerous misconceptions in our field.

The Grounding Electrode System (GES)

The connection to the earth is accomplished via a Grounding Electrode System (GES), as required by NEC 250.50. This system consists of one or more grounding electrodes specified in 250.52(A) that are in direct contact with the earth. These can include concrete-encased electrodes, ground rings, rod and pipe electrodes, and plate electrodes. All electrodes present at a building or structure must be bonded together to form a single, unified GES.

The Grounding Electrode Conductor (GEC)

The Grounding Electrode Conductor (GEC) is the conductor used to connect the system’s grounded conductor or the equipment to the Grounding Electrode System. Sizing the GEC correctly is crucial and is determined by NEC 250.66, based on the size of the ungrounded service-entrance conductors. The rules for GECs can be particularly complex, especially with transformer installations. For a deeper analysis, review the 2023 NEC grounding electrode conductor rules for transformer installations to ensure your next project is compliant.

Understanding Bonding and the Effective Ground-Fault Current Path

If grounding doesn’t clear faults, what does? Bonding. The entire purpose of bonding is to create what the NEC calls an “Effective Ground-Fault Current Path” (NEC 250.2). This is an intentionally created, low-impedance path designed to carry fault current from the point of a fault back to the electrical source. This causes a massive amount of current to flow, which is then detected and interrupted by the OCPD.

This path consists of equipment grounding conductors, metallic raceways, enclosures, and other conductive materials. It is the most critical life-safety element of the grounding and bonding system.

Key Bonding Jumpers: MBJ, SSBJ, and System Bonding Jumper

Several specific jumpers are required to complete the bonding picture:

• Main Bonding Jumper (MBJ): As per NEC 250.28, this is the connection at the service equipment between the grounded conductor (neutral) and the equipment grounding conductor bus. It’s the critical link that connects the non-current-carrying metal parts of the system to the grounded source conductor, allowing fault current to return to the source.
• Supply-Side Bonding Jumper (SSBJ): Typically required by NEC 250.102(C), this jumper is used on the supply side of a service disconnect. It bonds conductive equipment and enclosures (like metal service raceways) to ensure the integrity of the ground-fault path on the line side of the main overcurrent device.
• System Bonding Jumper: This is the connection between the grounded conductor and the equipment grounding conductor at a separately derived system (like a transformer). It performs the same function as the MBJ but for a locally derived source.

The Role of the Equipment Grounding Conductor (EGC)

The Equipment Grounding Conductor (EGC) is the workhorse of the bonding system. It can be a wire, a metal raceway, or armor of a cable. Its job is to connect the non-current-carrying metal parts of equipment, raceways, and other enclosures to the system’s grounded conductor at the service or the source of a separately derived system. When dealing with older installations, you may encounter situations without a dedicated EGC. Understanding how to properly ground non-grounding switches and receptacles is a vital skill for retrofit and repair work.

Step-by-Step: Verifying an Effective Ground-Fault Current Path

To ensure safety, you must be able to visually trace and verify the integrity of the effective ground-fault current path. Here is a simplified process:

1. Start at the Load: Identify a piece of equipment, such as a motor or receptacle.
2. Trace the EGC: Follow the EGC (e.g., green wire, EMT conduit) from the equipment’s metal frame back to the panelboard that supplies it.
3. Inspect Panelboard Connections: At the panel, verify the EGC is properly terminated on the equipment grounding bus.
4. Locate the Bonding Jumper: Trace the path from the EGC bus to the grounded conductor (neutral) bus. In service equipment, this is the Main Bonding Jumper (MBJ). In a subpanel, this connection should NOT exist, preventing objectionable current.
5. Return to the Source: Follow the grounded conductor from the service disconnect back to the utility transformer (or the source of a separately derived system).
6. Confirm Integrity: Throughout this path, ensure all connections are tight, free of corrosion, and that raceways used as EGCs have proper fittings (e.g., bonding bushings where required by 250.97).

Understanding these fault paths is not just about code—it’s about life safety. A ground fault can create a catastrophic arc flash hazard, which is why comprehensive safety training is crucial. Stay compliant with NFPA 70E 2024 training on ExpertCE to protect yourself and your team from electrical hazards.

Key Takeaways for the Field

• Grounding Connects to Earth: Its purpose is voltage stabilization and lightning protection, NOT fault clearing.
• Bonding Connects Things Together: Its purpose is to create a low-impedance path to facilitate the operation of an OCPD during a fault.
• The Earth is a Poor Conductor: Never rely on the earth as the effective ground-fault current path. NEC 250.4(A)(5) is clear on this.
• The EGC is for Safety: The Equipment Grounding Conductor’s sole purpose is to carry fault current. It should never carry current under normal operating conditions.
• Bonding at the Service is Key: The Main Bonding Jumper (MBJ) is the critical link that ties the whole safety system together at the service disconnect.

Frequently Asked Questions about NEC 2023 Grounding and Bonding

1. What is the main difference between grounding and bonding in the 2023 NEC?

The main difference lies in their purpose. Grounding connects the electrical system to the earth via a grounding electrode to stabilize voltage and protect from lightning. Bonding connects all non-current-carrying metallic parts of an electrical system together to create an “effective ground-fault current path,” which allows an overcurrent device to open and clear a fault.

2. What is the purpose of the Effective Ground-Fault Current Path in NEC 250?

Its purpose is purely for safety. It provides a low-impedance, intentionally-created path for fault current to travel from the point of a ground fault back to the electrical source. This high-magnitude fault current ensures the overcurrent protective device (OCPD), such as a breaker, trips quickly to de-energize the circuit and prevent fire or shock hazards.

3. Where is the Main Bonding Jumper (MBJ) installed and what does it do?

The Main Bonding Jumper (MBJ) is installed in the service equipment. According to NEC 250.28, it provides the required electrical connection between the grounded service conductor (the neutral) and the equipment grounding conductor(s). This connection is what ultimately joins the EGC system to the source, completing the effective ground-fault current path.

4. Can the earth be used as an Equipment Grounding Conductor (EGC)?

No. NEC 250.4(A)(5) explicitly states that the earth shall not be used as an effective ground-fault current path. The earth’s impedance is too high to allow sufficient current to flow to reliably trip a standard OCPD. An EGC, like a wire or metallic raceway, must be used to provide a low-impedance path for fault current.

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