Locating the Main Bonding Jumper: NEC Article 250 Rules

The main bonding jumper is a critical safety component in an electrical service, responsible for creating a reliable, low-impedance path for fault current to travel back to its source. According to the National Electrical Code (NEC), this connection is made between the grounded conductor (typically the neutral) and the equipment grounding conductor (EGC) system at the service equipment. Proper location and sizing are not just best practices; they are essential for ensuring that overcurrent protective devices operate swiftly during a ground fault. The primary purpose of the main bonding jumper is to establish an effective ground-fault current path, a foundational concept in electrical safety that protects both people and property. This connection must only be made at the service disconnect and nowhere else on the load side to prevent dangerous objectionable current from flowing on non-current-carrying metal parts. Correct sizing, determined by NEC Table 250.102(C)(1), is crucial for its ability to handle immense fault currents without failing.

What is a Main Bonding Jumper? Unpacking NEC Article 250

In any electrical system, understanding the concept of “bonding what is it” is fundamental for every journeyman electrician and master electrician. Bonding is the permanent joining of metallic parts to form an electrically conductive path. The main bonding jumper is a specific and vital application of this principle. Defined in NEC Article 100, it is the connection between the grounded conductor and the equipment grounding conductor (EGC) at the service. Its sole purpose is to complete the circuit during a ground fault—an unintentional connection between an ungrounded (hot) conductor and metallic equipment or enclosures. By connecting the EGC system back to the grounded service conductor, it creates a dedicated, low-impedance path for fault current to return to the utility transformer, causing the circuit breaker or fuse to open almost instantaneously. Without this jumper, fault current would seek other, less reliable paths, such as through the earth via a grounding rod, which has too high an impedance to allow enough current to flow to trip a standard overcurrent device. This critical link ensures the entire safety system functions as intended. For a deeper dive into these core principles, explore the differences between grounding vs. bonding as outlined in NEC 250.

The Critical Location: Where to Install the Main Bonding Jumper

The rules for locating the main bonding jumper are strict and clear: it must be installed only within the service equipment enclosure, typically at the main breaker panel that serves as the service disconnect. Per NEC 250.24(A)(5), grounding connections are prohibited on the load side of the service disconnecting means. This “one-point” rule is crucial to prevent objectionable current from flowing on the equipment grounding conductor and other non-current-carrying metal parts under normal operating conditions. When the neutral is bonded to the ground in a subpanel downstream from the service, it creates a parallel path for neutral current to return to the source. This means EGCs, conduits, and equipment enclosures can become energized with normal load current, creating a shock hazard and potential fire risk. Manufacturers typically provide a green bonding screw or a bonding strap with service-rated panels to facilitate this connection, and it is the installer’s responsibility to ensure it is correctly installed at the service and left out of any subpanels.

Main Bonding Jumper vs. System and Supply-Side Bonding Jumpers

A common point of confusion for electricians undergoing electrician training is distinguishing between the main bonding jumper, system bonding jumper, and supply-side bonding jumper. While they serve similar functions, their applications are distinctly different.

• Main Bonding Jumper (MBJ): As discussed, this is used only at the service entrance to connect the grounded conductor to the EGC system.
• System Bonding Jumper (SBJ): This jumper is used for a separately derived system, such as a generator or a transformer. It performs the same function as the MBJ—connecting the system’s grounded conductor to the equipment grounding conductors—but it does so at the source of the separately derived system, not at the building’s main service. This establishes a new ground reference for that system.
• Supply-Side Bonding Jumper (SSBJ): This conductor ensures electrical conductivity between metallic parts and enclosures on the supply side of the service disconnect. For example, if metal raceways are used to carry service entrance conductors, a supply-side bonding jumper ensures they are all effectively bonded together and connected to the grounding system.

Understanding these distinctions is critical for code compliance. To learn more, see our detailed comparison of the system bonding jumper vs. the supply-side bonding jumper.

How to Correctly Size Your Main Bonding Jumper

Properly sizing conductors, especially the main bonding jumper, is essential for safety. The jumper must be large enough to withstand the massive available fault current without vaporizing before the overcurrent device can operate. The process is straightforward and detailed in the NEC code book.

1. Determine the Size of Service Conductors: Identify the size of the largest ungrounded service entrance conductor(s) supplying the equipment. If using parallel conductors, calculate the equivalent total circular mil area per phase.
2. Reference NEC Table 250.102(C)(1): This table, “Grounded Conductor, Main Bonding Jumper, System Bonding Jumper, and Supply-Side Bonding Jumper for Alternating-Current Systems,” is the primary tool for sizing.
3. Select the Jumper Size: Find the row corresponding to your ungrounded conductor size in the left column of the table. The right column will specify the minimum required size for your main bonding jumper (in AWG or kcmil) for both copper and aluminum.
4. Apply the 12.5% Rule (if necessary): For very large services where the ungrounded conductors exceed 1100 kcmil (copper) or 1750 kcmil (aluminum), Note 1 of the table requires the bonding jumper to have a cross-sectional area of at least 12.5% of the total area of the largest ungrounded phase conductors.
5. Verify Material and Construction: According to NEC 250.28, the main bonding jumper can be a wire, busbar, screw, or similar suitable conductor and must be made of copper or another corrosion-resistant material. It must also be unspliced.

This sizing is different from sizing a Grounding Electrode Conductor (GEC), which is covered in a different table. You can review how to size a GEC using NEC Table 250.66 for comparison.

The Goal: Ensuring an Effective Ground-Fault Current Path

The ultimate goal of the main bonding jumper, and indeed the entire grounding and bonding system, is to create an effective ground-fault current path. The NEC defines this as an intentionally constructed, permanent, and low-impedance conductive path designed to carry current from the point of a ground fault back to the electrical supply source. This path must be able to safely conduct the massive amount of current available during a fault to facilitate the operation of the overcurrent protective device. The main bonding jumper is the lynchpin of this system. It is the bridge that connects the non-current-carrying metallic parts of an installation (via the EGC) to the grounded service conductor, which provides the direct line back to the source transformer. This completes the circuit, ensuring a fault is cleared in fractions of a second, protecting personnel from shock and equipment from severe damage.

Properly installing the main bonding jumper is not just about following rules; it’s about safeguarding lives. Understand the heart of the grounding system. Explore our NEC Article 250 training and our library of online electrical courses to master these critical safety concepts.

Related Resources

• Grounding vs. Bonding: NEC Article 250 Explained

Frequently Asked Questions (FAQ)

What is the primary purpose of the main bonding jumper in a breaker panel?

The primary purpose of the main bonding jumper is to connect the equipment grounding conductor (EGC) bus and the service enclosure to the grounded (neutral) conductor in the main service equipment. This creates an effective ground-fault current path, which is a low-impedance route for fault current to return to the source, ensuring the overcurrent device (breaker or fuse) trips quickly.

Can I install a main bonding jumper in a subpanel?

No. A main bonding jumper must only be installed at the service disconnect. Installing one in a subpanel would connect the neutral and ground conductors on the load side of the service, creating parallel paths for neutral current to flow on the EGCs. This is a code violation known as objectionable current and can create serious shock hazards.

How is a main bonding jumper different from a system bonding jumper?

A main bonding jumper is used at the service entrance of a building. A system bonding jumper serves the same function but is used for a separately derived system, such as a backup generator or transformer. The main bonding jumper connects the utility-derived neutral to ground, while the system bonding jumper connects the generator- or transformer-derived neutral to ground at the new source.

What NEC table do I use for sizing a main bonding jumper?

You must use NEC Table 250.102(C)(1) for sizing the main bonding jumper. The size is based on the size of the largest ungrounded service entrance conductors. This table is also used for sizing the grounded conductor, system bonding jumper, and supply-side bonding jumper.