Using NEC Table 250.102(C)(1) for Sizing Supply-Side Bonding Jumpers

Properly sizing supply-side bonding jumpers is a fundamental requirement of a safe electrical installation, and for this, electricians turn to the National Electrical Code (NEC). NEC Table 250.102(C)(1) is the definitive guide for these calculations, ensuring a low-impedance effective ground-fault current path. Sizing is based on the size of the ungrounded conductor or the equivalent area for parallel conductors. This table is crucial for any journeyman electrician or master electrician, as it governs the sizing for the grounded conductor, main bonding jumper, system bonding jumper, and supply-side bonding jumper. Understanding the difference between these components, such as the main bonding jumper vs. system bonding jumper, and their correct sizing is a core competency that prevents equipment damage and, more importantly, protects lives. This knowledge is a key part of any professional electrician training program and is essential for code compliance.

What is Bonding? The Foundation of Electrical Safety

Before diving into the specifics of the table, it’s vital to understand the core concept: bonding what is it? In simple terms, bonding is the act of connecting all non-current-carrying metallic components of an electrical system together. The goal, as outlined in NEC Article 250 requirements, is to create an uninterrupted, low-impedance path for fault current to travel back to its source. This path is known as the effective ground-fault current path. If a fault occurs, this path allows a massive amount of current to flow, which trips the overcurrent protective device (like a circuit breaker) and de-energizes the circuit. Without proper bonding, fault current could energize metal enclosures, raceways, and equipment, creating a severe shock hazard. Bonding ensures electrical continuity and conductivity among all these parts. It’s important not to confuse this with grounding. While closely related, grounding and bonding serve different primary purposes.

The Critical Role of the Supply-Side Bonding Jumper

A supply-side bonding jumper (SSBJ) is a conductor installed on the supply side of a service disconnect. Its job is to connect metallic parts—like conduits or enclosures—before the main breaker to ensure they are all effectively bonded together and to the grounded service conductor. This jumper is distinct from other components:

• Main Bonding Jumper (MBJ): A connection at the service disconnect that bonds the equipment grounding conductor bus to the grounded conductor (neutral) bus.
• System Bonding Jumper (SBJ): Performs a similar function to the MBJ but is used for separately derived systems, like a transformer.
• Equipment Grounding Conductor (EGC): A conductor that runs with the circuit conductors on the load side of the overcurrent device to bond non-current-carrying metal parts of equipment. EGCs are sized using NEC Table 250.122, not 250.102(C)(1).
• Grounding Electrode Conductor (GEC): This conductor connects the grounded service conductor, equipment, and service enclosure to the grounding electrode system (e.g., a grounding rod). A GEC is sized according to NEC Table 250.66 and is not intended to carry fault current, unlike a bonding jumper.

The SSBJ is critical because it is installed on the line side of service protection, where there is no overcurrent device to limit the magnitude of a potential fault. It must be robust enough to handle any fault current the utility can deliver.

How to Perform NEC 250.102(C)(1) Calculations

Sizing a supply-side bonding jumper using the table is a straightforward process that every electrician must master. The size of the jumper is directly related to the size of the ungrounded service entrance conductors. The larger the service conductors, the larger the potential fault current, and therefore, the larger the required bonding jumper.

1. Determine the Size of the Largest Ungrounded Conductor: Identify the size of the ungrounded conductors supplying the service. If the conductors are in parallel, you must calculate the total circular mil area per phase. For example, three parallel sets of 400 kcmil conductors have an equivalent area of 1,200 kcmil.
2. Reference NEC Table 250.102(C)(1): Locate the row in the table that corresponds to your ungrounded conductor size. The table has separate columns for copper and aluminum conductors.
3. Select the Minimum Bonding Jumper Size: The right-side columns of the table provide the minimum size required for the bonding jumper (and other conductors like the grounded conductor and main bonding jumper).

Example: A service is supplied by 500 kcmil copper ungrounded conductors.

• Step 1: The ungrounded conductor size is 500 kcmil copper.
• Step 2: In Table 250.102(C)(1), find the row “Over 350 through 600” for copper conductors.
• Step 3: The table indicates a minimum 1/0 AWG copper bonding jumper is required.

Beyond the Table: The 12.5 Percent Rule NEC

What happens when the service conductors are larger than what’s listed in the table? Note 1 to Table 250.102(C)(1) provides the answer with what is commonly known as the “12.5 percent rule NEC“.

If the total area of the ungrounded supply conductors exceeds 1100 kcmil for copper or 1750 kcmil for aluminum, the supply-side bonding jumper must have a cross-sectional area of at least 12.5% of the total area of the largest ungrounded supply conductors. This rule ensures that for very large services, the bonding jumper scales appropriately to handle massive fault currents.

Example Calculation: A service has parallel conductor installations with four sets of 600 kcmil copper conductors per phase.

• Total Area: 4 x 600 kcmil = 2400 kcmil.
• Check against Table Limit: 2400 kcmil is greater than 1100 kcmil, so the 12.5% rule applies.
• Apply the Rule: 2400 kcmil * 0.125 = 300 kcmil.
• Select Conductor: The minimum size required is 300 kcmil, which is a standard size according to Chapter 9, Table 8 of the nec code book.

Mastering these NEC 250.102(C)(1) calculations and understanding when to apply the 12.5% rule is a non-negotiable skill for passing a journeyman electrician or master electrician exam. Feeling confident with these calculations is key for any serious professional. Navigate the NEC tables like a pro. Check out our code book training and online electrical courses.

Important Considerations for Installation

Correctly sizing the conductor is only half the battle. Proper installation is just as critical for creating a reliable safety system. It is widely accepted in the industry, based on findings from organizations like the Electric Power Research Institute (EPRI), that approximately 80% of all power quality problems originate within a customer’s facility. This highlights the need for meticulous work on the customer’s side of the meter.

• Parallel Installations: Per NEC 250.102(C)(2), if you have parallel runs in multiple raceways, you have two options: install an individual bonding jumper in each raceway sized to the conductors in that specific raceway, or install a single common bonding jumper sized for the total equivalent area of the parallel conductors.
• Material and Method: Bonding jumpers must be made of copper or another corrosion-resistant material and installed using approved methods like listed pressure connectors or exothermic welding, as described in NEC 250.8.
• Distinguishing from Load-Side Bonding: Never confuse supply-side requirements with load-side bonding. Bonding jumpers on the load side of a breaker are sized differently, using Table 250.122.
• Grounded Conductor Sizing: Table 250.102(C)(1) is also used for grounded conductor sizing on the supply side, linking its capacity to the same potential fault currents as the bonding jumpers.

Primary Sources

• NFPA 70, National Electrical Code (NEC), 2023 Edition
• Schneider Electric, “Understanding the Sources of Power-Quality Disturbances,” 2016.

Frequently Asked Questions (FAQ)

What is the primary purpose of NEC Table 250.102(C)(1)?

The primary purpose of NEC Table 250.102(C)(1) is for sizing supply-side bonding jumpers, main bonding jumpers, system bonding jumpers, and grounded conductors for AC systems. Sizing is based on the size of the ungrounded service conductors to ensure the jumper can handle potential fault current.

When do I use the 12.5 percent rule NEC instead of Table 250.102(C)(1)?

You must use the 12.5 percent rule NEC when the size of your ungrounded conductors exceeds the maximums listed in the table—1100 kcmil for copper or 1750 kcmil for aluminum. For these large services, the bonding jumper must have an area of not less than 12.5% of the equivalent area of the ungrounded conductors.

Can I use Table 250.102(C)(1) to size an Equipment Grounding Conductor (EGC)?

No. An Equipment Grounding Conductor (EGC) is on the load side of an overcurrent device and must be sized according to NEC Table 250.122. Table 250.102(C)(1) is exclusively for conductors on the supply side, which must be capable of handling much larger fault currents because they are not protected by a service overcurrent device.

How are NEC 250.102(C)(1) calculations different for parallel conductor installations?

For parallel conductor installations, you have two options per 250.102(C)(2). You can either install one bonding jumper per raceway, sized based on the conductors in that raceway, or use a single common jumper sized based on the total equivalent area of all parallel conductors. If that total area exceeds the table’s limits, the 12.5% rule applies.