How to Size Grounding and Bonding Conductors per NEC Article 250?

Correctly sizing grounding and bonding conductors is a fundamental safety requirement governed by Article 250 of the National Electrical Code (NEC). These conductors provide a low-impedance path for fault current to return to the source, enabling overcurrent protection devices (OCPDs) like breakers and fuses to operate quickly and clear dangerous faults. Mis-sizing these conductors can lead to equipment damage, fire, and severe electric shock hazards.

Here are the core principles for sizing the most common grounding and bonding conductors:

• Main, System, & Supply-Side Bonding Jumpers: These critical components are sized based on the total circular mil area of the largest ungrounded service or source conductors. The primary reference is NEC Table 250.102(C)(1). It is a common mistake to size these based on the OCPD rating.
• Equipment Grounding Conductors (EGCs): The EGC, which runs with feeder and branch circuits, is sized based on the rating or setting of the OCPD protecting that circuit. The primary reference is NEC Table 250.122. If the ungrounded conductors are increased in size for voltage drop, the EGC must be proportionally increased as well.
• Grounding Electrode Conductors (GECs): This conductor connects the service equipment to the grounding electrode system (e.g., ground rods, concrete-encased electrode). It is sized based on the size of the ungrounded service-entrance conductors, using NEC Table 250.66.
• Specialized Bonding (Pools & Hot Tubs): For installations like hot tubs and pools, an equipotential bonding grid is required to minimize voltage gradients. As per NEC Article 680, this typically involves a No. 8 AWG solid copper conductor to bond all metal components and create the grid.

Conductor TypeSizing BasisPrimary NEC ReferenceMain/System/Supply-Side Bonding JumperSize of Ungrounded Source ConductorsTable 250.102(C)(1)Equipment Grounding Conductor (EGC)Rating of Overcurrent Protective DeviceTable 250.122Grounding Electrode Conductor (GEC)Size of Ungrounded Service ConductorsTable 250.66

Why Grounding and Bonding Sizing Matters

Grounding and bonding are arguably the most critical safety systems in any electrical installation. While ungrounded (hot) and grounded (neutral) conductors carry current during normal operation, the grounding and bonding system lies dormant, waiting to perform its life-saving function during a fault condition.

• Grounding is the act of connecting an electrical system to the earth. This helps stabilize the system voltage relative to ground and provides protection from lightning and other voltage surges.
• Bonding is the act of connecting all metallic parts of the electrical system (raceways, enclosures, equipment) that are not meant to carry current. This creates a continuous, low-impedance path for fault current to flow back to the source.

A properly sized and installed grounding and bonding system ensures that if a fault occurs (e.g., a hot wire touches a metal enclosure), a massive amount of current will flow, instantly tripping the circuit breaker or blowing the fuse. An undersized conductor can act like a fuse itself—melting and opening the fault path before the OCPD can operate, leaving the equipment energized and extremely dangerous.

Design professionals, plan reviewers, and field inspectors scrutinize these calculations during permitting and construction. Errors here are a common source of plan rejection and inspection failures, leading to costly project delays.

What does NEC article 250 cover?

NEC Article 250 provides the comprehensive requirements for grounding and bonding of electrical systems. Its fundamental purpose is to safeguard people and property from electrical hazards by creating a stable, safe electrical environment. It achieves this by detailing the rules for limiting voltages from lightning and surges and by providing a low-impedance path to clear ground faults.

The article is systematically organized into ten parts, each addressing a specific aspect of the grounding and bonding system:

• Part I: General: Scope and definitions.
• Part II: System Grounding: Requirements for grounding AC systems.
• Part III: Grounding Electrode System and Grounding Electrode Conductor: Rules for connecting the system to the earth, including sizing the GEC.
• Part IV: Enclosure, Raceway, and Service Cable Connections: How to bond service equipment.
• Part V: Bonding: General requirements for bonding at services and other locations.
• Part VI: Equipment Grounding and Equipment Grounding Conductors (EGCs): Rules for grounding non-current-carrying metal parts of equipment, including sizing the EGC.
• Part VII: Methods of Equipment Grounding: Acceptable types of EGCs (e.g., wires, conduit).
• Part VIII: Direct-Current Systems: Specific rules for DC grounding.
• Part IX & X: Rules for instruments, meters, and systems over 1000 volts.

Understanding the distinction between grounding (earth connection) and bonding (connecting metal parts together) is the key to correctly applying the rules within Article 250.

According to NEC Article 250, what are the correct methods for sizing the main bonding jumper, system bonding jumper, and supply-side bonding jumper for a 1200A, 480/277V separately derived system?

The main bonding jumper, system bonding jumper, and supply-side bonding jumper are sized based on the size of the ungrounded source conductors, not the overcurrent device rating. For a 1200A separately derived system, you must determine the total circular mil area of your parallel source conductors and use NEC 2023 Table 250.102(C)(1) to find the minimum required jumper size.

Let's walk through a typical example for a 1200A, 480/277V system.

1. Determine the Ungrounded Conductor Size: A 1200A service is typically installed with multiple parallel conductors per phase. A common configuration is three sets of 500 kcmil copper conductors per phase.
2. Calculate the Total Circular Mil Area:
   • One 500 kcmil conductor = 500,000 circular mils.
   • Total area per phase = 3 x 500,000 cmil = 1,500,000 cmil (or 1500 kcmil).
3. Apply NEC Table 250.102(C)(1): This table sizes the jumper based on the "Size of Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors."
   • Find the row for copper conductors that covers 1500 kcmil. This falls into the "Over 1100 through 1750" kcmil category.
4. Check Note 1 (The 12.5% Rule): This is a critical step that often overrides the table value. Note 1 to Table 250.102(C)(1) states: "If the ungrounded supply conductors are larger than 1100 kcmil copper... the bonding jumper shall have an area not less than 12.5 percent of the area of the largest ungrounded supply conductor or equivalent area for parallel supply conductors."
   • Calculation: 1,500,000 cmil x 0.125 = 187,500 cmil.
5. Select the Conductor: You must select a standard conductor with a circular mil area equal to or greater than 187,500 cmil.
   • Using NEC Chapter 9, Table 8:
     • 3/0 AWG Copper = 167,800 cmil (Too small)
     • 4/0 AWG Copper = 211,600 cmil (Correct)

Therefore, for a 1200A system using three parallel sets of 500 kcmil copper conductors, the minimum required size for the system bonding jumper (per NEC §250.28(D) and §250.30(A)(1)) is 4/0 AWG copper. The same methodology applies to main bonding jumpers at services and supply-side bonding jumpers.

I need to size an equipment grounding conductor for a 400A feeder protected by a 400A breaker. Using NEC Table 250.122, what is the minimum size copper EGC required?

The minimum size copper Equipment Grounding Conductor (EGC) for a circuit protected by a 400A breaker is 3 AWG copper. This is determined directly from NEC 2023 Table 250.122.

The EGC's function is to carry enough fault current to trip the OCPD, so its size is directly related to that device's rating.

How to Use NEC Table 250.122:

1. Identify the OCPD Rating: In this case, it is a 400A circuit breaker.
2. Locate the Rating in the Table: Find the row in the "Rating or Setting of Automatic Overcurrent Device..." column that corresponds to 400A.
3. Read the Required Conductor Size: Follow that row across to the "Size (AWG or kcmil)" column for the appropriate material (Copper or Aluminum).

Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)Size (AWG or kcmil)...Copper2006300440035002...

Important Considerations and Nuances:

• Proportional Sizing (NEC §250.122(B)): If the ungrounded phase conductors for this 400A feeder were increased in size to account for voltage drop, the EGC must also be increased in size proportionally. For example, if the phase conductors were increased from 350 kcmil to 500 kcmil (a ~43% increase in circular mil area), the EGC's circular mil area must also be increased by ~43%.
• Not Required to Be Larger (NEC §250.122(A)): The EGC is never required to be larger than the circuit conductors supplying the equipment.
• Parallel Conductors (NEC §250.122(F)): If the 400A feeder were run in two parallel conduits, a full-sized EGC (3 AWG Copper) would be required in each conduit.

I'm installing a hot tub on a wood deck. What are the specific NEC bonding requirements for the tub's metal components and the equipotential bonding grid? Does the deck structure itself require bonding?

For a hot tub on a wood deck, all accessible metal parts of the tub and associated electrical equipment must be bonded together with a minimum No. 8 AWG solid copper conductor. The wood deck structure itself is non-conductive and does not require bonding. A key factor is whether the hot tub is a listed, self-contained unit, which often waives the requirement for an extensive equipotential bonding grid on the deck surface.

The specific requirements are found in NEC 2023 Article 680, Part IV (Spas and Hot Tubs), which references the general bonding rules in Part II (Permanently Installed Pools).

Here is a breakdown of the requirements:

1. Component Bonding (NEC §680.43(D) & §680.26(B)):
   • All metal parts of the spa or hot tub structure.
   • Metal shells of underwater lighting fixtures.
   • Metal fittings within or attached to the spa structure (e.g., drains, jets).
   • Electrical equipment associated with the spa (pumps, heaters, blowers).
   • Metal raceways and enclosures.
   • The bonding conductor must be a minimum of 8 AWG solid copper. Stranded wire is not permitted for this application unless it is part of a listed assembly.
2. Equipotential Bonding Grid (Perimeter Surfaces - NEC §680.26(B)(2)):
   • The code requires an equipotential bonding grid to be installed on or under the surface around the hot tub, extending 3 feet out horizontally from the inside walls.
   • This grid is intended to prevent dangerous voltage differences between the water and the surrounding deck area.
   • CRITICAL EXCEPTION (NEC §680.42(B)): An equipotential bonding grid for perimeter surfaces is NOT required for an outdoor hot tub if it is a listed, self-contained spa or hot tub that is installed on or above the ground.
   • Real-World Application: The vast majority of modern residential hot tubs are listed self-contained units. Therefore, for a typical installation on a wood deck, you only need to perform the component bonding described in step 1 and do not need to install a wire grid on the deck. Always verify the hot tub's listing and the manufacturer's installation instructions.
3. Wood Deck Structure:
   • Wood is not a conductive material. The NEC bonding requirements apply only to conductive (metal) parts.
   • The wood framing, decking, and supports of the deck do not require bonding.

In summary, for a typical self-contained hot tub on a deck, the primary task is to run a No. 8 solid copper wire connecting the motor(s), heater, and any other accessible metal components back to the bonding lug on the tub's control pack.

Additional Considerations for Grounding & Bonding

Common Mistakes and Misinterpretations

• Using the Wrong Table: A frequent error is using Table 250.122 (EGCs) to size a main bonding jumper or a grounding electrode conductor. Always remember: EGCs are based on the OCPD (Table 250.122), while GECs and bonding jumpers are based on the size of the service/source conductors (Table 250.66 and Table 250.102(C)(1), respectively).
• Forgetting to Upsize the EGC: When ungrounded conductors are upsized for voltage drop, plan reviewers and inspectors will always check that the EGC was upsized proportionally per NEC §250.122(B).
• Misunderstanding the 12.5% Rule: For large services (over 1100 kcmil copper), the 12.5% rule for bonding jumpers is mandatory and results in a larger conductor than the one listed in the corresponding table row.
• Improper Hot Tub Bonding: Assuming a full equipotential grid is always needed. For most modern installations, the exception for listed, self-contained units simplifies the work significantly.

Jurisdictional Variations: State and Local Amendments

While the NEC provides the model code, many states and local jurisdictions adopt it with amendments. For example:

• The California Electrical Code (CEC) is based on the NEC but may have specific amendments related to seismic requirements or state-specific administrative rules.
• New York City and other dense urban areas often have local amendments that can affect conduit types, conductor sizing, or specific installation methods.
• Some jurisdictions may have stricter requirements for grounding in corrosive environments or may require a specific type of grounding electrode not mandated by the NEC.

It is absolutely essential for architects, engineers, and installers to consult the adopted code for the specific project location, not just the model NEC.

Coordination for Permitting and Inspection

Proper grounding and bonding requires coordination across disciplines:

• Architects: Must provide adequate space for service equipment, clearances around panels (per NEC §110.26), and pathways for grounding electrode conductors to reach the earth.
• Structural Engineers: Must identify available steel for use as a grounding electrode and coordinate connection points.
• Electrical Engineers: Are responsible for all sizing calculations, specifying conductor types, and showing the complete grounding and bonding system on drawings.
• Inspectors: Will visually verify every connection point, including the exothermic weld to building steel, connections at the ground bar, and bonding of all required equipment. An inaccessible or incorrect connection will fail an inspection.

Frequently Asked Questions (FAQ)

Can I use the metal frame of a building as an equipment grounding conductor?

‍No. While building steel can be used as a grounding electrode (part of the grounding electrode system), it cannot be used as an EGC to clear a fault for a feeder or branch circuit, as specified in NEC §250.121.

What's the difference between a main bonding jumper and a system bonding jumper?

‍A Main Bonding Jumper (MBJ) is used at a service disconnect to connect the grounded conductor (neutral) to the equipment grounding conductor bus. A System Bonding Jumper (SBJ) performs the same function but for a separately derived system, like a transformer. They are sized using the same method (NEC §250.102(C)).

Does the NEC require a specific color for grounding conductors?

‍Yes. NEC §250.119 requires equipment grounding conductors to be bare, or have a continuous green outer finish, or be green with one or more yellow stripes.

When do I need to install a supplemental grounding electrode?

‍If a single rod, pipe, or plate electrode has a resistance to earth of 25 ohms or more, NEC §250.53(A)(2) requires that it be supplemented by one additional electrode of any type specified in §250.52. The second electrode must be installed at least 6 feet away.

Is a separate EGC wire required in rigid metal conduit (RMC)?

‍No, not always. Per NEC §250.118(2), rigid metal conduit (RMC) is recognized as an effective equipment grounding conductor. However, many engineers specify a separate EGC wire as a best practice for reliability, and some jurisdictions or project specifications may require it.

Why is the bonding conductor for a hot tub solid and not stranded?

‍NEC §680.26(B) explicitly requires the bonding conductor to be solid, unless it is part of a listed assembly or is a stranded conductor with a terminal listed for direct burial. The primary reason is for mechanical strength, corrosion resistance, and ensuring a reliable, long-term connection in a wet and potentially corrosive environment.