NEC Article 250 - Grounding and Bonding

Overview

NEC Article 250 establishes the requirements for grounding and bonding of electrical systems and equipment. This article is fundamental to electrical safety, providing protection against electrical shock, fire, and equipment damage.

Key Concepts

Grounding vs. Bonding

• Grounding: Connection to earth through a grounding electrode system
• Bonding: Connection between metallic parts to ensure electrical continuity and conductivity

Most Referenced Sections

250.66 - Size of Alternating-Current Grounding Electrode Conductor

Determines the minimum size of the grounding electrode conductor based on the service entrance conductor size.

Grounding Electrode Conductor Sizing Table

250.122 - Size of Equipment Grounding Conductors

One of the most frequently referenced tables in electrical work, establishing minimum equipment grounding conductor sizes.

Equipment Grounding Conductor Sizing Table

Grounding Systems

250.20 - Alternating-Current Systems to Be Grounded

Systems that must be grounded:

• Single-phase, 2-wire: 120V systems
• Single-phase, 3-wire: 120/240V systems
• Three-phase, 4-wire wye: 208Y/120V, 480Y/277V systems

250.24 - Grounding Service-Supplied Alternating-Current Systems

Service Grounding Requirements:

• Grounded conductor: Must be connected to grounding electrode
• Equipment grounding: All service equipment must be grounded
• Grounding electrode conductor: Sized per 250.66

250.50 - Grounding Electrode System

All available grounding electrodes must be bonded together:

• Metal underground water pipe
• Metal frame of building/structure
• Concrete-encased electrode (Ufer ground)
• Ground ring
• Rod and pipe electrodes
• Plate electrodes

Equipment Grounding

250.118 - Types of Equipment Grounding Conductors

Acceptable equipment grounding conductors:

• Copper, aluminum, or copper-clad aluminum conductor
• Rigid metal conduit (RMC)
• Intermediate metal conduit (IMC)
• Electrical metallic tubing (EMT)
• Flexible metal conduit (FMC) - limited conditions
• Liquidtight flexible metal conduit (LFMC) - limited conditions
• Armor of Type AC cable
• Copper sheath of mineral-insulated cable

250.134 - Equipment Fastened in Place or Connected by Permanent Wiring Methods

Fixed equipment grounding methods:

• Equipment grounding conductor
• Grounded circuit conductor (limited applications)
• Raceway system serving as equipment grounding conductor

Bonding Requirements

250.92 - Services

Service equipment bonding requirements:

• Bonding jumpers: Connect service raceways and enclosures
• Main bonding jumper: Connects grounded conductor to equipment grounding system
• Size: Per Table 250.66 based on service conductors

250.94 - Bonding for Other Systems

Requirements for bonding:

• Communications systems
• CATV systems
• Network-powered broadband systems
• Intersystem bonding termination required

250.104 - Bonding of Piping Systems and Exposed Structural Steel

Metal Water Piping:

• Interior metal water piping: Must be bonded to service equipment
• Bonding conductor size: Per Table 250.66
• Connection points: Near service equipment entrance

Gas Piping:

• CSST systems: Require bonding per manufacturer instructions
• Rigid gas piping: Bonding may be required by local codes

Special Applications

250.146 - Connecting Receptacle Grounding Terminal to Box

Methods for grounding receptacles:

• Equipment grounding conductor: Direct connection to receptacle
• Self-grounding receptacles: Listed devices with grounding contact
• Grounding pigtail: When box is grounded through raceway

250.148 - Continuity and Attachment of Equipment Grounding Conductors to Boxes

Requirements for box grounding:

• Splicing: Equipment grounding conductors must be spliced with approved methods
• Connection to box: Direct connection required for metal boxes
• Pigtail method: Acceptable for maintaining continuity

Calculation Examples

Example 1: Service Grounding Electrode Conductor

Service: 200A with 3/0 AWG copper service entrance conductors Grounding electrode conductor: 4 AWG copper (per Table 250.66)

Example 2: Equipment Grounding Conductor

Circuit: 30A branch circuit with 10 AWG conductors Equipment grounding conductor: 10 AWG copper (per Table 250.122)

Example 3: Upsized Circuit with Voltage Drop Compensation

Original circuit: 20A with 12 AWG conductors, 12 AWG EGC Upsized for voltage drop: 10 AWG conductors Required EGC: 10 AWG (proportionally increased per 250.122(B))

Common Violations

1. Improper EGC sizing - Using undersized equipment grounding conductors
2. Missing bonding jumpers - Not bonding metallic raceways and enclosures
3. Improper electrode connections - Poor connections to grounding electrodes
4. Missing intersystem bonding - Not providing bonding for other systems

Interactive Tools

Calculator tools will be available in a future update.

Related Code Sections

• Article 680 - Swimming Pools, Fountains, and Similar Installations
• Article 517 - Health Care Facilities
• Article 547 - Agricultural Buildings
• Table 250.66 - Grounding Electrode Conductor Sizing
• Table 250.122 - Equipment Grounding Conductor Sizing

Study Questions

1. What is the minimum size grounding electrode conductor for a 400A service with 600 kcmil conductors?
2. When must equipment grounding conductors be increased in size?
3. What grounding electrodes must be bonded together when present?
4. What is the difference between grounding and bonding?

This article summary is based on the 2023 National Electrical Code. Always consult the current adopted code in your jurisdiction and verify with local authorities having jurisdiction.