Electrician Prep

NEC Article 250 Grounding and Bonding: Visual Study Guide

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NEC Article 250 Grounding and Bonding: Visual Study Guide

Article 250 is one of the most challenging topics on the Texas journeyman electrician exam, and it is also one of the most important for electrical safety. Grounding and bonding account for 7.5% of the NEC Knowledge portion, plus additional grounding-related questions appear in other topic areas like services and wiring methods.

The difficulty comes from the number of terms that sound similar but mean different things, the multiple tables used for sizing, and the interconnected requirements that reference other code articles. This guide breaks Article 250 into its key components and shows you how each piece fits into the bigger picture.

The Big Picture: Why Grounding and Bonding Exist

Before diving into code sections, understand the two separate purposes that Article 250 serves:

Grounding connects the electrical system to the earth. This establishes a reference voltage (so 120V is actually 120V relative to earth) and provides a path for lightning and static discharge. The grounding electrode conductor (GEC) and grounding electrode system handle this job.

Bonding connects all metallic parts of the electrical system together to create a low-impedance fault current path back to the source. When a fault occurs (a hot wire touches a metal enclosure, for example), bonding ensures enough current flows to trip the overcurrent device quickly. The equipment grounding conductor (EGC) and bonding jumpers handle this job.

These are fundamentally different functions, and the exam tests whether you understand the distinction.

Key Terms You Must Know

Article 250 uses precise terminology. Confusing these terms is one of the most common reasons candidates miss questions.

Grounding Electrode Conductor (GEC): The conductor that connects the system grounded conductor (neutral) or equipment to the grounding electrode system. Sized from Table 250.66 based on the size of the largest ungrounded service-entrance conductor.

Equipment Grounding Conductor (EGC): The conductor that connects non-current-carrying metal parts of equipment to the system grounded conductor, the grounding electrode conductor, or both at the service. Sized from Table 250.122 based on the rating of the overcurrent device ahead of the circuit.

Grounded Conductor: The system conductor that is intentionally grounded (the neutral in most systems). This is NOT the same as the grounding conductor.

Main Bonding Jumper (MBJ): The connection between the grounded conductor (neutral) and the equipment grounding conductor at the service. This is the critical link that allows fault current to return to the source through the bonding path.

System Bonding Jumper: Similar to the MBJ but used at separately derived systems (like transformers).

Bonding Jumper (Supply Side): A conductor on the supply side of the service overcurrent device that bonds metallic raceways, enclosures, and other components. Sized from Table 250.66.

The Grounding Electrode System (250.50)

NEC 250.50 requires that all grounding electrodes present at a building be bonded together to form the grounding electrode system. You cannot choose one and ignore the others.

Types of Grounding Electrodes

| Electrode Type | NEC Section | Requirements | |---|---|---| | Metal underground water pipe | 250.52(A)(1) | At least 10 ft in direct contact with earth. Must be supplemented by an additional electrode. | | Metal building frame | 250.52(A)(2) | Must be effectively grounded by one of several specified methods | | Concrete-encased electrode (Ufer ground) | 250.52(A)(3) | At least 20 ft of #4 AWG copper or larger rebar encased in at least 2 in. of concrete in direct contact with earth | | Ground ring | 250.52(A)(4) | At least 20 ft of #2 AWG bare copper in direct contact with earth, encircling the building at a depth of at least 30 in. | | Rod and pipe electrodes | 250.52(A)(5) | At least 8 ft long, driven to at least 8 ft depth. If resistance exceeds 25 ohms, a second rod is required. | | Plate electrodes | 250.52(A)(7) | At least 2 sq ft of surface exposed to soil |

Exam tip: The requirement that a metal water pipe electrode must be supplemented by an additional electrode (250.53(D)(2)) is a frequently tested point. Even if the water pipe is a good electrode, you always need a backup.

Sizing the Grounding Electrode Conductor (Table 250.66)

The GEC is sized based on the largest ungrounded service-entrance conductor or equivalent area for parallel conductors. This table is heavily tested.

| Size of Largest Ungrounded Conductor (Copper) | Size of GEC (Copper) | Size of GEC (Aluminum) | |---|---|---| | #2 AWG or smaller | #8 AWG | #6 AWG | | #1 AWG or 1/0 AWG | #6 AWG | #4 AWG | | 2/0 AWG or 3/0 AWG | #4 AWG | #2 AWG | | Over 3/0 AWG through 350 kcmil | #2 AWG | 1/0 AWG | | Over 350 kcmil through 600 kcmil | 1/0 AWG | 3/0 AWG | | Over 600 kcmil through 1100 kcmil | 2/0 AWG | 4/0 AWG | | Over 1100 kcmil | 3/0 AWG | 250 kcmil |

Key rule: The GEC is never required to be larger than 3/0 AWG copper or 250 kcmil aluminum, regardless of the service size. This is a common exam question.

Exception for rod, pipe, and plate electrodes: The GEC to a rod, pipe, or plate electrode is not required to be larger than #6 AWG copper or #4 AWG aluminum (250.66(A)). This exception saves money on long runs to ground rods.

Sizing the Equipment Grounding Conductor (Table 250.122)

The EGC is sized based on the rating of the overcurrent device (fuse or breaker) protecting the circuit, not on the conductor ampacity.

| Rating of OCPD (Amps) | Copper EGC | Aluminum EGC | |---|---|---| | 15 | #14 AWG | #12 AWG | | 20 | #12 AWG | #10 AWG | | 30 | #10 AWG | #8 AWG | | 40 | #10 AWG | #8 AWG | | 60 | #10 AWG | #8 AWG | | 100 | #8 AWG | #6 AWG | | 200 | #6 AWG | #4 AWG | | 300 | #4 AWG | #2 AWG | | 400 | #3 AWG | #1 AWG | | 500 | #2 AWG | 1/0 AWG | | 600 | #1 AWG | 2/0 AWG | | 800 | 1/0 AWG | 3/0 AWG | | 1000 | 2/0 AWG | 4/0 AWG |

Common exam pitfall: Students sometimes confuse Table 250.66 and Table 250.122. Remember:

  • Table 250.66 = GEC (grounding electrode conductor), sized by service conductor size
  • Table 250.122 = EGC (equipment grounding conductor), sized by OCPD rating

A helpful mnemonic: 66 for service, 122 for circuit.

Grounding at the Service (250.24)

At the service, the system must be grounded by connecting the grounded conductor (neutral) to the grounding electrode system through the grounding electrode conductor. This connection happens at the service disconnect or the service entrance equipment.

The main bonding jumper (MBJ) connects the neutral bus to the equipment grounding bus at the service panel. This connection is what allows fault current on equipment grounding conductors to return to the source.

Critical rule: The neutral-to-ground bond (MBJ) is made at the service only, not at downstream panels. Subpanels must keep the neutral and ground buses separate (250.24(A)(5)). Making this bond at a subpanel creates parallel paths for normal neutral return current, which is a code violation and a safety hazard.

Separately Derived Systems (250.30)

A separately derived system is one that has no direct electrical connection to conductors of another system. The most common example is a transformer with a separately derived secondary.

Separately derived systems require their own:

  • System bonding jumper
  • Grounding electrode conductor
  • Connection to the grounding electrode system

The system bonding jumper for a separately derived system is sized using the same method as the main bonding jumper: 12.5% of the area of the largest ungrounded conductor of the derived system, or from Table 250.66.

Exam question pattern: The exam often asks where the grounding connection for a separately derived system must be made. The answer is at the source of the separately derived system (typically at the transformer secondary).

Bonding Requirements

Bonding at the Service (250.92)

The following must be bonded at the service:

  • Service raceways
  • Service cable armor or sheath
  • All metallic enclosures containing service conductors
  • Metal water piping and structural metal building framing that is interconnected to form a building frame

Bonding methods at the service include: bonding to the grounded service conductor, threaded connections, threadless couplings made tight, and listed bonding fittings (250.92(B)).

Key detail: Standard locknut connections are NOT sufficient for bonding at the service. You need bonding bushings, bonding locknut assemblies, or other listed methods. This is a frequently tested point.

Bonding of Water Piping and Structural Steel (250.104)

Metal water piping systems must be bonded to the service equipment enclosure, the grounded conductor at the service, the GEC, or the grounding electrode system. The bonding jumper is sized from Table 250.66 based on the service conductor size.

Structural metal building frames that are not used as a grounding electrode must also be bonded, using a conductor sized from Table 250.66 or connected to the EGC for the circuit that is likely to energize the frame.

Common Exam Question Patterns

Pattern 1: Size the GEC. Given the service entrance conductor size, look up the GEC in Table 250.66. Watch for the exception about rod and pipe electrodes (#6 AWG copper maximum).

Pattern 2: Size the EGC. Given the OCPD rating of a branch circuit, look up the EGC in Table 250.122.

Pattern 3: Identify where the neutral-to-ground bond goes. At the service, not at subpanels.

Pattern 4: Grounding electrode system requirements. Know which electrodes are required and that a metal water pipe must be supplemented.

Pattern 5: Bonding at the service. Standard locknuts are insufficient. Bonding bushings or other listed methods are required.

Pattern 6: Separately derived system grounding. Know that a system bonding jumper and GEC are required at the source.

Study Strategy for Article 250

  1. Start with the definitions. Make sure you know the difference between GEC and EGC, grounding and bonding, and the various bonding jumper types.

  2. Memorize the two sizing tables. Table 250.66 and Table 250.122 are the most frequently tested tables from Article 250. Practice looking them up until you can find the right value in under 10 seconds.

  3. Understand the grounding electrode system. Know which electrode types exist, when supplemental electrodes are required, and the installation requirements for each type.

  4. Focus on the service. Most Article 250 questions center on what happens at the service: the MBJ, the GEC, bonding of service raceways, and the neutral-to-ground connection.

  5. Practice with real questions. Article 250 concepts are best learned through practice problems that force you to apply the rules to specific scenarios.

Practice grounding and bonding questions in our exam simulator

Sources:

  1. National Fire Protection Association - NFPA 70: National Electrical Code, 2023 Edition
  2. Texas Department of Licensing and Regulation (TDLR) - Electrician Exam Information
  3. TDLR - Electrician Exam Statistics, Fiscal Year 2025
  4. International Association of Electrical Inspectors (IAEI) - NEC 2026 Significant Code Changes
  5. PSI Exams - TDLR Electrician Exam Candidate Information