Understanding NEC Grounding and Bonding: Article 250 Deep Dive

Why Article 250 Matters

Article 250 — Grounding and Bonding — is one of the longest and most tested articles in the entire NEC. On most licensing exams, you can expect 10-15% of questions to come from this single article.

Understanding grounding and bonding is not just exam knowledge — it’s critical for electrical safety. Proper grounding provides a path for fault current and helps protective devices operate correctly.

Key Definitions

Before diving into the requirements, understand these fundamental terms from Article 100:

• Ground: The earth
• Grounded: Connected to ground or to a conductive body that extends the ground connection
• Grounding Electrode: A conducting object through which a direct connection to earth is established
• Bonding: The permanent joining of metallic parts to form an electrically conductive path
• Equipment Grounding Conductor (EGC): The conductive path that provides a ground-fault current path

Grounding Electrode System (250.50)

The NEC requires all of the following electrodes, when present, to be bonded together to form the grounding electrode system:

1. Metal underground water pipe (in contact with earth for 10 feet or more)
2. Metal frame of the building or structure
3. Concrete-encased electrode (Ufer ground)
4. Ground ring
5. Rod and pipe electrodes
6. Plate electrodes

Exam tip: If a metal underground water pipe is used as a grounding electrode, it must be supplemented by an additional electrode per 250.53(D)(2).

Grounding Electrode Conductor (GEC) Sizing

The GEC is sized using Table 250.66 based on the largest ungrounded service-entrance conductor or equivalent area for parallel conductors.

Key points:

• Sole connection to rod, pipe, or plate electrode: GEC need not be larger than 6 AWG copper or 4 AWG aluminum
• Connection to concrete-encased electrode: GEC need not be larger than 4 AWG copper
• Must be installed in one continuous length without splice (with exceptions)

Equipment Grounding Conductors (250.122)

EGCs are sized using Table 250.122 based on the rating of the overcurrent protective device ahead of the circuit:

• 15A circuit: 14 AWG copper minimum
• 20A circuit: 12 AWG copper minimum
• 30A circuit: 10 AWG copper minimum
• 60A circuit: 10 AWG copper minimum
• 100A circuit: 8 AWG copper minimum

Important Exception

When conductors are increased in size for voltage drop or other reasons, the EGC must be proportionally increased as well.

Main Bonding Jumper (250.28)

The main bonding jumper connects the grounded conductor (neutral) to the equipment grounding conductor at the service:

• Sized per Table 250.66 or the 12.5% rule for parallel conductors
• Must be installed at each service disconnect
• This is the only point where neutral and ground should be connected in a system

Common Exam Questions

Question Types You’ll See:

1. Sizing questions: “What size GEC is required for a 400A service with 500 kcmil copper conductors?”
2. Application questions: “Where is the main bonding jumper installed?”
3. Code reference questions: “What is the minimum burial depth for a ground ring?”
4. Exception questions: “When is a GEC permitted to be spliced?”

Tips for Article 250 Questions:

• Always check the table references — most sizing questions can be answered directly from Tables 250.66 and 250.122
• Remember the distinction between grounding (earth connection) and bonding (metallic path for fault current)
• Pay attention to the difference between services, feeders, and branch circuits — grounding requirements differ
• Know when the neutral can and cannot serve as an equipment ground

Practice Makes Perfect

Article 250 questions appear on every NEC-based exam. Regular practice with questions referencing specific sections of Article 250 will build both your confidence and your ability to navigate this critical article quickly during the exam.