Understanding NEC Article 250: Grounding and Bonding Essentials

NEC Article 250 covers grounding and bonding requirements. It is one of the longest and most complex articles in the codebook, and it shows up heavily on the Texas Master Electrician exam.

This post is organized in two parts. The first part covers exactly what you need for the exam — the tables, the sizing rules, and the concepts they test. The second part, "Going Deeper," covers the real-world practices and field knowledge that make you a better electrician.

If you're cramming for the exam, focus on Part 1. If you want to actually understand grounding, read both.

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Part 1: What the Exam Tests

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Grounding vs. Bonding

This is one of the most commonly tested concepts. Know the difference cold.

Key exam concept: Bonding is what actually clears faults. The EGC and bonding create the low-impedance path that lets enough fault current flow to trip the breaker. A ground rod alone won't do it — earth resistance is too high.

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EGC Sizing (Table 250.122)

The equipment grounding conductor is sized based on the overcurrent device rating protecting the circuit. Use Table 250.122.

Exam trap: The EGC is sized based on the overcurrent device, NOT the conductor size. Don't confuse Table 250.122 with Table 250.66.

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GEC Sizing (Table 250.66)

The grounding electrode conductor is sized based on the largest service-entrance conductor. Use Table 250.66.

Important exception: The GEC to a concrete-encased electrode (Ufer ground) never needs to be larger than #4 AWG copper per NEC 250.66(B). This is a frequently tested exception.

Maximum size: The GEC never needs to be larger than #3/0 copper or 250 kcmil aluminum. The exam may describe a massive service to trick you into oversizing it.

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Main Bonding Jumper (250.28)

The main bonding jumper connects the grounded conductor (neutral) to the equipment grounding conductor at the service disconnect. This is the critical connection that ties the entire grounding system together.

This connection is only made at the service — never at subpanels.

At a subpanel, the neutral and ground buses must be separated. If you bond them at a subpanel, you create parallel paths for normal return current to flow on the EGC. The exam tests this frequently.

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Grounding Electrodes (250.52)

The exam expects you to know the types of grounding electrodes and their requirements.

Three rules the exam loves:

1. All electrodes present must be bonded together per NEC 250.50. You don't get to pick one and ignore the rest.

2. A water pipe electrode must always be supplemented by an additional electrode per NEC 250.53(D)(2). You can never use it alone.

3. Metal gas piping cannot be used as a grounding electrode per NEC 250.52(B). But it must be bonded to the grounding electrode system per 250.104(B).

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Ground Rod Rules (250.53)

NEC 250.53(A)(2) requires that a single ground rod electrode achieve 25 ohms or less of resistance to ground. When a single rod does not meet this threshold, a supplemental electrode must be installed.

Practical application: NEC 250.53(A)(2) Exception permits an alternative approach. When a supplemental ground rod is installed at least 6 feet from the first rod, the 25-ohm resistance requirement does not need to be verified. This is the method used on the vast majority of residential and commercial installations, since proper ground resistance testing requires specialized equipment (a fall-of-potential tester) that costs $2,000 to $5,000 and is not commonly available on most job sites.

For the exam: Know that the code requires 25 ohms or less for a single rod (250.53(A)(2)), and that installing a second rod at least 6 feet away eliminates the need to test (250.53(A)(2) Exception). Both facts are testable.

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Bonding Requirements (250.104)

Know which table to use for sizing bonding jumpers — the exam tests this.

Exam trap: Water pipe bonding jumpers and gas pipe bonding jumpers use different tables. Water uses 250.66. Gas uses 250.122. They will test this.

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Common Exam Questions

Q: A circuit is protected by a 100-amp breaker. What is the minimum EGC? Look up 100 amps in Table 250.122. Answer: #8 AWG copper.

Q: A 400-amp service uses 500 kcmil copper conductors. What is the minimum GEC? Look up 500 kcmil in Table 250.66. Answer: #1/0 AWG copper.

Q: What is the maximum required GEC size to a concrete-encased electrode? Per 250.66(B): #4 AWG copper.

Q: Can metal gas piping be used as a grounding electrode? No, per 250.52(B). But it must be bonded per 250.104(B).

Q: At a subpanel, should the neutral and ground be bonded together? No. Only at the service disconnect.

Q: A metal water pipe is used as a grounding electrode. Is a supplemental electrode required? Yes, always, per 250.53(D)(2).

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Sections to Tab in Your Codebook

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Part 2: Going Deeper — Real-World Grounding

This section covers practical field knowledge that goes beyond the exam. If you're working as an electrician or want to understand why the code requires what it does, keep reading.

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Why Two Ground Rods Is Standard Practice

The code says you need 25 ohms or less from a single ground rod. But here's the reality most electricians know: the equipment to properly test ground rod resistance costs $2,000 to $5,000.

A proper fall-of-potential test requires a specialized ground resistance tester like a Fluke 1625 or Megger DET4T. You also need two additional test stakes driven into the ground at specific distances from the electrode. It's time-consuming, requires open ground, and most electricians don't own the equipment.

So what happens in practice? Electricians drive two ground rods and move on. NEC 250.53(A)(2) Exception allows this — if you install a supplemental rod, you don't need to prove the first one meets 25 ohms. Two rods spaced at least 6 feet apart satisfies the code without any testing.

That's why you see two ground rods at virtually every residential service in Texas. It's not because every electrician tested the first rod and found it over 25 ohms. It's because driving a second $15 rod is far cheaper and faster than renting a $3,000 tester.

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The Concrete-Encased Electrode (Ufer Ground) in Practice

The Ufer ground is one of the most effective grounding electrodes available, and it's worth understanding why.

Concrete absorbs moisture from the surrounding earth and retains it. This gives the electrode excellent contact with the soil. The large surface area of concrete in contact with earth provides very low impedance — often under 5 ohms, compared to 25-100+ ohms for a ground rod in dry soil.

On new construction, this is often the best grounding electrode you'll have. Running 20 feet of bare #4 AWG copper or tying into the rebar in the footing before the concrete pour is straightforward. Many inspectors prefer seeing a Ufer ground because it provides consistently low resistance regardless of soil conditions.

If you're working new construction, always look for the opportunity to install a concrete-encased electrode. It's cheap, effective, and the code requires you to use it if it's present.

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Bonding Building Steel and Rebar

In commercial and industrial buildings with structural steel frames, the metal frame can serve as a grounding electrode under 250.52(A)(2) — but only if it's effectively grounded.

"Effectively grounded" means the steel is either in direct contact with the earth for at least 10 feet, or it's connected to a concrete-encased electrode. In practice, most steel-frame buildings have their columns sitting on concrete footings with rebar, so the connection is often already there.

When the building frame qualifies as an electrode, all the structural steel must be bonded together and connected to the grounding electrode system. This creates a massive, interconnected grounding network throughout the building. It's one of the reasons commercial buildings tend to have much better grounding than residential.

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Water Pipes vs. Gas Pipes: The Critical Distinction

Water pipes can serve as grounding electrodes if they have 10 feet of earth contact. But they must always be supplemented because the water utility could replace a section of metal pipe with plastic at any time. Your grounding electrode could disappear without you knowing it.

All interior metal water piping must also be bonded to the grounding electrode system per 250.104(A). This prevents the piping from becoming energized during a fault.

Gas pipes are a completely different story. Metal gas piping is explicitly prohibited from being used as a grounding electrode per 250.52(B). The reason is straightforward — you never want fault current flowing through gas piping. A spark at a gas pipe joint could cause an explosion.

However, metal gas piping must be bonded to the grounding electrode system per 250.104(B). This prevents the gas pipe from becoming energized if a fault occurs nearby. The bonding jumper is sized from Table 250.122 based on the overcurrent device rating of the circuit that could energize the piping.

The distinction is important: bonding the gas pipe protects people by preventing it from becoming energized. But you never use it as a path for grounding the electrical system.

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Soil Conditions and Why They Matter

Ground rod effectiveness depends heavily on what's in the ground around it.

In parts of Texas with sandy or rocky soil, a single ground rod might have 100+ ohms of resistance. Even two rods in parallel might only get you to 50 ohms. But a Ufer ground in the same location could be under 5 ohms.

This is exactly why the code requires you to use a concrete-encased electrode when it's available. It's not optional — if the building has a concrete foundation with rebar or embedded copper, that electrode must be part of the grounding electrode system.

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Separately Derived Systems

A separately derived system — like a transformer or generator with no direct connection to the utility ground — has its own grounding requirements under NEC 250.30.

The system needs its own grounding electrode conductor, its own bonding jumper, and its own connection to a grounding electrode. The system bonding jumper connects the grounded conductor to the equipment ground at the source.

Think of it this way: every separately derived system is like a miniature service entrance. It needs the same grounding and bonding connections that the main service has.

In practice, this comes up most often with step-down transformers in commercial buildings. Each transformer that creates a new voltage system needs its own grounding electrode connection.

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The Bottom Line

Grounding and bonding is a topic that rewards deeper understanding. For the exam, focus on the tables, the sizing rules, and the key concepts in Part 1. For your career, the real-world knowledge in Part 2 will make you a better, safer electrician.

The electricians who truly understand grounding — not just the code sections, but the physics behind why it works — are the ones who catch problems that others miss.

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Keep Studying

Test your knowledge with our free practice quizzes — the grounding and bonding topic has questions pulled directly from Article 250 concepts.

Review the Formula Reference Sheet for quick conductor sizing reminders, and follow the 4-Week Study Plan to stay on track.

For more exam-style practice with detailed explanations, check out the Master Electrician Future Kit.

Have a grounding question from the field or the exam? Visit our Support page for help.