NEC · Article 250 Pt III

Grounding Electrode System (250.50–250.70)

The NEC does not let you choose your favorite electrode and ignore the rest — every electrode present at a structure must be bonded into one unified grounding electrode system.

NEC 2017 2020 2023 Last updated 2026-06-27

What the grounding electrode system is

A grounding electrode is any conductor or conductive object that makes an intentional connection between the electrical system and the earth. The NEC, in Part III of Article 250 (Sections 250.50 through 250.70), goes further than simply requiring one electrode — it requires all qualifying electrodes that exist at a structure to be interconnected and used together as a single grounding electrode system. That "use it if it's there" rule is the central concept the journeyman exam tests in this area.

The logic is that multiple electrodes bonded together create a lower combined resistance to earth than any single electrode could achieve on its own, and distributed connections at different physical locations help equalize voltage gradients in the soil around the structure. The system approach also ensures that no one electrode can become isolated from the intentional ground reference.

Electrodes that must be used when present

Section 250.50 lists the electrode types that must be included in the grounding electrode system whenever they are present at or within a structure. Each type has its own installation requirements covered in Sections 250.52 through 250.60:

  • Metal underground water pipe. A metal water pipe that is in contact with the earth for at least a minimum continuous length (10 feet is the threshold) qualifies. Because water service pipes can be replaced with plastic, recent NEC editions require supplementing the water pipe electrode — it cannot stand alone.
  • Metal in-ground support structure. Structural metal that is in direct contact with the earth — typically the steel frame of a building — serves as an electrode when present.
  • Concrete-encased electrode (Ufer ground). Rebar or bare copper wire embedded in concrete that is in direct contact with the earth — such as a foundation footing — qualifies when it meets the minimum length requirement. The NEC sets specific thresholds for the total length of rebar or wire that must be present. This electrode is widely considered one of the most effective because concrete in contact with moist soil achieves consistently low earth resistance.
  • Ground ring. A bare copper conductor encircling the structure, buried at a minimum depth in the earth, can serve as a ground ring electrode when it meets the minimum conductor size and burial requirements.
  • Rod and pipe electrodes. Copper-clad steel ground rods (or listed alternatives) and iron or steel pipes of sufficient diameter and length are common "made" electrodes installed where other electrode types are not present or need supplementing.
  • Plate electrodes. Metal plates with sufficient surface area buried in contact with the earth can serve as a listed electrode type.

Bonding the electrodes together

Having multiple electrodes at a structure is not enough — they must be electrically interconnected. Section 250.50 requires that all grounding electrodes at a building or structure be bonded together to form the system. The conductor used to tie individual electrodes to each other, or to the grounding electrode conductor that runs back to the service, is called a bonding jumper. These jumper connections are made at or near the service equipment or at each electrode, depending on the installation.

The grounding electrode conductor (GEC), sized from Table 250.66 per Section 250.66, is the conductor that connects the service equipment to the grounding electrode system as a whole. Once the GEC reaches the electrode system, bonding jumpers tie the individual electrodes together. Taps from the main GEC run to each electrode, or a single conductor can loop through multiple electrode connections — the NEC permits both approaches.

The supplemental electrode rule for rods, pipes, and plates

A rod, pipe, or plate electrode that cannot achieve a measured resistance to earth of 25 ohms or less must be supplemented by at least one additional electrode. The supplemental electrode must be spaced at least 6 feet away from the first to gain meaningful improvement in the combined resistance. This requirement appears in Section 250.53.

Because testing resistance requires specialized equipment and field conditions vary, the NEC also provides a straightforward alternative: simply install a second electrode regardless of resistance. In practice, most electricians install two rods spaced the required distance apart and skip the resistance test — a widely accepted and code-compliant approach. The exam tests both the resistance threshold and the "two rods" alternative, so know both options.

Why this section is on the journeyman exam

Grounding electrode system questions cluster around three areas on the exam. First: which electrodes must be used when present. The "use it if it's there" rule catches candidates who assume they can choose just one. Second: the supplemental electrode rule for rods, including the 6-foot spacing and the 25-ohm threshold. Third: the water pipe electrode's requirement to be supplemented — a nuance added to require a backup in case water service is converted to plastic.

Understanding the electrode system also sets the context for the GEC sizing rules in Section 250.66. The reduced-size allowances in 250.66(A) and 250.66(B) apply only when a GEC connects solely to a rod or a concrete-encased electrode respectively. Once those electrodes are part of a bonded system that includes other electrode types, the full Table 250.66 size applies. See the GEC sizing article for the sizing rules that flow from the electrode system you just assembled.

Practice questions on this rule

Real journeyman-exam practice questions from Loomi's bank — answer first, then reveal the explanation.

Q1 NEC 250.53(C)

An electrician is bonding a ground rod, a metal water pipe electrode, and a concrete-encased electrode together to form a grounding electrode system. Rebar is exposed and convenient at the connection point. Can it be used as the interconnecting bonding jumper?

  1. A Yes, because rebar is already an approved grounding electrode material
  2. B No, rebar shall not be used as a conductor to interconnect the electrodes
  3. C Yes, but only if it is exothermically welded at both ends
  4. D Yes, as long as it is at least 20 ft long
Show answer & explanation

Correct: B — No, rebar shall not be used as a conductor to interconnect the electrodes

250.53(C) explicitly states rebar shall not be used as a conductor to interconnect the electrodes of a grounding electrode system, even though rebar is a recognized electrode type on its own. The 20-ft-length option and the exothermic-welding option describe conditions relevant to other grounding electrode rules, not an exception to this prohibition.

Q2 NEC 250.52(A)(3)

A slab-on-grade foundation has a plastic vapor barrier installed between the concrete and the earth, with qualifying rebar embedded 2 in. deep in the concrete. Can this rebar serve as a concrete-encased electrode?

  1. A No, because the vapor barrier means the concrete is not in direct contact with the earth
  2. B Yes, because 2 in. of concrete cover is all that matters, regardless of vapor barriers
  3. C Yes, as long as the rebar itself touches the vapor barrier
  4. D No, because concrete-encased electrodes are never permitted in slab-on-grade construction
Show answer & explanation

Correct: A — No, because the vapor barrier means the concrete is not in direct contact with the earth

Per 250.52(A)(3), concrete installed with insulation, vapor barriers, films, or similar items separating it from the earth is not considered to be in direct contact with the earth, so it cannot qualify. Cover depth alone doesn't satisfy the rule, and slab-on-grade foundations are not categorically excluded — only the vapor-barrier condition disqualifies this one.

Q3 NEC 250.52(B)

A contractor wants to bond the metal underground gas piping system as required, and also wants to know if it can double as the service's grounding electrode. What does 250.52(B) say?

  1. A It may serve as the grounding electrode if no other electrode is available
  2. B It may serve as the grounding electrode only if it is at least 10 ft long
  3. C It may serve as the grounding electrode as long as it is also bonded per 250.104(B)
  4. D It shall not be used as a grounding electrode, regardless of the required bonding under 250.104(B)
Show answer & explanation

Correct: D — It shall not be used as a grounding electrode, regardless of the required bonding under 250.104(B)

250.52(B)(1) bans metal underground gas piping as a grounding electrode outright. The bonding requirement in 250.104(B) is a separate, mandatory obligation and does not change or waive the ban on using the piping as an electrode — there's no length or last-resort exception.

Q4 NEC 250.52(A)

A rod-type grounding electrode made of zinc-coated steel is being installed. Per 250.52(A)(5), what is the minimum diameter required unless the rod is listed?

  1. A 19.05 mm (3/4 in.)
  2. B 12.7 mm (1/2 in.)
  3. C 9.53 mm (3/8 in.)
  4. D 15.87 mm (5/8 in.)
Show answer & explanation

Correct: D — 15.87 mm (5/8 in.)

250.52(A)(5) requires rod-type electrodes of stainless steel, copper, or zinc-coated steel to be at least 15.87 mm (5/8 in.) in diameter unless listed. The other diameters are plausible-sounding hardware sizes but do not match the code minimum.

All Grounding and Bonding practice questions (Article 250)

Frequently asked questions

What is a grounding electrode conductor (GEC)?
The grounding electrode conductor is the wire that connects the service's grounded conductor and equipment to the grounding electrode system — the rods, concrete-encased electrodes, water pipe, or structural steel that put the installation in contact with the earth. It is sized from Table 250.66 based on the service-entrance conductors.
Does a new residence always need a ground rod?
Not necessarily as the only electrode — but rods are almost always installed because the NEC requires you to use every electrode type that is present at the structure. If there is no metal water pipe, no structural steel, and no concrete-encased electrode, a rod or other made electrode becomes required. In most residential work, a concrete-encased electrode is present in the foundation, and the NEC requires it to be used and supplemented by a second electrode such as a rod.
What resistance does a single ground rod need to achieve?
A single rod, pipe, or plate electrode must achieve a resistance to ground of 25 ohms or less — otherwise a supplemental electrode must be added. In practice, most installers simply add the second electrode rather than testing resistance, because NEC allows that approach as an alternative to measuring.
What is a concrete-encased electrode and why does the NEC favor it?
A concrete-encased electrode — sometimes called a Ufer ground after the engineer who popularized it — uses rebar or bare wire embedded in a concrete foundation footing that is in contact with the earth. Because concrete is slightly conductive and the footing has a large surface area in contact with the soil, a properly installed Ufer ground typically achieves lower resistance to earth than a single rod, making it one of the most effective and reliable electrode types the NEC recognizes.
Related articles
Grounding & Bonding The full Article 250 topic hub on the journeyman exam. Grounding Electrode Conductor Sizing (250.66) How to size the conductor that connects your electrode system to the service. Equipment Grounding Conductor Sizing (250.122) The companion conductor that runs with circuits — sized differently. Neutral vs Ground Where the grounded conductor and the electrode system meet at the service.
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