9.2 The Grounding Electrode System — 250.50 through 250.53

All Present Electrodes Must Be Bonded — 250.50

250.50 Grounding Electrode System. All grounding electrodes as described in 250.52(A)(1) through (A)(7) that are present at each building or structure served shall be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 250.52(A)(4) through (A)(8) shall be installed and used.

Exception: Concrete-encased electrodes of existing buildings or structures shall not be required to be part of the grounding electrode system where the steel reinforcing bars or rods are not accessible for use without disturbing the concrete.

Read this rule literally. You do not choose one electrode. Every electrode present must be bonded into the system. If the building has a metal underground water pipe, a rebar footing, and driven rods, all three are bonded together.

Note also the ranges. All of (A)(1) through (A)(7) must be bonded if present, but where none exist you may install any of (A)(4) through (A)(8) — the ground ring, rod and pipe, other listed electrodes, plate electrodes, or other local metal underground systems. You cannot invent a water pipe or a footing.

The Recognized Electrodes — 250.52(A)

(A)(1) Metal Underground Water Pipe. A metal underground water pipe in direct contact with the earth for 10 feet (3.0 m) or more, including any metal well casing bonded to the pipe, and electrically continuous (or made electrically continuous by bonding around insulating joints or sections or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductors. Interior metal water piping located more than 5 feet (1.5 m) from the point of entrance to the building shall not be used as a part of the grounding electrode system or as a conductor to interconnect electrodes, with an exception for industrial and commercial buildings under qualified supervision.

(A)(2) Metal In-Ground Support Structure. One or more metal in-ground support structures in direct contact with the earth vertically for 10 feet (3.0 m) or more, with or without concrete encasement. Where multiple metal in-ground support structures are present at a building, it shall be permissible to bond only one into the grounding electrode system.

(A)(3) Concrete-Encased Electrode. An electrode encased by at least 2 inches (50 mm) of concrete, located horizontally near the bottom or vertically within that portion of a concrete foundation or footing that is in direct contact with the earth, consisting of at least 20 feet (6.0 m) of one or more bare or zinc-galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 1/2 inch (13 mm) in diameter, installed in one continuous 20-foot length; or, if in multiple pieces, connected together by the usual steel tie wires, exothermic welding, welding, or other effective means to create a 20-foot or greater length; or consisting of at least 20 feet of bare copper conductor not smaller than 4 AWG. Where multiple concrete-encased electrodes are present, it shall be permissible to bond only one into the grounding electrode system.

(A)(4) Ground Ring. A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 20 feet (6.0 m) of bare copper conductor not smaller than 2 AWG.

(A)(5) Rod and Pipe Electrodes. Rod and pipe electrodes shall not be less than 8 feet (2.44 m) in length.

• (a) Pipe or conduit electrodes shall not be smaller than trade size 3/4 and, where of steel, shall have the outer surface galvanized or otherwise metal-coated for corrosion protection.
• (b) Rod-type electrodes of stainless steel and copper or zinc coated steel shall be at least 5/8 inch (15.87 mm) in diameter, unless listed.

(A)(6) Other Listed Electrodes. Other listed grounding electrodes shall be permitted.

(A)(7) Plate Electrodes. Each plate electrode shall expose not less than 2 square feet (0.186 m squared) of surface to exterior soil. Electrodes of bare or electrically conductive coated iron or steel plates shall be at least 1/4 inch (6.4 mm) in thickness. Solid, uncoated electrodes of nonferrous metal shall be at least 0.06 inch (1.5 mm) in thickness.

(A)(8) Other Local Metal Underground Systems or Structures. Other local metal underground systems or structures such as piping systems, underground tanks, and underground metal well casings that are not bonded to a metal water pipe.

Not Permitted as Electrodes — 250.52(B)

The following shall not be used as grounding electrodes:

1. Metal underground gas piping systems;
2. Aluminum;
3. The structures and structural reinforcing steel described in 680.26(B)(1) and (B)(2) — pool equipotential bonding components.

The gas-pipe prohibition is absolute and frequently tested. Note the distinction: a gas pipe may not be an electrode, but 250.104(B) requires metal piping systems likely to become energized — including gas piping — to be bonded. Bonding a gas line is required; using it as an electrode is prohibited.

Installation — 250.53

250.53(A)(1) Below Permanent Moisture Level. Rod, pipe, and plate electrodes shall be installed below permanent moisture level where practicable and shall be free from nonconductive coatings such as paint or enamel.

250.53(A)(2) Supplemental Electrode Required. A single rod, pipe, or plate electrode shall be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(8). The supplemental electrode shall be permitted to be bonded to one of the following: the rod, pipe, or plate electrode; the grounding electrode conductor; the grounded service-entrance conductor; the nonflexible grounded service raceway; or any grounded service enclosure.

Exception: If a single rod, pipe, or plate grounding electrode has a resistance to earth of 25 ohms or less, the supplemental electrode shall not be required.

The economics of that exception are worth understanding: measuring the resistance requires a three-point fall-of-potential test with specialized equipment, so in practice almost everyone simply drives a second rod. Driving two rods is the default, not because two rods are required, but because proving one is sufficient costs more than the second rod.

250.53(A)(3) Supplemental Electrode. Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall be not less than 6 feet (1.8 m) apart. An Informational Note adds that the effectiveness of rod, pipe, and plate electrodes increases by spacing them twice the length of the longest rod — so 16 feet apart for 8-foot rods is better practice than the 6-foot minimum.

250.53(B) Electrode Spacing. Where more than one of the electrodes of the type specified in 250.52(A)(5) or (A)(7) are used, each electrode of one grounding system shall not be less than 6 feet (1.8 m) from any other electrode of another grounding system. Two or more grounding electrodes that are bonded together shall be considered a single grounding electrode system.

250.53(C) Bonding Jumper. The bonding jumper used to connect the grounding electrodes together to form the grounding electrode system shall be installed in accordance with 250.64(A), (B), and (E), shall be sized in accordance with 250.66, and shall be connected in the manner specified in 250.70.

250.53(D)(1) Continuity. Continuity of the grounding path or the bonding connection to interior piping shall not rely on water meters or filtering devices and similar equipment.

250.53(D)(2) Supplemental Electrode Required. A metal underground water pipe shall be supplemented by an additional electrode of a type specified in 250.52(A)(2) through (A)(8). Where the supplemental electrode is a rod, pipe, or plate type, it shall comply with 250.53(A). The supplemental electrode shall be bonded to one of the following: the grounding electrode conductor; the grounded service-entrance conductor; the nonflexible grounded service raceway; any grounded service enclosure; or the metal in-ground support structure.

The water pipe always needs a supplement — there is no 25-ohm exception for it.

250.53(E) Supplemental Electrode Bonding Connection Size. Where the supplemental electrode is a rod, pipe, or plate electrode, that portion of the bonding jumper that is the sole connection to the supplemental grounding electrode shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire.

250.53(F) Ground Ring. The ground ring shall be installed not less than 30 inches (750 mm) below the surface of the earth.

250.53(G) Rod and Pipe Electrodes. The electrode shall be installed such that at least 8 feet (2.44 m) of length is in contact with the soil. It shall be driven to a depth of not less than 8 feet except that, where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical or, where rock bottom is encountered at an angle up to 45 degrees, the electrode shall be permitted to be buried in a trench that is at least 30 inches (750 mm) deep. The upper end of the electrode shall be flush with or below ground level unless the aboveground end and the grounding electrode conductor attachment are protected against physical damage as specified in 250.10.

250.53(H) Plate Electrode. Plate electrodes shall be installed not less than 30 inches (750 mm) below the surface of the earth.

Alaska Ground Conditions

Alaska presents three real electrode challenges, and the Code accommodates all three without needing state amendments.

Rock. Much of Southeast and coastal Alaska sits on shallow bedrock. 250.53(G) is written for exactly this: drive at up to a 45-degree oblique angle, or bury the rod in a trench at least 30 inches deep.

Permafrost and frozen ground. Frozen soil has dramatically higher resistivity than the same soil thawed, because the conduction mechanism is ionic and requires liquid water. A rod that measures a comfortable resistance in August can measure many times that in February. Practical responses that remain within the Code include: installing more electrodes bonded together per 250.50 and 250.53(B); using deeper electrodes to reach below the active layer where feasible; and using listed electrodes under 250.52(A)(6), which is the pathway for chemically enhanced or electrolytic rods.

Shallow active layer. Where the seasonal thaw layer is thin, a horizontal ground ring under 250.52(A)(4) — 20 feet or more of bare 2 AWG or larger copper at least 30 inches deep — or a plate electrode under 250.52(A)(7) may perform better than a vertical rod, because both keep more conductor surface in the layer that actually thaws.

What the Code does not do is set a maximum resistance for the grounding electrode system as a whole. The 25-ohm figure applies only to the single-electrode supplemental exception in 250.53(A)(2). There is no NEC requirement that a completed grounding electrode system measure any particular resistance, and no Alaska amendment adding one. Where a project specification imposes a resistance target, that is a contractual requirement, not a Code requirement.