Grounding ElectrodesGrounding Electrodes

Definition of a Grounding ElectrodeDefinition of a Grounding Electrode

Article 100 –

Grounding Electrode:A conducting object through which a direct connection to earth is established.

250.4250.4 General Requirements forGeneral Requirements for Grounding and BondingGrounding and Bonding

250.4(A)(1) Electrical System GroundingElectrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operations.– Informational Note: An important consideration for limiting the

imposed voltage is the routing of bonding and grounding electrode conductors so that they are not any longer than necessary to complete the connection without disturbing the permanent parts of the installation and so that unnecessary bends and loops are avoided.

250.52 250.52 Grounding ElectrodesGrounding Electrodes

(A) Electrodes Permitted for Grounding Metal Underground Water Pipe Metal Frame of Building or Structure Concrete Encased Electrode Ground Ring Rod Electrodes and Pipe Electrodes Other Listed Electrodes Plate Electrode Other Local Metal Underground Systems or

Structures

250.52(A)(1)250.52(A)(1)Metal Underground Water PipeMetal Underground Water Pipe

Shall be in direct contact with a minimum 10 ft of earth

Shall include any well casing effectively bonded to the pipe

Shall be electrically continuous by bonding around insulating joints

250.52(A)(2)250.52(A)(2)Metal Frame of Building or StructureMetal Frame of Building or StructureProvides two conditions under which the metal frame

of a building or structure can be judged suitable: 10 ft or more of a single structural metal member in

direct contact with the earth or in concrete directly contacting the earth

Use of hold-down bolts securing the structural steel to a concrete encased electrode that complies with 250.52(A)(3) and is located in the support footing or foundation.

250.52(A)(3)250.52(A)(3) Concrete-Encased ElectrodeConcrete-Encased Electrode

A concrete-encased electrode shall consist of at least 6.0 m (20 ft) of either: One or more bare or galvanized or other electrically conductive coated

steel reinforced bars or rods not less than 13mm(1/2 in.) in diameter, installed in one continuous length, or if in multiple pieces connected together by the usual tie wire or other effective connection method

Bare copper conductor not smaller than 4 AWG Encased by at least 50mm (2 in.) of concrete Electrode shall

be:- at least 20 ft in length- reinforcing bars or rods not less than ½” in diameter- bare copper conductor not less than #4 AWG

Reinforcing bars may be bonded together by the usual steel tie wires

250.52(A)(4)250.52(A)(4) Ground RingGround Ring

A ground ring encircling the building or structure in direct contact with the earth, consisting of at least 6m (20 ft)of bare copper conductornot smaller than 2 AWG.

250.52(A)(5)250.52(A)(5) Rod and Pipe ElectrodesRod and Pipe Electrodes

Electrodes of Pipe - 2.44m (8 ft) minimum length - 3/4 in. “trade size” minimum

diameter - be of iron or steel and shall have the outer

surface galvanized or otherwise metal-coated for corrosion protection

Electrodes of Rod - 8 ft minimum length

- 5/8 in. (0.625”) minimum diameter - 1/2 in. (0.500”) minimum diameter for

stainless steel, and copper-coated or zinc-coated steel when listed

250.52(A)(7)250.52(A)(7) Plate ElectrodesPlate Electrodes Plate electrodes shall expose not less than

0.186m2 (2 ft2) of surface to exterior soil Electrodes of bare or conductively coated iron

or steel plate shall be at least 6.4m (1/4 in.) thick

Solid, uncoated, non-ferrous electrodes shall be at least 1.5mm (.06 in.) thick

250.52(A)(8)250.52(A)(8) Other Local Metal Underground Other Local Metal Underground Systems or StructuresSystems or Structures

May include piping systems, underground tanks, and underground well casings not effectively grounded to metal water pipe.

250.53 250.53 Grounding Electrode Grounding Electrode System Installation SummarySystem Installation Summary

If practicable, rod, pipe and plate electrodes shall be installed below permanent moisture level

Grounding electrodes shall be spaced not less than 1.83m (6 ft) apart where more than one electrode is specified Informational Note: the paralleling efficiency of rods is increased by spacing

them twice the length of the longest rod. A bonding jumper shall be sized and spaced according to NEC If a metal underground water pipe is used, it shall meet NEC

requirements A bonding jumper that is the sole connection to the supplemental

grounding electrode shall not be required to be larger than 6 AWG copper wire.

Ground rings and plate electrodes shall be installed not less than 750mm (30 in.) below the surface of earth.

ANSI/NEMA GR 1-2007ANSI/NEMA GR 1-2007Zinc Coated Ground Rod Zinc Coated Ground Rod

ElectrodesElectrodes

Diameter Ranges for Zinc Coated Steel Rods

Finish Diameter Range, Inches Trade Sizes Minimum Maximum

5/8 0.625 0.640 3/4 0.750 0.765

Note: The above dimensions may vary if ground rod is certified to a U.S. nationally recognized standard.

ANSI/NEMA GR 1–2007ANSI/NEMA GR 1–2007Copper-Bonded Ground Rod Copper-Bonded Ground Rod

ElectrodesElectrodesDiameter Ranges for Copper-Bonded Steel

Rods

Finish Diameter Range, Inches Trade Sizes Minimum Maximum 1/2 0.500 0.507 5/8 0.555 0.565 3/4 0.673 0.683

1 0.907 0.917

ANSI/NEMA GR 1–2007ANSI/NEMA GR 1–2007Copper-Bonded Ground Rod Copper-Bonded Ground Rod

ElectrodesElectrodes

Thread Specifications Trade Sizes Thread Length Thread B Profile

(in. +1/8, -1/16) (UNC) 1/2 1-1/16 9/16-12 5/8 1-3/16 5/8-11 3/4 1-1/4 3/4-10

1 1-11/16 1-8

Properties of ANSI/NEMA GR-1 Properties of ANSI/NEMA GR-1 Zinc-Coated Ground ElectrodesZinc-Coated Ground ElectrodesZinc-Coated Steel Straightness variation ≤ 0.25 inches in 5 ft Steel core tensile strength ≥ 80k psi Zinc deposit shall be minimum 0.0034 inches Legibly marked with manufacturer’s

identification ASTM Standards -

ASTM A 370 – Tensile strength ASTM A 123 or ASTM A 153 – Zinc coating ASTM E 376 – Coating thickness measurement

Properties of ANSI/NEMA GR-1 Properties of ANSI/NEMA GR-1 Copper-Bonded Ground ElectrodesCopper-Bonded Ground Electrodes

Copper-Bonded Steel Straightness variation ≤ 0.25 inches in 5 ft Steel core tensile strength ≥ 80k psi Copper deposit shall be minimum 0.010 inches Legibly marked with manufacturer and catalog

or equivalent designation ASTM Standards -

ASTM A 370 – Tensile strength ASTM E 376 – Coating thickness measurement

Resistance of ElectrodesResistance of Electrodes Water pipe – no NEC requirements Building steel – no NEC requirements Concrete encased – no NEC requirements Ground ring – no NEC requirements Rod, Pipe, and Plate – 25 ohms or less

A second rod is required if the first rod’s resistance is non-compliant

NEC Section 250.53 Supplemental Electrodes for metal underground

water pipe - Where of pipe, rod or plate – NEC 250.53(A) (2) is applicable

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