Grounding Grounding Electrodes Electrodes
Mar 29, 2015
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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