2Definition of a Grounding Electrode Article 100 –Grounding Electrode:A conducting object through which a direct connection to earth is established.
3250.4 General Requirements for Grounding 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.
4250.52 Grounding Electrodes (A) Electrodes Permitted for Grounding Metal Underground Water PipeMetal Frame of Building or StructureConcrete Encased ElectrodeGround RingRod Electrodes and Pipe ElectrodesOther Listed ElectrodesPlate ElectrodeOther Local Metal Underground Systems or Structures
5250.52(A)(1) Metal Underground Water Pipe Shall be in direct contact with a minimum 10 ft of earthShall include any well casing effectively bonded to the pipeShall be electrically continuous by bonding around insulating joints
6250.52(A)(2) Metal Frame of Building or Structure Provides 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 earthUse of hold-down bolts securing the structural steel to a concrete encased electrode that complies with (A)(3) and is located in the support footing or foundation.
7250.52(A)(3) Concrete-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 methodBare copper conductor not smaller than 4 AWGEncased 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 AWGReinforcing bars may be bonded together by the usual steel tie wires
8250.52(A)(4) Ground RingA 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.
9250.52(A)(5) Rod 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 protectionElectrodes 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
10250.52(A)(7) Plate Electrodes Plate electrodes shall expose not less than 0.186m2 (2 ft2) of surface to exterior soilElectrodes of bare or conductively coated iron or steel plate shall be at least 6.4m (1/4 in.) thickSolid, uncoated, non-ferrous electrodes shall be at least 1.5mm (.06 in.) thick
11250.52(A)(8) Other Local Metal Underground Systems or Structures May include piping systems, underground tanks, and underground well casings not effectively grounded to metal water pipe.
12250.53 Grounding Electrode System Installation Summary If practicable, rod, pipe and plate electrodes shall be installed below permanent moisture levelGrounding electrodes shall be spaced not less than 1.83m (6 ft) apart where more than one electrode is specifiedInformational 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 NECIf a metal underground water pipe is used, it shall meet NEC requirementsA 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.
13ANSI/NEMA GR 1-2007 Zinc Coated Ground Rod Electrodes Diameter Ranges for Zinc Coated Steel RodsFinish Diameter Range, InchesTrade Sizes Minimum Maximum5/3/Note: The above dimensions may vary if ground rod is certified to a U.S. nationally recognized standard.
14ANSI/NEMA GR 1–2007 Copper-Bonded Ground Rod Electrodes Diameter Ranges for Copper-Bonded Steel RodsFinish Diameter Range, Inches Trade Sizes Minimum Maximum1/5/3/
15ANSI/NEMA GR 1–2007 Copper-Bonded Ground Rod Electrodes Thread SpecificationsTrade Sizes Thread Length Thread B Profile (in. +1/8, -1/16) (UNC)1/ /16 9/16-125/ /16 5/8-113/ /4 3/4-10/
16Properties of ANSI/NEMA GR-1 Zinc-Coated Ground Electrodes Zinc-Coated SteelStraightness variation ≤ inches in 5 ftSteel core tensile strength ≥ 80k psiZinc deposit shall be minimum inchesLegibly marked with manufacturer’s identificationASTM Standards -ASTM A 370 – Tensile strengthASTM A 123 or ASTM A 153 – Zinc coatingASTM E 376 – Coating thickness measurement
17Properties of ANSI/NEMA GR-1 Copper-Bonded Ground Electrodes Copper-Bonded SteelStraightness variation ≤ 0.25 inches in 5 ftSteel core tensile strength ≥ 80k psiCopper deposit shall be minimum inchesLegibly marked with manufacturer and catalog or equivalent designationASTM Standards -ASTM A 370 – Tensile strengthASTM E 376 – Coating thickness measurement
18Resistance of Electrodes Water pipe – no NEC requirementsBuilding steel – no NEC requirementsConcrete encased – no NEC requirementsGround ring – no NEC requirementsRod, Pipe, and Plate – 25 ohms or lessA second rod is required if the first rod’s resistance is non-compliantNEC SectionSupplemental Electrodes for metal underground water pipe- Where of pipe, rod or plate – NEC (A) (2) is applicable
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