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Grounding Electrodes
Definition of a Grounding Electrode
Article 100 –
Grounding Electrode:
A conducting object through which
a direct connection to earth is
established.
250.4
General Requirements for
Grounding and Bonding
250.4(A)(1) Electrical System Grounding
Electrical 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 Grounding 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)
Metal 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)
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 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)
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 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) Ground 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 conductor
not smaller than 2 AWG.
250.52(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 metalcoated 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 zinccoated steel when listed
250.52(A)(7) Plate 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)
Other Local Metal Underground
Systems or Structures
May include piping systems, underground
tanks, and underground well casings not
effectively grounded to metal water pipe.
250.53 Grounding Electrode
System 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 1-2007
Zinc Coated Ground Rod
Electrodes
Diameter Ranges for Zinc Coated Steel Rods
Trade Sizes
5/8
3/4
Note:
Finish Diameter Range, Inches
Minimum
Maximum
0.625
0.640
0.750
0.765
The above dimensions may vary if ground rod is certified to a U.S.
nationally recognized standard.
ANSI/NEMA GR 1–2007
Copper-Bonded Ground Rod
Electrodes
Diameter Ranges for Copper-Bonded Steel
Rods
Trade Sizes
1/2
5/8
3/4
1
Finish Diameter Range, Inches
Minimum
Maximum
0.500
0.507
0.555
0.565
0.673
0.683
0.907
0.917
ANSI/NEMA GR 1–2007
Copper-Bonded Ground Rod
Electrodes
Thread Specifications
Trade Sizes
1/2
5/8
3/4
1
Thread Length Thread B Profile
(in. +1/8, -1/16)
(UNC)
1-1/16
9/16-12
1-3/16
5/8-11
1-1/4
3/4-10
1-11/16
1-8
Properties of ANSI/NEMA GR-1
Zinc-Coated Ground Electrodes
Zinc-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
Copper-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 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 noncompliant
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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Grounding Electrodes.