ERICO Practical Guide to Electrical Grounding - Manual (Page 90)

aluminum. Copper-clad aluminum is constructed of a
minimum of 10% copper which is metallurgically bonded
to the aluminum core. The GEC is permitted to be a solid
or stranded conductor and it can be an insulated, covered or
bare conductor. Solid conductors provide less surface area
to corrode and subsequently are used when installed in
corrosive locations. However, stranded conductors in
general are easier to work with so they are used more
frequently. With stranded conductors of a given size, the
greater the number of strands, the smaller each strand is and
the conductor is more flexible. Copper is by far the most
common choice for grounding electrode conductors but
copper-clad aluminum may be used to reduce the likelihood
of repeated theft of the copper GEC. The major
disadvantage to using aluminum is the installation
restriction in damp or wet locations. See installation
provisions below.
Installation:
In general, grounding electrode conductors are required to
be installed in one continuous length, without splices or
joints. As noted above however, the GEC can be spliced by
means of irreversible compression-type connectors listed
for the use or by means of the exothermic welding process
(CADWELD). Also as noted above, the GEC can be
installed directly on a building structure, if a No. 6 AWG or
larger, and not subject to physical damage. If the GEC is
going to be subject to physical damage it should be installed
in a raceway or cable armor for protection. Section 250-92
(a) prohibits the use of aluminum or copper-clad aluminum
grounding electrode conductors when they are installed in
direct contact with masonry, the earth, or where they are
subject to corrosive conditions. Another important
restriction for aluminum or copper-clad aluminum GEC’s is
the prohibition against their use outdoors within 18 inches
of the earth. This requirement effectively precludes the use
of aluminum or copper-clad aluminum for connection to
“made” electrodes installed outdoors.
Size:
The size of the grounding electrode conductor is based on
the size of the largest service-entrance conductor that
supplies the building or structure. When the service
conductors are installed in parallel, the size of the GEC is
based on the size of the equivalent area of a single
conductor. For example, if a 3-phase, 4-wire service
consists of two, 500 kcmil conductors per phase, in parallel,
the size of the GEC would be based on the equivalent area
of a single phase,1,000kcmil,( 500kcmil x 2 conductors).
Table 250-94 of the NEC contains the minimum size for
aluminum, copper-clad aluminum and copper grounding
electrode conductors. The table includes sizes for circuits
from No. 2 AWG copper and No. 1/0 AWG aluminum up
to 1100 kcmil copper and 1750 kcmil aluminum or copper-
clad aluminum. Designers and installers of electrical
systems should note that no matter what the size of the
service, the GEC is never required to be larger than a 3/0
AWG copper or a 250 kcmil aluminum or copper-clad
aluminum conductor. The reason for this limitation is that
the grounding electrode is unable to dissipate any more
current into the earth than can be carried by these
conductors. So even if the conductor size were increased,
the effectiveness of the grounding electrode system would
not be improved. As noted in Chapter 3, there may be
particular applications where design personnel oversize the
grounding electrode conductor because of the size of the
facility or the nature of the equipment which may be used
in the facility. For large facilities where outdoor equipment
and exposed conductors are used, available fault current
and maximum clearing times must be considered. IEEE Std
80 gives guidance for choosing conductor size and material.
THE GROUNDING ELECTRODE
Many different types of grounding electrodes are available,
some “natural” and some “made”. The natural types
include metal underground water pipe, the metal frame of
the building (if effectively grounded), copper wire or
reinforcing bar in concrete foundations or underground
structures. “Made” electrodes are specifically installed to
improve the system grounding or earthing. Made electrodes
include rods or pipe driven into the earth, metallic plates
buried in the earth or a copper wire ring encircling the
structure. Note that underground gas piping is not permitted
to be used as a grounding electrode. Likewise, aluminum
electrodes are prohibited by the NEC.
Other rules for the above electrodes also may apply. Those
in effect at the time of this writing include:
1. All water pipe electrodes must be in contact with
the earth for at least 10 feet and must be supple-
mented by an additional electrode as listed above.
(If the water pipe happens to be disconnected or if a
section of plastic pipe is installed at a later date, the
supplemental electrode would still be effective.)
2. The copper conductor in the concrete foundation or
footer must be #4 AWG or larger and must be at
least 20 feet if it is to be used as a grounding
electrode. If rebars are used, they must be 1/2 inch
(#4) or larger, bare or coated with an electrically
conductive material and at least 20 feet long. The
foundation must be in direct contact with the earth.
Practical Guide to Electrical Grounding 82
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