ERICO Practical Guide to Electrical Grounding - Manual (Page 58)

devices. This would include fittings such as bonding-type
locknuts and grounding bushings. These fittings are
designed to make good contact with the metal enclosure
and help to ensure good electrical continuity.
Bonding Other Enclosures. In addition to the service
equipment enclosures,
other noncurrent-carrying
enclosures are also required to be bonded by the NEC.
Section 250-75 requires that metal raceways, cable trays,
cable armor, cable sheaths, enclosures, frames, fittings and
any other metal noncurrent-carrying parts be bonded if they
are to serve as grounding conductors. This requirement
applies regardless of whether a supplementary equipment
grounding conductor is present. The purpose of this rule is
to ensure that these metallic components cannot become
energized because they are isolated from a low impedance
ground path. If these components were not properly bonded
and they were to become energized due to some fault
condition, the overcurrent device may not operate. This
would result in personnel being put at risk to serious
electrical shock hazards. This section also contains an
important requirement when making any electrical
connection. Prior to making any bonding or grounding
connection, installers must ensure that they have removed
any nonconductive coatings, such as paint, enamel or other
similar coatings, from the metal surface to which they are
making a connection. Failure to do so could drastically
increase the impedance of the ground path.
Bonding Over 250 Volts. Installers of electrical systems
frequently overlook the bonding requirements for electrical
circuits which operate at over 250 volts to ground. Section
250-76 requires that such circuits be bonded to ensure
electrical continuity of metal raceways or cable armors or
sheaths. The permissible methods which can be used to
achieve the required bonding are: threaded connections,
threadless couplings and connectors, bonding jumpers or
other approved devices. These methods are the same as
those used for service equipment with the exception of the
grounded conductor which is not permitted for over 250-
volt applications. Another installation requirement which
installers of electrical systems need to be especially aware
of is the use of 250 volt circuits where oversized concentric
or eccentric knockouts are present. If these types of
knockouts are encountered, one of the methods listed above
must be utilized to achieve the required bonding. An
exception to Section 250-76, however, does permit alternate
bonding methods where such knockouts are not
encountered or where they are encountered in a box or
enclosure which has been tested and the enclosure or box is
listed for the use. In such cases, any of the following
methods may be used in lieu of those listed above for
bonding circuits of over 250 volts to ground: threadless
couplings and connectors for metal sheath cables, double
locknut installations for RMC, IMC, fittings with shoulders
which seat firmly against the enclosure for EMT, flexible
metal conduit (FMC), and cable connectors, and other
listed fittings. Keep in mind that if the box or enclosure has
been listed for use with these concentric or eccentric
locknuts it will be identified or labeled as such. If a box or
enclosure is encountered and such identification is not
provided, one of the methods listed above must be used and
the exception is not applicable.
Bonding of Piping Systems and Exposed Structural
Steel. Section 250-80 of the NEC contains requirements
for bonding interior metal water piping systems, other
piping systems and structural steel. Section 250-81(a)
requires that metal underground water pipe which is in
direct contact with the earth for at least 10 ft be included as
part of the grounding electrode system. Installers of
electrical systems should note that even if for some reason
the metal water piping is not used as part of the grounding
electrode system it is still required to be bonded per Section
250-80. The purpose of such bonding is to ensure that the
metal water piping throughout the building or structure is at
the same potential to ground as the service ground. Keeping
the water piping at the same potential helps to ensure that
an electrical shock hazard could not exist if the metal piping
were to become inadvertently energized. Section 250-80 (a)
permits the bonding to occur to the service equipment
enclosure, the service grounded conductor, the grounding
electrode conductor or to the one or more grounding
electrodes that comprise the grounding electrode system.
Installers and designers of electrical systems should also
note that a 1996 NEC change now requires that the metal
water piping in areas served by a separately derived system
also be bonded to the grounded conductor of the separately
derived system. The most frequently encountered source of
separately derived systems is an isolation transformer. Keep
in mind that due to the magnetic coupling of the
transformer windings, grounds cannot be transferred across
such systems. A new grounding electrode system must be
established for each separately derived system. See Section
250-26 for a complete list of the requirements for
grounding separately derived systems.
Part (b) of Section 250-80 covers other interior piping
systems that are required to be bonded. Any interior piping
systems, such as, domestic well water, or any piping which
contains a liquid or a gas, and “may become energized,”
shall be bonded. Once again the permissible bonding
locations are to the service equipment enclosure, the
grounding electrode conductor, the service grounded
conductor or the one or more grounding electrodes that
Practical Guide to Electrical Grounding 50
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