| Definitions:
Bonding
(Bonded): "The permanent
joining of metallic parts to form an electrically
conductive path that will ensure electrical continuity
and the capacity to conduct safely any current likely to
be imposed." N
Bonding
jumper, Main: "The
connection between the grounded circuit conductor and
the equipment grounding conductor at the service."N
|
Main bonding jumper.
The main bonding jumper is one of the most critical
elements in the safety grounding system. This conductor is the
link between the grounded service conductor, the equipment
grounding conductor and in some cases, the grounding electrode
conductor. The primary purpose of the main bonding jumper is
to carry the ground-fault current from the service enclosure
as well as from the equipment grounding system that is
returning to the source. In addition, where the grounding
electrode conductor is connected directly to the grounded
service conductor bus, the main bonding jumper ensures that
the equipment grounding bus is at the same potential as the
earth.
Figure 5-1. Main bonding jumper
For a grounded system, Section 250-28
requires that an unspliced main bonding jumper be used to
connect the equipment grounding conductor(s) and the
service-disconnect enclosure to the grounded conductor of the
electrical system. The connection is required to be made
within the enclosure for each service disconnect.
An example of this is where two or more
service disconnecting means in individual enclosures are
grouped at one location. This type of installation often is
made with a wireway or a short section of busway installed
downstream from the metering equipment. In other cases, a
wireway or short section of busway is installed ahead of
metering and is supplied by a service lateral or
service-entrance conductors. Sets of service-entrance
conductors supply each of the service disconnecting means.
Service disconnecting means are installed from the wireway or
auxiliary gutter. (If there are nipples between the
disconnecting means and the metal or nonmetallic trough, the
trough meets the definition of a wireway from Article 362
rather than an auxiliary gutter from Article 374.) Section
250-28 requires a main bonding jumper be installed in each
service disconnect enclosure. As previously mentioned, Section
250-24(b) requires that the grounded service conductor be
brought to each service disconnecting means and be bonded to
the enclosure. The main bonding jumper is the means to
accomplish this requirement.
Figure
5-2. Main bonding jumper—multiple enclosures
The rules are a little different where more
than one service disconnecting means is in a common enclosure.
This equipment usually consists of listed switchboards,
panelboards or motor control centers.. Where more than one
service disconnecting means is located in an assembly listed
for use as service equipment, Section 250-28 Exception No. 1
permits the grounded service conductors to be run to a single
grounded conductor bus in the enclosure and then be bonded to
the assembly enclosure. This means that only one main bonding
jumper connection is required to be installed from the common
grounded conductor bus to the assembly enclosure. The sections
of the assembly are bonded together by means of an equipment
grounding conductor bus or by being bolted together.
Exception No. 2 to Section 250-28(b)
permits alternate means for bonding of high-impedance grounded
neutral systems. See Chapter Four of the IAEI Soares Book on
Grounding for methods and requirements for grounding
high-impedance grounded neutral systems. Also see NEC®
Sections 250-36 and 250-186 for the specific requirements and
allowances.
The main bonding jumper is permitted to
consist of a wire, bus, screw or other suitable conductor. It
must be fabricated of copper or other corrosion-resistant
material. Aluminum alloys are permitted where the environment
is acceptable. In addition, where the main bonding jumper
consists of a screw, it must have a green finish that is
visible with the screw installed. This green finish assists in
identifying the bonding-jumper screw from the other screws
that are on or near the neutral bus. See Sections 250-28(a)
and (b).
Figure
5-3. Main bonding jumper for listed assembly
Functions of Main Bonding Jumper.
The main bonding jumper performs three major functions:
1. Connecting the grounded service
conductor to the equipment grounding bus or conductor and the
service enclosure.
2. Providing the low-impedance path for the
return of ground-fault currents to the grounded service
conductor. The main bonding jumper completes the ground-fault
return circuit from the equipment through the service to the
source as is illustrated in Figure
5-4.
3. Connecting the grounded service
conductor to the grounding electrode conductor. Under certain
conditions given in Section 250-24(a)(4), it is permitted to
connect the grounding electrode conductor to the equipment
grounding terminal bar rather than to the terminal bar for the
grounded service conductor. This scheme is common on larger
switchboard services and is necessary for proper operation of
certain types of equipment ground fault protection systems.
See Chapter 15 of the IAEI Soares Book on Grounding for
additional information on this subject.
Size of main bonding jumper in listed
enclosures.
Where listed service equipment consisting of a
switchboard, panelboard or motor control center is installed,
the main bonding jumper that is provided with the equipment is
rated for the size of conductors that would normally be used
for the service. The method for sizing of the main bonding
jumper in listed service equipment is found in Underwriters
Laboratories Safety Standard for the equipment under
consideration and is verified by the listing agency.
Therefore, if a main bonding jumper that is a bus bar, strap,
conductor, or screw is furnished by the manufacturer as part
of the listed equipment, it may be used without calculating
its adequacy. Section 384-3(c) requires the equipment
manufacturer to provide the main bonding jumper.
Size of main bonding jumper at single
service-disconnect or enclosure.
Since the main bonding jumper must carry the full
ground-fault current of the system back to the grounded
service conductor (which may be a neutral), its size must
relate to the rating of the service conductors which supply
the service. The minimum size of the main bonding jumper is
found in Table 250-66 as required by Section 250-28(d). This
relationship is based on the conductor’s ability to carry
the expected amount of fault current for the period of time
needed for the overcurrent device to open and stop the flow of
current.
For example, where 250 kcmil aluminum
service-entrance conductors are installed, the main bonding
jumper is found to be No. 4 copper or No. 2 aluminum by
reference to Table 250-66.
The size of the main bonding jumper does
not directly relate to the rating of the service overcurrent
device. Do not attempt to use Table 250-122 for this purpose.
Table 250-122 gives the minimum size of equipment grounding
conductors for feeders and circuits on the load side of the
service.
Figure
5-5. Main bonding jumper at single disconnect
Sizing of main bonding jumper for
parallel service conductors.
Where service conductors are installed in parallel,
(connected together at each end to form a larger conductor)
the total circular mil area of the conductors connected in
parallel for one phase are added together to determine the
minimum size main bonding jumper required. See Section
250-28(d). For example, where three 250 kcmil conductors are
connected in parallel per phase, they are treated as a single
750 kcmil conductor. By reference to Table 250-66 the main
bonding jumper, if aluminum service-entrance conductors are
used, is 1/0 copper or 3/0 aluminum.
Where the service-entrance conductors are
larger than the maximum given in Table 250-66, Section
250-28(d) requires the main bonding jumper to be not less than
12½ percent (0.125) of the area of the largest phase
conductors.
This is illustrated by the following
example:
Three 500 kcmil copper conductors are
installed in parallel as service-entrance conductors.
3 x 500 kcmil = 1500 kcmil.
1500 x .125 = 187,500 circular mils.
Since a 187,500 circular mil conductor is
not a standard size, we next refer to Chapter 9, Table 8 to
find the area of conductors.
The next conductor exceeding 187,500
circular mils is a No. 4/0 AWG conductor which has an area of
211,600 circular mils. It is always necessary to go to the
next larger size conductor since the 12½ percent size is the
minimum size permitted.
Follow a similar procedure for determining
the minimum size main bonding jumper required for other sizes
of parallel service-entrance conductors.
Figure 5-6. Main
bonding jumper for parallel runs
Bonding of service conductor
enclosures.
Special rules are provided for bonding enclosures on the
line side of the service disconnecting means. This is due to
the fact that this equipment does not have overcurrent
protection on its line side such as feeders and branch
circuits have. Fault current of sufficient magnitude must flow
during a short period of time to allow the fuse on the line
side of the utility transformer to open. The level of fault
current and particularly the duration the current may flow
could be much larger than would flow in a feeder or branch
circuit as there is not an overcurrent device in series with
the conductor.
The basic rule is that all metallic
enclosures that contain a service conductor must be bonded
together. The bonding ensures that none of the equipment
enclosures can become isolated electrically and become a shock
hazard should a line-to-ground fault occur. The bonding also
provides a low impedance path for fault current to flow in so
the fuse or circuit breaker on the line side of the electric
utility transformer will open.
Sizing of equipment bonding jumper on
line (supply) side of service.
Equipment bonding jumpers on the line side of the service
and main bonding jumper must be sized to comply with Table
250-66. This is required by Section 250-102(c). For example,
where 250 kcmil copper conductors are installed as
service-entrance conductors, Table 250-66 requires a No. 2
copper or 1/0 aluminum bonding jumper.
Where the sum of the circular mil area of
the service-entrance phase conductors exceeds 1100 kcmil
copper or 1750 kcmil aluminum, the equipment bonding conductor
must be not less than 12½ percent (0.125) of the area of the
ungrounded phase conductors.
Figure
5-7. Size of equipment bonding jumper on line side of
service
Sizing of equipment bonding jumper
for parallel conductors.
Two methods are provided for bonding service raceways that
are installed in parallel. The first method is to add the
circular mill area of the service-entrance conductors per
phase together and treat them as a single conductor. The
bonding jumper size is determined from Table 250-66 and is
connected to each conduit bonding bushing in a
"daisy-chain fashion." This method often results in
an equipment bonding jumper that is quite large and difficult
to work with.
For example, if five 250 kcmil copper
conductors are installed in parallel for a phase, the
equipment bonding jumper for bonding the metal raceways must
not be smaller than 3/0 copper.
Figure
5-8. Size of equipment bonding jumper on line side of
service
This is determined as follows:
Five x 250 kcmil = 1250 kcmil.
1250 kcmil x .125 = 156,250 circular mils.
The next larger conductor found in Chapter
9, Table 8 is 3/0 with an area of 167,800 circular mils.
In this case, a 3/0 copper equipment
bonding conductor must be connected from the grounded service
conductor or equipment grounding bus to each metal raceway in
series (daisy-chain fashion from one raceway to another).
A more practical method of performing the
bonding for services supplied by multiple raceways may be to
connect an individual bonding conductor between each raceway
and the grounded service conductor terminal bar or equipment
grounding bus. This is permitted by Section 250-102(c). This
will usually result in a smaller equipment bonding conductor
which is easier to install.
Again, using the example above and
referring to Table 250-66, the minimum size equipment bonding
conductor for the individual raceways containing 250 kcmil
copper service-entrance conductors is No. 2 copper or 1/0
aluminum. A properly sized equipment bonding jumper is
installed from the terminal bar for the grounded service
conductor or from the equipment grounding terminal bar to each
conduit individually.
Different conductor material.
Section 250-28(d) provides instructions on sizing the main
bonding jumper or equipment bonding jumper on the supply side
of the service where different conductor materials are used
for the service-entrance conductors and the bonding jumper.
The procedure involves assuming the phase conductors are of
the same material (copper or aluminum) as the bonding jumper
and that they have an equivalent ampacity to the conductors
that are installed. This is illustrated as follows:
Assume aluminum phase conductors and a
copper bonding jumper are installed.
Three 750 kcmil Type THW aluminum
conductors are installed.
From Table 310-16, 385 amperes x 3 = 1155
amperes. The smallest type THW copper conductor that has an
equivalent rating is 600 kcmil with an ampacity of 420.
Next, determine the total circular mil area
of the copper conductors.
Three x 600 kcmil = 1800 kcmil.
1800 kcmil x .125 = 225 kcmil.
The next standard size is 250 kcmil copper
which is the minimum size bonding jumper permitted to bond
equipment at or ahead of the service equipment in this
example.
Bonding service equipment enclosures.
The Code requires that electrical continuity of service
equipment and enclosures that contain service conductors be
established and maintained by bonding. The items required to
be bonded together are stated as follows in Section 250-92(a):
(1) The service raceways, cable-trays,
cablebus framework or service cable armor or sheath.
(2) All service equipment enclosures
containing service conductors, including meter fittings, boxes
or the like, interposed in the service raceway or armor.
(3) Any metallic raceway or armor which
encloses the grounding electrode conductor. (This subject is
covered in detail in Chapter 7 of this text.)
An exception to this requirement for
bonding at service equipment is mentioned in Section
250-92(a)(1). It refers to Section 250-84 which has rules on
underground service cables that are metallically connected to
the underground service conduit. The Code points out that if a
service cable contains a metal armor, and if the service cable
also contains an uninsulated grounded service conductor which
is in continuous electrical contact with its metallic armor,
then the metal covering of the cable is considered to be
adequately grounded.
Figure
5-9. Bonding service equipment enclosures
Use of neutral for bonding on line
side of service.
Section 250-94(1) permits the use of the grounded service
conductor (may be the neutral) for grounding and bonding
equipment on the line side of the service disconnecting means.
This is also permitted by Section 250-142(a)(1). (Two other
applications of this bonding are explored in later chapters of
the IAEI Soares Book on Grounding.) Often, connecting the
grounded service conductor to equipment such as meter bases,
current transformer enclosures, wireways and auxiliary gutters
is the most practical method of bonding these enclosures.
Usually, self-contained meter sockets and
meter-main combination equipment are produced with the
grounded conductor terminals or bus (often a neutral) bonded
directly to the enclosure. The enclosure is then effectively
bonded by the connection of the grounded circuit conductor to
these terminals. No additional bonding conductor connection to
the meter enclosure is required. Current from a ground fault
to the meter or meter-main enclosure will return to the source
by the grounded service conductor (may be a neutral) and,
hopefully, will allow enough current to flow in the circuit to
operate the overcurrent protection on the line side of the
utility or other transformer.
Figure
5-10. Use of neutral for bonding on line side of service
In addition, meter enclosures installed on
the load side of the service disconnecting means are permitted
to be grounded (bonded) to the grounded service conductor
provided that:
(a) Service ground-fault protection is not
installed; and
(b) The meter enclosures are located near
the service disconnecting means. (No distance is used to
clarify what is meant by the word "near."), and
(c) The size of the grounded circuit
conductor is not smaller than the size specified in Table
250-122 for equipment grounding conductors. See Section
250-142(b) Exception No. 2.
Means of bonding at service
equipment.
The methods for bonding at service equipment are outlined
in Section 250-94. These requirements for bonding are more
restrictive at services than downstream from the service. The
reason this is so important is service equipment and
enclosures may be called upon to carry heavy fault currents in
the event of a line-to-ground fault. The service conductors in
these enclosures have only short-circuit protection provided
by the overcurrent device on the line side of the utility
transformer. Only overload protection is provided at the load
end of the service conductor by the overcurrent device. This
is one of the reasons the Code limits the length of service
conductors inside of a building.
Figure 5-11. Methods
of bonding service equipment
Bonding of these enclosures is to be done
by one or more of the following methods from Section 250-94:
(1) Bonding to the grounded service
conductor through the use of exothermic welding, listed
pressure connectors such as lugs, listed clamps, or other
listed means. These connections cannot depend solely upon
solder.
(2) Threaded couplings and threaded bosses
in a rigid or intermediate metal conduit system where the
joints are made up wrench-tight. Threaded bosses include hubs
that are either formed as a part of the enclosure or are
supplied as an accessory and installed according to the
manufacturer’s instructions.
(3) Threadless couplings and connectors are
permitted where they are made up tight for rigid and
intermediate metal conduit and electrical metallic tubing and
metal-clad cables.
(4) Other approved devices such as
bonding-type locknuts and bushings.
Figure
5-12. Bonding fittings
Bonding jumpers are required to be used
around concentric or eccentric knockouts that are punched or
otherwise formed so as to impair an adequate electrical path
for ground-fault current. It is important to recognize that
concentric and eccentric knockouts in enclosures such as
panelboards, wireways and auxiliary gutters have not been
investigated for their ability to carry fault current. Where
any of these knockout rings remain at the conduit connection
to the enclosure, they must always be bonded around to ensure
an adequate fault-current path.
The Code states here that "Standard
locknuts or bushings shall not be the sole means for the
bonding required by this section." This statement does
not intend to prevent the use of "standard" locknuts
and bushings, it is just that they cannot be relied upon as
the sole means for the bonding that is required by this
section. "Standard" locknuts are commonly used
outside the enclosure on conduit that is bonded with a bonding
bushing or bonding locknut inside the enclosure. Standard
locknuts are used to make a good, reliable mechanical
connection as required by Section 300-10.
Parallel bonding conductors.
Section 250-102(c) requires that where service-entrance
conductors are paralleled in two or more raceways or cables
and the equipment bonding jumper is routed with the raceways
or cables, the equipment bonding jumper must be run in
parallel.
In this case again, the size of the bonding
jumper for each raceway is based upon the size of the
service-entrance conductor in the raceway by referring to
Table 250-66.
Figure
5-13. Parallel bonding conductors
Grounding and bonding of remote
metering.
As mentioned before, Section 250-92(a) requires all
equipment containing service conductors to be bonded together
and to the grounded service conductor. This includes remote
(from the service equipment) meter cabinets and meter sockets.
Grounding and bonding of equipment such as
meters, current transformer cabinets and raceways to the
grounded service conductor at locations on the line side of
and remote from the service disconnecting means increases
safety.
This equipment should never be grounded
only to a grounding electrode such as a ground rod. Figures 5-14 and 5-15 show
why. If a ground-fault occurred at this line-side equipment,
and it is not bonded as required, the only means for clearing
a ground fault would be through the grounding electrodes and
earth. Given the relatively high impedance and low
current-carrying capacity of this path through the earth and
high resistance of grounding electrodes such as rods, little
current will flow in this path. This leaves the equipment
enclosure(s) at a dangerous voltage above ground potential
just waiting to shock or possibly electrocute a person or
animal that may contact it. The voltage drop across this
portion of the circuit can easily be calculated by using Ohms
Law. (Resistance times the current gives the voltage.) There
are many records of livestock being electrocuted while
contacting electrical equipment that was improperly grounded.
Sections 250-2 and 250-54 require that the earth not be used
as the sole equipment grounding conductor or fault-current
path.
The most practical method for grounding and
bonding this line-side equipment is to bond the grounded
service conductor to it. As can also be seen in Figures 5-14 and 5-15, a
ground fault to the equipment will have a low impedance path
back to the source through the grounded service conductor.
This will allow a large current to flow in the circuit to
cause the overcurrent protection on the line side of the
transformer to clear the fault.
Supplementary grounding electrodes.
In accordance with Section 250-54, it is permissible to
install a grounding electrode at the remote meter location
shown in Figures 5-14 and 5-15 to
supplement the grounded service conductor. This Code section
refers specifically to grounding electrodes supplementing the
equipment grounding conductors. Some electric utilities
require a grounding electrode at meter equipment installed
remote from service equipment such as on poles. The Code in
Section 230-66 makes it clear that individual meter socket
enclosures are not to be considered service equipment. The
same is true for metering equipment installed in remote
current-transformer enclosures. As mentioned earlier, it is
critically important that these meter enclosures be properly
bonded as they contain service conductors.
This additional grounding electrode will
attempt to keep the equipment at the earth potential that
exists at the meter location. In addition, the electrodes at
the remote meter and at the service location are bonded
together by the grounded service conductor installed between
the metering and service equipment. This brings the
installation into compliance with Section 250-58 which
requires a common grounding electrode or where two or more
electrodes are installed, they must be bonded together.
As previously stated, these grounding
electrodes should never be used as the only means for
grounding or bonding these enclosures or to carry fault
current.
More extensive discussion of this subject
is found in Chapter Six of the IAEI Soares Book on
Grounding.
Bonding of multiple service
disconnecting means.
Installation of multiple services as permitted by Section
230-2(a) through (d) and installations of services that have
multiple disconnecting means can take several forms.
Additional services are permitted by Section 230-2 for:
(a) Fire pumps, emergency, legally
required, standby, optional standby or parallel power
production systems.
(b) By special permission, for multiple
occupancy buildings where there is no available space for
service equipment that is accessible to all occupants, or, for
a single building or structure that is large enough to make
two or more services necessary.
(c) Capacity requirements; where the
service capacity requirements exceed 2,000 amperes at 600
volts or less, where load requirements of a single-phase
installation is greater than the serving utility normally
provides through a single service, or by special permission
(related to capacity requirements).
(d) Different characteristics of the
services such as different voltages, frequencies, or phases,
or for different uses, such as for different rate schedules.
The basic rule for sizing of the equipment
bonding jumper for bonding these various configurations is
found in Section 250-102(c). This section requires that the
bonding jumpers on the line side of each service the main
bonding jumper be sized from Table 250-66. Also, the size of
the bonding jumper for each raceway is based on the size of
service-entrance conductors in each raceway. As discussed
earlier, conductors larger than given in Table 250-66 are
required for larger services. Since different sizes of
service-entrance conductors may be installed at various
locations, the minimum size of the equipment bonding conductor
and main bonding jumper is based on the size of the
service-entrance conductors at each location.
For example, the appropriate size of
bonding jumper for the installation in Figure
5-16 with the assumed size of conductors is as follows:
(all sizes copper)
| Service-Entrance
Conductor |
Bonding
Jumper |
| a. 500 kcmil in service mast |
1/0 |
| b. 1000 kcmil in
wireway |
2/0 |
c. 300 kcmil to 300 ampere service |
No. 2 |
| d. 3/0 to 200 ampere service |
No. 4 |
| e. No. 2 to 125 ampere
service |
No. 8 |
A practical method for bonding the current
transformer enclosure and wireway (sometimes referred to as a
"hot gutter") is to connect the grounded service
conductor directly to the current transformer enclosure or
wireway. This may be done by bolting a multi-barrel lug
directly to the wireway and connecting the neutral or grounded
service conductors to the lug. Be sure to remove any
nonconductive paint or other coating that might insulate the
connector from the enclosure.
As previously discussed, the grounded
service conductor must also be extended to each service
disconnecting means and be bonded to the enclosure.
Excerpted from Chapter 5 of the IAEI Soares
Book on Grounding, 7th Edition
Phil has written and illustrated several
authoritative books on electrical systems. He is acting
chair of NEC CMP-5, chairman of IAEI Neon
Installation Manual, and a member of the Inspector
Certification Exam Committee. He is a former member of
NFPA Standards Council and NEC Technical Correlating
Committee, past chair of NFPA Electrical Section, former
member of NEC CMP-1 and 17, and past chairman of CMP-19. |
