Article 250 of the National Electrical
Code provides the minimum requirements for grounding and
bonding. The title of the article was revised in the 2005 NEC development process to reflect what is actually covered by the
article. In the past, it seems many viewed the word grounding as an umbrella description, and viewed the concepts for both
bonding and grounding rules under the derivative of the word ground.
This is probably why there are some very apparent
misunderstandings about grounding and bonding, and which rules
apply when.
Photo
1. Bonding requirements for metal piping systems
Definitions
Grounding is the process of connecting something to ground
(the earth). For example, the NEC defines the term grounded
conductor as "a system or circuit conductor that is
intentionally grounded." Working hand in hand with that
definition is the word grounded which is defined as
"connected to earth or to some conducting body that
serves in place of the earth" (see figure
1).
On the other hand, the words bonding (or bonded) are defined as "the permanent joining
of metallic parts to form an electrically conductive path that
ensures electrical continuity, and the capacity to conduct
safely any current likely to be imposed" (see figure
2).
So, in simplest form, grounding or grounded means connecting to the earth, and bonding means
connecting metallic parts together to establish continuity and
conductivity between them. Both concepts have to be applied
for safety in electrical installations and systems (see figure
3). This article focuses on the requirements for bonding
metal piping systems as required by the NEC.
Bonding in General
Part V of Article 250 is titled "Bonding." This is
where the major requirements relating to bonding are found
within the article. To emphasize the point that many view
bonding and grounding as one and the same, look at Figure
250.4 in Article 250. Notice how the caption of the figure
says "Figure 250.4 Grounding," yet when one looks at
the figure, the box on the right hand side of the graphic is
titled "Bonding" and is interlocked with most of the
boxes on the left (see figure
4). This demonstrates graphically that grounding and
bonding work cooperatively through many of the requirements in
the article. It sounds like an opportunity for a Code change proposal, doesn’t it? The title of the figure should
be "250.4 Grounding and Bonding" just like the title
and scope of the article to reflect what is actually covered
within.
Bonding Metal
Water Piping Systems
Let’s take a closer look at the Code rules for
bonding metal water piping systems. When applying Code rules to installations and systems, it is important to
understand the reasons behind the requirements. The
requirements for bonding metal water piping systems, by
definition, are to establish required continuity and
conductivity with a capacity to handle any fault current
likely to be imposed. If metal water piping systems are not
bonded to the electrical system in a way that meets the
requirements of the Code, they can present a fire or
shock hazard within the building or structure because they
could become energized. This portion of the article covers
metal water piping systems, but similar rules apply to other
metal piping systems as well, such as metal gas piping
systems. We’ll discuss those requirements later in the
article.
Section 250.104(A) of Article 250 indicates
that metal water piping systems installed in or attached to a
building or structure are required to be bonded as required in
(A)(1), (A)(2), or (A)(3) of that section. Note the word system is used within this rule. This means the metal piping system
must be bonded. Short sections of metallic piping in a
nonmetallic piping system typically are not required to be
bonded because they do not constitute a full metal piping
system. Because many plumbing fixtures use nonmetallic
components, bonding both the hot and cold piping systems helps
ensure that both are electrically continuous.
Bonding jumper(s) for metal water piping
systems are required to be installed in accordance with
Sections 250.64(A), (B), and (E). These sections provide
installation rules for bonding jumpers. Connections of bonding
jumpers to piping system(s) [point(s) of attachment] are
required to be accessible (see photo
2).
Location of
Bonding Jumper Connection
The bonding jumper for a metal water piping system is required
to be installed to establish electrical continuity and
conductivity (an effective fault current path) to one of the
following locations:
1. The service equipment enclosure
2. The grounded conductor at the service
3. The grounding electrode conductor
where of sufficient size
4. One or more grounding electrodes used
for the service—see figure
5.
The bonding jumper is required be sized in
accordance with Table 250.66 except as permitted in
250.104(A)(2) and (A)(3). For example, if a building with an
600-ampere service has two 400-kcmil copper conductors in
parallel per phase, then applying Table 250.66 for 800 kcmil
reveals that the minimum size required for the water piping
system bonding jumper cannot be smaller than 2/0 copper or 4/0
aluminum or copper-clad aluminum. Bonding jumper connections
are required to meet the provisions in 250.8, meaning the
bonding jumper(s) are required to be connected using the
exothermic welding process, listed pressure connectors, listed
clamps, or other listed means. The 2005 UL General
Information for Electrical Equipment Directory states,
"Clamps suitable for use on copper water tubing are
marked for such use" (see photos 3 and 4).
Grounding
Electrode Conductor Often Serves Two Purposes
Sometimes the metal water piping grounding electrode conductor
for a building or structure also serves as the bonding jumper
for the metal water piping system. The sizing rules are the
same for both bonding jumpers. A major requirement to observe
is that the connection to a metal water piping system that is
a grounding electrode, in accordance with 250.52(A)(1), must
be made not more than 1.52 (5 ft) from its point of entrance
to the building or structure. This provides a reasonable
dimension so that if metal water piping is replaced with
nonmetallic piping, interrupting the electrical continuity,
the change is more obvious. The primary reason for this is to
keep the length where the water piping serves as the
connection to the grounding electrode to a minimum within the
building or structure. If the metal water piping is not a
grounding electrode, the connection of the bonding jumper in
accordance with 250.104(A) does not have be located within the
first 1.52 (5 ft) of the water piping system entry to the
building or structure.
Multiple Occupancy
Buildings
The NEC includes a metal water piping system bonding
alternative for multiple occupancy buildings that is found in
250.104(A)(2). This alternative is restricted for use only
where a certain condition can be satisfied: Metal water piping
systems to each individual occupancy are required to be
metallically isolated from all other occupancies, by using
nonmetallic water piping. If this isolation exists, the Code then permits a bonding jumper to be installed to the supply
feeder panelboard or switchboard supplying that occupancy with
a conductor sized in accordance 250.122 based on the rating of
the overcurrent device protecting the feeder (see figure
6). For example, if a multiple occupancy building had a
600-ampere service supplying six dwelling units, and there was
a 100-ampere panelboard in each occupancy, the bonding jumper
could be installed to the equipment grounding terminal bus in
the panelboard in that occupancy. The bonding jumper would be
permitted to be sized based on the rating of the overcurrent
device protecting the feeder (100 amperes) which in this
example would be a minimum of an 8 AWG copper bonding jumper.
Remember, the metal water piping system in each of the
occupancies must be metallically isolated from the other
occupancies before this alternative water piping system
bonding method may be used.
Separate Buildings
or Structures
A separate building or structure supplied by a feeder(s) or
branch circuit(s) [see 225.30 & .32] and in which metal
water piping systems are installed, is required to be bonded.
The water piping system is required to be bonded to the
building or structure disconnecting means enclosure where it
is located at the building or structure, to the equipment
grounding conductor run with the supply conductors, or to one
or more grounding electrodes used. The bonding jumper is
required to be sized in accordance with 250.66 based on the
size of the feeder or branch-circuit conductors supplying the
building or structure (see figure 7).
Bonding
Requirements for Other Metal Piping
Metallic piping systems other than water are also required to
be bonded, for the same reasons previously discussed. This
piping includes, but is not limited to, metal gas piping,
medical gas piping, and pneumatic piping. The rules for
bonding other metal piping are provided in 250.104(B). The National
Fuel Gas Code, NFPA 54, in Section 6.13 also requires
metal gas piping systems to be bonded. In fact, its
requirements are very similar to those in the NEC. (See
the sidebar that includes extracted text from NFPA 54-2002).
However, one must understand that metal gas
piping systems are not permitted to be used as grounding
electrodes. Section 250.52(B) prohibits this practice, but the
fine print note following this section reminds Code users that bonding requirements for such piping are provided
in 250.104(B). These bonding requirements are not optional.
"Other metal piping" is required to be bonded to any
of the following locations:
1. The service equipment enclosure
2. The grounded conductor at the service
3. The grounding electrode conductor
where of sufficient size
4. One or more grounding electrodes used
(see figure 8)
Minimum sizing requirements for bonding
jumpers for other metal piping systems are different from
those for metal water piping systems. Bonding jumper(s) are
required to be sized using 250.122, based on the rating of the
overcurrent device protecting the branch circuit that is
likely to energize the piping system. For example, if a gas
furnace is supplied by a 30-ampere branch circuit that
includes a 10 AWG copper equipment grounding conductor, the 10
AWG equipment grounding conductor for this circuit shall be
permitted as the bonding means. Remember: We’re dealing with
bonding, not grounding (see photos 5 and 6). The
connection point(s) of the bonding jumper to the other metal
piping system(s) are required to be accessible.
Figure
8. Bonding jumper connection locations for other metal
piping systems
Separately Derived
Systems
Metal water piping systems are required to be bonded to the
grounded conductor of each separately derived system. The
connection is required to be made at the nearest point
available on the metal water piping system(s) in the area
served by the separately derived system. This connection is
required to be made at the same point where the grounding
electrode conductor is connected to the system. The metal
water piping system bonding jumper from each separately
derived system is required to be sized in accordance with
Table 250.66, based on the largest ungrounded phase conductor
of the separately derived system (see figure
9).
A bonding jumper connected directly to a
separately derived system provides a direct path to the source
for ground-fault currents, and will attempt to help facilitate
overcurrent device operation in the event a circuit supplied
by the derived system secondary comes in contact with the
metal piping system. For all of the specific bonding
requirements for metal water piping systems to separately
derived systems, refer to 250.104(D) and the exceptions.
Where a common grounding electrode
conductor tap concept is used for multiple separately derived
systems in accordance with 250.30(A)(4), the metal water
piping system bonding requirements in 250.104(D)(3) must be
applied to the installation.
There are also bonding requirements for
exposed structural metal that is interconnected to form a
building frame, and is likely to become energized. The
installation and sizing requirements for bonding jumper(s) are
identical to those for metal water piping systems. Code rules for bonding structural metal are provided in 250.104(C)
and (D)(2) [see photo
7].
For more detailed information about the
bonding requirements of the NEC, please refer to
chapter 8 in IAEI’s Soares Book on Grounding and Bonding (9th edition). As always, consult the local
authority having jurisdiction in your area for any local codes
or regulations that would be required in addition to the
minimum requirements of the NEC.
Extracted text from
NFPA 54-2002
6.13 Electrical Bonding and Grounding.
6.13.1 Each aboveground portion of a gas piping
system that is likely to become energized shall be electrically
continuous and bonded to an effective ground-fault current path.
Gas piping shall be considered to be bonded when it is connected
to gas utilization equipment that is connected to the equipment
grounding conductor of the circuit supplying that equipment.
6.13.2 Gas piping shall not be used as a
grounding conductor or electrode. |
Michael J. Johnston is IAEI’s director of education, codes and standards and an IAEI principal member on CMP-5. Johnston was formerly employed as an electrical field inspections supervisor for the city of Phoenix, Arizona. He is fully certified in many areas. He is a member of the IBEW. He achieved both journeyman E-2 and master electrician E-1 licenses in the state of Connecticut. Additionally, he holds all IAEI certifications. He also holds ICC Electrical Inspections Certification. He is a member of the UL Electrical Council. |
