Part III of Article 250 provides the
important concept of a grounding electrode system,
where all grounding electrodes are required to be bonded
together and function as a system as indicated in Section
250.50. Rather than reliance on a single grounding electrode
to perform its function over the life of the electrical
installation, the NEC requires the formation of a
system of electrodes "that are present at each building
or structure served." There is no doubt that building a
system of electrodes adds a level of reliability and helps
ensure system performance over a long period of time, usually
the duration of the life of a building or structure. It is
important to understand that this requirement applies
generally to all buildings or structures and is not limited in
application to just commercial or industrial installations.
Residential construction is included in these requirements.
Several questions have been raised about
recent revisions to NEC 250.50, which provides
the minimum requirements for establishing a grounding
electrode system. Let’s take a closer look at what changed
in the Code and what these changes mean to the
electrical industry as a whole, beginning with a review of the
purpose of and requirements contained in 250.50. This rule
clearly calls for all grounding electrodes present at each
building or structure served to be used to form a grounding
electrode system. If there are multiple grounding electrodes
of any of the types listed in 250.52(A)(1) through (A)(6),
they all are required to be used to form the grounding
electrode system (see table 1).
250.52(A)(1) Metal Underground Water
Pipe |
| 250.52(A)(2)
Metal Frame of the Building or Structure |
| 250.52(A)(3)
Concrete-Encased Electrode |
| 250.52(A)(4)
Ground Ring |
| 250.52(A)(5)
Rod and Pipe Electrodes |
| 250.52(A)(6)
Plate Electrodes |
Table 1. These grounding electrodes are
required to be used where present. If any of these electrodes
are inherent to the building or structure, they shall be used
in the grounding electrode system.
Note that there are no options here or
choices of any combinations or quantities of grounding
electrodes that can be used to establish this required system.
The Code says all of them shall be used. As with many
other aspects of building construction, this may involve some
planning and coordination. The primary purpose of this rule is
to require all grounding electrodes to function as a system.
This is the grounding foundation of the electrical power
system for the building or structure. The goal is to establish
the most effective connection to the earth. Grounding
electrode effectiveness is dependent on it.
Let’s compare Section 250.50 of NEC-2002
with how it was revised for NEC-2005.
NEC-2002
250.50 Grounding Electrode System.
If available on
the premises at each building or structure served, each item
in 250.52(A)(1) through (A)(6) shall be bonded together
to form the grounding electrode system. Where none of these
electrodes are available, one or more of the electrodes
specified in 250.52(A)(4) through (A)(7) shall be installed
and used [emphasis added].
NEC-2005
250.50 Grounding Electrode System.
All grounding electrodes as described in 250.52(A)(1) through (A)(6) that are
present at each building or structure served shall be
bonded together to form the grounding electrode system.
Where none of these grounding electrodes exist, one or
more of the grounding electrodes specified in 250.52(A)(4)
through (A)(7) shall be installed and used [emphasis added;
see table 2].
250.52(A)(4) Ground Ring |
| 250.52(A)(5)
Rod and Pipe Electrodes |
| 250.52(A)(6)
Plate Electrodes |
| 250.52(A)(7)
Other Local Metal Underground Systems or Structures |
Table 2. Any one or more of the grounding
electrodes in this table are required to be installed and used
where any of the electrodes identified in 250.52(A)(1) through
(A)(6) are not there to use.
The More Things
Change the More Things Stay the Same
The language in Section 250.50 in NEC-2002 and in
previous editions resulted in varying interpretations and
inconsistencies in how the requirement was being applied to
installations in the field. This is one of the principle
reasons that necessitated the revision. The other primary
reason is to ensure that the benefits of all grounding
electrodes present are utilized. There really is no difference
in what has always been required by the Code. For some,
this may have been an eye opener or even a major change. This
is probably owing to the vague language used in this section
previously. The revised wording clarifies that when it comes
to establishing a grounding electrode system, there are few
options. If grounding electrodes are inherent to the
construction of a building or structure, they are required to
be used for the grounding electrode system—all of them.
Grounding
Electrodes to be Used Where Present
Previous editions of the Code used the words "if
available" in this section which was the primary reason
for the wide range of applications of this rule. The NEC
Manual of Style indicates that the Code shall
not contain requirements that are vague or unenforceable, and
the word "available" is one such word listed in the Style
Manual that should be avoided for that reason. Many in the
industry feel that there has been a significant change in
requirements of this section when in reality the requirement
has not been changed, just clarified. The challenge is for
those jurisdictions or other entities that have the
responsibility for interpreting the rules and applying them in
the field to installations and systems. This rule has been
clarified to be more in step with what was always the initial
objective. The words "if available" were removed and
the section was revised to provide needed clarification and
guidance as to which grounding electrodes are required to be
used to form the system. The Code says "all,"
which should remove any doubt as to what is required.
Where the words "if available"
were used in previous editions of the Code in this
section, they were to mean where there is an electrode that
meets the criteria of any of the grounding electrodes
specified in Section 250.52(A) (1) through (6), then it should
be used in the grounding electrode system. Let’s face it,
"if available" was not the best language to use in a
mandatory requirement of the Code.
Existing Buildings
or Structures
It is also recognized that buildings or structures will exist
in which electrical power systems will be used and installed.
For those situations this section of the Code has been
revised to include an exception that relaxes the requirements
for using the concrete-encased grounding electrode. The
technical committee was very specific to clearly indicate in
the exception that concrete-encased electrodes of pre-existing
buildings or structures are not required to be part of the
grounding electrode system where using them would involve
disturbing existing concrete. Otherwise the Code requires them to be used in the grounding electrode system,
which could include planning and coordination to make this
happen, just as planning and coordination is required for many
other aspects of the building construction. For many
jurisdictions this is not an issue because it has become the
common practice in new construction. For many contractors as
well as jurisdictions this will involve a different approach
or an adjustment, but should result in improved effectiveness
of an essential safety component of the electrical system, the
grounding electrode system.
Let’s look at an example to help put
things in perspective. A similar installation that should be
inspected is the equipotential bonding grid connections to the
rebar system for a swimming pool installation. The
installation and inspection are critical, and generally
coordination is established to accomplish both. This
inspection is generally required prior to the pool concrete
being installed. Jurisdictions typically coordinate this
inspection to verify the required bonding connections to the
rebar are installed. There is no option for this requirement
and no words such as "if available" are used in
these rules associated with these equipotential bonding grid
requirements. It is not optional and is required for safety.
It’s important. In comparison, at some point in time during
construction of the building or structure the reinforcing
steel in concrete footings is accessible (not available) for
making this installation, connection and inspection. The Code
is silent on the coordination of the work and always has been.
That is the responsibility of the contractors. Many inspection
jurisdictions have had success with cross-training inspectors
who already verify reinforcing rods in building footings for
compliance with the structural code to also verify the
concrete-encased grounding electrode conductor connection is
also installed. This is just one of several ways to handle
this from the inspection standpoint. There are many other ways
to handle this, but generally it just involves a small effort
in planning to yield the best result for safety.
Electrode
Effectiveness
Concrete-encased grounding electrodes normally have low
resistance in the connection to the earth and are proven
reliable as either the sole electrode or as part of a
grounding electrode system. The findings of Herbert G. Ufer*
that concrete-encased metal objects were effective in
providing improved grounding under adverse soil conditions
suggests that the reinforced framework of footings and
structural members buildings would provide an effective
grounding means and function. Ensuing tests of these
electrodes in high, medium, and low resistivity soils indicate
that the grounding capability of such reinforced footings (per
unit) is equivalent to that of other conventional grounding
electrodes and superior to them under high soil resistivity
conditions. In addition, where a much larger number of
concrete footings and structural column footings are used,
they provide more effective grounding under all soil
conditions than previous grounding systems.
For more detailed information on studies
relating to concrete-encased electrodes see 1970 IEEE Vol.
IGA-6, No. 4 July/August by Eugene J. Fagan and Ralph E. Lee
and the "Investigation of footing-type grounding
electrodes for electrical installations" by *Herbert G.
Ufer (IEEE Trans. Power Apparatus and Systems, Vol. 83 p.
1042-1048 October 1964).
Let’s address a couple of other common
questions since we’re into this topic fairly deep now
anyway. Where a footing is separated from the earth by a
plastic or other insulating (vapor) barrier, the
concrete-encased electrode is not present (see photos 1 and 2). The Code defines grounding electrode as "a device that
establishes an electrical connection to the earth." The
other challenge that will be encountered in the field is the
expanded use of encapsulated or coated reinforcing bars in
footings. Obviously, footings utilizing coated rebar would not
be suitable for use as concrete-encased grounding electrodes.
A Closer Look
The National Association of Home Builders has recently
submitted seven excellent questions related to the
requirements for concrete-encased grounding electrodes. Let’s
take a closer look at these questions and address each one
specifically.
1. Are more than a total of 20 feet of
reinforcing steel located in a footing required for compliance
with Sections 250.50 and 250.52? The 20 feet can include
pieces of rebar that are tied together in the usual manner. In
other words, if more than 20 feet of reinforcing steel is
located in the footing does all of the steel need to be bonded
together to form the electrode, or will a minimum of 20 feet
of the steel be the maximum length required? This is important
to understand as having footings at different levels (step
footings) may occur. This also may include concrete pads for
the support for interior columns, or many concrete piers to
support the entire structure. If all of the steel needs to be
bonded together this would require a bonding conductor to jump
from one footing to the other and possibly the pads, or to
each separate pier.
Answer: The Code establishes the
minimum requisite, which means one is required to do at least
that much. The answer is yes; a single section of rebar in a
length of 6.0 m (20 ft) satisfies the requirements of Section
250.52(A)(3). The minimum length of 6.0 m (20 ft) can be
accomplished by multiple reinforcing bars being bonded
together using the usual steel tie wire as indicated in the
last sentence of this section. To answer the second part of
this question, if there are multiple concrete-encased
electrodes in the same overall concrete footing that are not
tied together by the usual tie wire, then by the current
minimum requirements in the Code, they should be used
and bonded together. This situation can be compared to
conditions where multiple water pipe grounding electrodes are
in the same structure. They all are required to be used in the
grounding electrode system, without exception.
2. If steel reinforcement is not normally
contained in the footing design, can the installation of only
20 feet of ½ inch rebar in the footing serve as the primary
grounding electrode?
Answer: Yes. A length of reinforcing bar
not less than 6.0 m (20 ft) meeting the criteria in Section
250.52(A)(3) can be installed within and near the bottom of a
concrete footing that has little or no reinforcing rods and
would meet the Code criteria for concrete-encased
grounding electrode. However, the NEC does not require
a length of reinforcing bar to be installed in a concrete
footing where it is not required by any structural code.
3. If reinforcing steel and a metal water
pipe are used as the grounding electrode(s) for the system, is
a supplemental ground rod required, even if the 25 ohms or
less resistance cannot be achieved?
Answer: No. A grounding electrode system
does not have to meet the "25 ohms or less"
resistance requirements contained in Section 250.56. A single
electrode of the rod, pipe, or plate type is required to meet
the "25 ohms or less" requirement in 250.56. The
concrete-encased grounding electrode can supplement the water
pipe electrode as required by Section 250.53(D)(2). This
section indicates that the water pipe electrode must be
supplemented by any of the types specified in 250.52(A)(2)
through (A)(7). The concrete-encased electrode is included. A
rod, pipe, or plate grounding electrode would not be required
in this particular case.
4. If no footing is used in the
construction of the building, will the typical current method
of using the metal water piping and/or ground rods meet the
provisions for the grounding electrode system?
Answer: Yes. The requirements of 250.50
hold consistent in this application. If there were no
concrete-encased electrode to use, it would not have to be
installed and used. Section 250.50 requires grounding
electrodes that are there for use, to be used. If no
electrodes exist, then one or more of the grounding electrodes
in Section 250.52(A)(4) through (A)(7) must be installed and
used.
5. Where there may be a total of 20 feet of
rebar in a footing but it consists of a few small pieces
separated by many feet, would these small, remotely placed
pieces be required to be bonded together to serve as an
electrode?
Answer: No. The last sentence of the rule
in 250.52(A)(3) is permissive text according to the NEC
Style Manual. The smaller sections of reinforcing bars
could be bonded together using the usual steel tie wire to
form a 6.0 m (20 ft) length and then used, but it would not be
a requirement. This section also offers the alternative of
creating a concrete-encased grounding electrode by installing
a 6.0 m (20 ft) length of copper wire sized not less than 4
AWG copper, but this is also not a requirement, just another
option. Where the reinforcing bars are inherent to the
construction and meet the criteria provided in Section
250.52(A)(3), then they become required electrodes to be used
in the system.
6. Are new footings containing steel
reinforcement for an addition to an existing building exempt
from these provisions?
Answer: Possibly. I always try to stick to
more direct answers, but when the answer will be primarily
dependant on the judgment of the AHJ, it is the best approach.
There is no "yes" or "no" answer for this
question. The answer is going to be based on how each local
jurisdiction handles existing installations or alterations to
existing buildings or structures through their own specific
administrative provisions or local code rules.
7. If a new addition is constructed as
noted above and the existing electric service is upgraded,
would the reinforcing steel now need to be used as an
electrode, even if the panel is located remote from the new
addition?
Answer: Based on the information provided
in this question, the answer would be yes, the
concrete-encased electrode installed in the new footings would
be required to be used as part of the grounding electrode
system for the new service being installed. This situation
will be handled individually by each local jurisdiction
enforcing the code in that particular area. Each existing
installation is going to have to be handled case by case.
There are going to be situations where it would be practical
to accomplish using a concrete-encased grounding electrode in
a remodel or retrofit that includes installing new or
additional building footings. There will be other instances
where it will be less practical, if it is possible at all.
Judgment by the AHJ in those situations is going to have to be
applied.
Making the Best
Decision
There were many different discussions and interpretations of
the words "if available" as previously used in this
section. Some contended that if a building is constructed,
then at some point during the construction, the
concrete-encased electrode is required to be installed and
made part of the grounding electrode system. This is one
reason many jurisdictions adopt local amendments to the NEC
to require concrete-encased electrodes to be installed. This
eliminated any doubt as to whether or not it was required.
Many jurisdictions feel that it should not be viewed as a
coordination problem between subcontractors on the jobsite. In
contrast, there are those who feel that if the concrete
footings or foundation is already in place prior to the
electrical contractor’s presence on the construction site,
access to the rebar system for establishing a concrete-encased
electrode is not practical, and therefore is unavailable. Is
this the best approach? There are varying opinions when
addressing this question—each opinion is directly
proportional to the knowledge of the fundamental purposes of
grounding an individual has. The grounding electrode system is
an important, vital part of the electrical distribution safety
system and decisions relative to the grounding electrode
system and what grounding electrodes are used as part of this
system should be carefully considered. The revision to Section
250.50 should add considerable consistency to a requirement
that appears to have had a wide variety of inconsistent
application. This is a good thing for the industry, not to
mention the benefit to the electrode system in the way of
electrode effectiveness and lower resistances in the
connection to ground.
Summary
Section 250.50 has only been revised to clarify what was
previously required. Remember the goal when establishing a
grounding electrode or grounding electrode system for a
building or structure should be to strive for the best
possible connection to the earth. Concrete-encased electrodes
have a proven record of providing this. The grounding
electrode is an essential component of the electrical safety
system, just as a footing is an essential component of the
building’s structural integrity. Buildings or structures
require a solid foundation on which they should be built just
as the grounding electrode system serves as the foundation for
the electrical power service or system. Many jurisdictions are
not having an issue with this revision and understand it.
Hopefully this article has brought a more thorough perspective
of the revision. The revision to the NEC was not
intended to be a significant change in requirements, just to
clarify what was already required. It is apparent the
inconsistencies in installations and inspections were created
by the use of the words "if available." A look back
at previous editions of the NEC reveals that the words
"if available" have been used in the grounding
electrode rules back as far as the late 1920s and early 1930s.
The answers to these questions have been addressed based on
the minimum requirements of the NEC. As always, you
should check with the local authority having jurisdiction for
any local requirements that may amend or modify the minimum NEC requirements.
Michael J. Johnston is IAEI’s
director of education 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. |
