Understanding the Zone
Area Classification Method
in the NEC
By Edward M. Briesch and Mark C. Ode
T
he division classification method has been used
in the United States since being incorporated into
NEC 1920 to cover electrical and electronic equipment and wiring in extra-hazardous areas where fire
or explosion hazards may exist from flammable and
combustible liquids, vapors, or gases. The use of the
term extra-hazardous locations implied that an electrical installation was already a hazardous installation
but where used in areas, rooms, or compartments with
some chemicals, it was more hazardous than usual. In
NEC 1931, the term classifications was added to the
NEC along with Class I, Class II, and III. The term extra-hazardous has been dropped from the NEC. These
areas are now called hazardous (classified) locations
which differentiates the hazardous chemical areas with
electrical equipment from hazardous health areas.
The NEC 1996 was the first edition to address the
new hazardous (classified) method of zone classification. The NEC 1996 identified the newly introduced
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zone classification method as Class I, Zone 0, 1, and 2
and provided a basis to build upon in subsequent editions of the NEC.
This new method of protection was based primarily
upon the international method of zone protection and
covered only limited concepts of the international technique. Definitions for this new method of zone protection were provided in a new Article 505 but contained
limited information and application techniques. The
NEC 1996 tied the division type of hazardous location
articles together with the new zone type of protection
techniques but the information was rather difficult to
find since the new zone protection text was inserted
into various places within the general hazardous locations articles. Even though the bulk of the information
was in Article 505, it was necessary to extract some of
the zone classification information from Articles 500
and 501 primarily covering Class I, Division 1 and 2
hazardous (classified) locations.
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UNDERSTANDING THE ZONE AREA CLASSIFICATION METHOD IN THE NEC
In NEC 1999, all references to the zone concept of
hazardous (classified) locations were removed from
Article 500 for general hazardous location concepts and
from most of Article 501 for Class I, Division 1 and 2
installations. A fine print note was left in 500-1 to provide the user with direction to Article 505 containing
the bulk of the information on the zone concept of
area classification.
This single article concept for zone applications provided a more user-friendly method to access pertinent
information where applying the zone concept. Article
505 provides information on equipment and wiring
methods that are permitted in a Zone 0, Zone 1, and
Zone 2 classified area. All other applicable rules would
apply to these installations, except as amended by Article 505 or amended by those requirements in Article
504 dealing with intrinsically safe installations.
The zone concept was further enhanced in NEC 2002
by changing the basic layout of Article 505 to fit the
new numbering sequence required by the NEC Style
Manual. All definitions were moved to 505.2; 505.3
covers other applicable rules located in other articles;
and 505.4 provides general information pertaining to
zone installations. This was done to provide consistency of section numbering with other similar articles.
Further changes to Article 505 made it completely
independent of Articles 500 and 501. Section 505.15
for wiring methods in a zone area was expanded to
match its counterpart in 501.4 for wiring methods in a
Class I, Division 1 and 2 area. Section 505.16 was inserted to cover the sealing requirements for zone methods rather than just referring back to 501.5 as was done
in the previous Codes.
Location and General Requirements for
Article 505
In zone applications, hazardous locations are classified in accordance with the properties of flammable
liquids, gases, or vapors that may be present in the area
where electrical equipment is installed. The liquids,
gases, or vapors must be likely to form ignitable concentrations and the quantities of the material must be
sufficient to pose a hazard when mixed with a sufficient quantity of air. These conditions are similar or
often the same as the conditions required for a similar
location being designed and installed in the division
concept.
Each area should be considered individually in determining the classification and care should be taken
to not over-classify, as well as to not under-classify. Electrical equipment should be installed and connected in
an area that is outside the hazardous (classified) location; however, where this not possible or practical, then
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special electrical equipment must be installed in the
hazardous (classified) location. All of the factors that
are normally associated with division area classification
would apply to a zone classification, such as temperature, density or molecular weight of the substance, air
circulation, quantity, pressure, and so forth.
Highly oxygenated areas are outside the scope of
both the zone classification and the division classification systems. These areas have had the ignition and
burning characteristics of materials changed by exceeding the normal volume of oxygen of air that is mixed
with the flammable gas or vapor. Where highly oxygenated areas are encountered, the user should refer to
the specialized documents that deal with these types
of areas and comply with the requirements for installation of electrical equipment in those areas.
Areas where pyrophoric materials are present or
handled are also outside the scope of both the zone
classification and the division classification systems.
Pyrophoric materials can be ignited just by introducing the material to air. Where these chemicals are used,
installation of special electrical equipment is usually
not necessary. However, care should be taken since
there may be other combustible chemicals in the area
that may require special electrical equipment.
Since much of the electrical equipment built for the
European and other world markets incorporate metric
measurement and sizing, it may be necessary to provide adapters to convert from the standard NPT threads
that are used in the United States to the metric threaded
entries or fittings provided with the equipment. If
equipment with metric threads is provided for use in
the United States, the NEC requires that metric to NPT
adapters be included with the equipment or be marked
to identify the threaded entries as metric threads.
NPT threads provide a ¾ inch per foot taper. Most
standard conduit threading equipment available in the
United States uses NPT thread. By conforming to a standard threaded system and providing a standard to
metric adapter, electrical equipment with metric threads
can be readily and safely adapted. Good electrical and
mechanical continuity of conduit connections by the
proper threaded adaptor to the enclosure can prevent
sparking between the fitting, the conduit, and the enclosure during an electrical fault. The proper fitting
can also ensure the integrity of the explosionproof or
flameproof path by permitting the venting and subsequent cooling of the hot flaming gases after an explosion occurs within the enclosure.
Definitions of Zone Classification
Zone 0 Locations
A Zone 0 location involves ignitable concentrations
of flammable gases or vapors that are present conNOVEMBER/DECEMBER 2002
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UNDERSTANDING THE ZONE AREA CLASSIFICATION METHOD IN THE NEC
tinuously or present for long periods of time. This
classification includes locations inside tanks in which
flammable or combustible liquids, gases, or vapors
erminals
Photo 1. Flameproof Control Switch with Increased Safety TTerminals
are stored or locations such as between the inner or
outer roof sections of a floating roof tank containing
volatile flammable liquids. If venting is provided to
allow the gases or vapors to escape from inside the
tank during the process of filling, there may be an
area outside of the tank near the opening of the vent
that is considered to be a Zone 0 location. In some
cases, the area around the vent outlet would be considered to be a Class I, Zone 1 location, depending
upon the amount of air circulation and the type of
material involved.
For Zone 0 locations, it is recommended that electrical equipment be located in an area outside the Zone
0 location. This is not always feasible since measurement equipment and other electrical equipment may
be necessary for the function and monitoring of the
system. In a Zone 0 location, intrinsically safe apparatus and the associated intrinsically safe circuit are the
only wiring methods and protection technique that are
permitted to be installed.
Zone 1 Locations
A Zone 1 location, similar to Class I, Division 1 location, is one in which there may be ignitable concentrations during normal operating conditions or ignitable concentrations exist frequently from repair or
maintenance of the equipment. A Zone 1 would be a
location where equipment breakdown or faulty operation of the equipment could release ignitable con88
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centrations of gases or vapors and simultaneous failure of electrical equipment. Areas that are adjacent to
a Class I, Zone 0 location where ignitable concentrations of gases or vapors could migrate are also considered to be Zone 1 locations.
The normal classification for Zone 1 locations does
not take into consideration catastrophic leaks. It does,
however, consider small leaks from valves, pump packing glands, and other similar leakage considered common for equipment handling hazardous materials.
Zone 2 Locations
A Zone 2 location and a Division 2 location are essentially the same and have similar definitions. It is a
location where vapors or gases are not likely to occur
during normal operation but if a leak does occur, it
would normally be for a short duration of time.
A Zone 2 location is one where the liquids, gases, or
vapors are handled, processed, or used in an area but
are normally confined within closed piping or containers that can only leak as a result of a rupture or some
other abnormal condition.
Positive mechanical ventilation is often used to prevent the buildup of ignitable concentrations of gases
or vapors and, thus, to reclassify or declassify the area.
Should the ventilation system break down, an ignitable concentration could require classification as a Zone
2 location.
An area that is adjacent to a Class I, Zone 1 location
from which ignitable concentrations of flammable gases
or vapors could migrate would be a Zone 2 location,
unless positive mechanical ventilation has been provided and there are safeguards to ensure against ventilation failure.
Photo 2. Intrinsic Safe Telephone
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UNDERSTANDING THE ZONE AREA CLASSIFICATION METHOD IN THE NEC
Protection Techniques for Zone
Locations
Many of the protection techniques for zone locations
in the NEC are modeled after the International
Electrotechnical Commission (IEC) standards and the
International Standards Organization (ISO) standards.
These international organizations have been primarily
responsible for providing standards on electrical equipment for many countries.
Underwriters Laboratories Inc. (UL) has further
Photo 3. Battery Operated Intrinsic Safety (IS) Gas Detector
adapted these IEC documents with minor changes into
a multi-part American National Standards Institute
(ANSI) document, ANSI/UL 2279-2001, the Standard for
Electrical Equipment for Use in Class I, Zone 0, 1, and
2 Hazardous (Classified) Locations. This UL standard
is used for testing and listing of electrical equipment
for use in zone classified areas and provides the manufacturer and the user with a detailed description of the
particular protection techniques that can be used for
these areas.
Section 505.4(B), fine print notes provides standard
references for important information involving hazardous locations. Each protection technique permitted in
a zone installation is covered in 505.8 and in fine print
notes that immediately follow many of these protection techniques.
1. Flameproof (Type “d”) Equipment
Flameproof (Type “d”) is a type of protection that is
similar to explosionproof equipment protection that has
been one of the most common protection techniques
used in the United States. In this protection technique,
the equipment is in an enclosure that has been tested to
withstand an internal explosion of a gas- or vapor-to-air
mixture that can penetrate into the interior of the enclosure. The enclosure must contain the flame and the exwww.iaei.org
plosion pressure without damaging the enclosure and
without permitting the flame to leave the enclosure
through any joints or other openings in the enclosure
and without igniting the surrounding gases.
2. Purged and Pressurized (Type “p”) Equipment
Purged and pressurized (Type “p”) is a type of protection that uses inert gas or instrument quality air as a protective gas to purge the inside of the enclosure of any hazardous quantity of flammable gases or vapors. The enclosure is then kept pressurized at a pressure high enough
above the outer atmosphere surrounding the enclosure to
prevent the flammable gas or vapor from re-entering the
enclosure. This guards against an ignitable concentration
of gas accumulating within the enclosure and then being
ignited by an arcing or sparking part or hot electrical equipment. This protection technique also permits a general type
enclosure to be used rather than an explosionproof or
flameproof enclosure, as would normally be the case.
3. Intrinsic Safety (Types “ia” or “ib”) Equipment
Intrinsic Safety (Types “ia” or “ib”) is a type of protection technique that uses an apparatus that limits the
maximum level of current and voltage measured as
energy (usually in millijoules) under normal or fault
conditions that can be delivered into the hazardous
location. The intrinsically safe associated apparatus
providing this energy limitation has a further identification marking on the equipment that indicates the
type of intrinsic safety. The marking will be “ia” or “ib”
and should be located on the nameplate of the apparatus. This equipment should provide a level of safety
that, even in a double fault condition (“ia”) or a single
fault condition (“ib”), there will not be enough ignition energy to ignite the gas or vapor in that area.
4. Protection (Type “n”) Equipment
Protection (Type “n”) equipment is a protection technique applied to electrical equipment which does not
have enough ignition energy to ignite the gas or vapor
during normal operation and is not likely to have a
fault that could cause ignition. This type of protection
is similar to the nonincendive circuits and equipment
used in Class I, Division 2 locations. If there is a possibility of a fault occurring within the system, then some
other method of protection should be chosen or extra
protection against faults should be incorporated.
5. Oil Immersion (Type “o”) Equipment
Oil immersion (Type “o”) equipment is a type of
protection in which the electrical equipment is immersed in a protective fluid of nonconductive silicone
or mineral oil. The fluid level should be such that the
electrical parts and any arcing or sparking parts of the
oil-encased equipment are immersed and the liquid
always covers the electrical parts. This ensures that the
gas or vapor located above the arcing or sparking parts
or the gas or vapor located exterior to the enclosure
NOVEMBER/DECEMBER 2002
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UNDERSTANDING THE ZONE AREA CLASSIFICATION METHOD IN THE NEC
Photo 4. Open View of Control Station
Photo 6. Close Up of Increased Safety Equipment Terminals
Photo 5. Close Up Shot of Control Station
Photo 7. Control Station Protection Method “edm” Increased Safety
Enclosure with Flameproof Switches and Enscapsulated Indicating Light
cannot be ignited by the electrical arcing parts within
the oil.
6. Increased Safety (Type “e”) Equipment
Increased safety (Type “e”) equipment is a type of
protection applied to electrical equipment that does
not produce arcs or sparks during normal operation
and under certain abnormal conditions. This equipment has additional security against the possibility of
excessive temperature and the equipment is assembled
very carefully to ensure that arcing or sparking from
part to part or connection to connection will not occur. The increased safety concept is often combined
with flameproof and other protection techniques at the
termination point outside of the enclosure to permit a
connection point.
7. Encapsulation (Type “m”) Equipment
Encapsulation (Type “m”) equipment is a type in
which any parts that could cause ignition of an explosive atmosphere by either excessive temperature or by
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arcing or sparking has been encapsulated in a compound of some sort. This encapsulation will prevent
the flammable or combustible material from migrating
into the enclosure in a large enough volume to form
an ignitable atmosphere at the point of excessive temperature or electrical arc within the equipment.
8. Powder Filling (Type “q”) Equipment
Similar to encapsulation equipment, powder filling
(Type “q”) equipment protection incorporates a quartz
or glass powder as a filling material. This method surrounds the hot or arcing electrical parts and doesn’t
permit the ignition of the gas or vapor at a point either
inside or outside the equipment enclosure.
Special Precaution with Zone Equipment
and Area Classification
The proper use and maintenance of zone equipment
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UNDERSTANDING THE ZONE AREA CLASSIFICATION METHOD IN THE NEC
Dual classification permits these two different classification methods with their different wiring methods
to exist at a single facility but to provide a clear line of
delineation between the two different systems. A user
could reclassify a Class I, Division 1 or Division 2 area
as a Class I, Zone 0, Zone 1 or Zone 2 area provided all
of the reclassified space is classified using a single flammable gas or vapor source.
Grouping and Classification of Areas
Based on the Chemicals Used.
Photo 8. Spark Test Apparatus for Determination of MIC Ratio
is as critical as the ability to properly classify the area
for the electrical installation for the facility. It is important that all personnel involved in the design, installation, maintenance, and inspection of zone areas be very
familiar with the zone concept.
If abnormally low temperatures are encountered at
the installation, such as below -20°C (-4°F), special
equipment may need to be installed. If these low temperatures are encountered, it should be noted that
many liquids are well below their flash points and are
no longer a hazardous concentration issue; however,
low temperature may also adversely affect the equipment operation, unless the equipment has been designed to operate in extremely low temperatures.
For Class I, Zone 0, 1, and 2 areas, NEC 1996, 1999,
and 2002 require classification of areas, selection of
equipment, and selection of wiring methods to be
handled under the supervision of a qualified, registered
professional engineer. The use of a qualified, registered
professional engineer for these systems should help
ensure a degree of accountability for the installation
and will remain a requirement until a significant number of trained personnel familiar with the equipment
design and installation are available.
Care should be taken where an installation that has
both division and zone locations exists at the same facility. There may be some applications where the zone
system is located adjacent to an existing division area,
and this may require different area classification methods for the same plant or facility. This dual classification of a facility is acceptable; however, there are certain cautions that the user must observe. Class I, Zone
0 and Zone 1 locations must not abut or overlap any
Class I, Division 1 or 2 locations. It is permissible to
have Class I, Division 2 and Class I, Zone 2 areas that
are adjacent to each other.
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For the purposes of testing and approval of the electrical equipment suitable for the zone area and to help in
area classification of the location itself, air mixtures that
have not been oxygen enriched have been classified
into three groups. The three group classifications for
zone applications are based upon the maximum experimental safe gap (MESG) or the minimum igniting
current (MIC) ratio or both. The MIC ratio is based
upon the use of methane gas as a base for comparison
to the other gases in question.
These group classifications are subdivided into
Groups IIC, IIB, and IIA according to the nature of the
gas or vapor and the type of protection technique. Protection techniques, such as increased safety “e”, encapsulation “m”, purged and pressurized “p”, and powder filled “q” equipment, are not dependent upon the
ignition properties of the gases in the group. Their inherent designs will preclude the gases or vapors from
entering the enclosures or being ignited by an arcing
or hot electrical part.
Group IIC is an atmosphere containing acetylene,
hydrogen, or flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor
mixed with air that may burn or explode when ignited,
having either an MESG less than or equal to 0.50 mm
or an MIC ratio less than or equal to 0.45.
Group IIB is an atmosphere containing acetaldehyde,
ethylene, or flammable gas, flammable liquid-produced
vapor, or combustible liquid-produced vapor mixed
with air that may burn or explode when ignited, having either an MESG greater than 0.50 mm and less than
or equal to 0.90 mm or an MIC ratio greater than 0.45
and less than or equal to 0.80.
Group IIA is an atmosphere containing acetone,
ammonia, ethyl alcohol, gasoline, methane, propane,
or flammable gas, flammable liquid-produced vapor,
or combustible liquid-produced vapor mixed with air
that may burn or explode when ignited, having either
an MESG greater than 0.90 mm or an MIC ratio greater
than 0.80.
Equipment can be listed for the specific gas or vapor, specific mixtures of gases or vapors, or any specific combination of gases or vapors. This permits elecNOVEMBER/DECEMBER 2002
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UNDERSTANDING THE ZONE AREA CLASSIFICATION METHOD IN THE NEC
trical equipment to be
designed, tested, and
listed based upon a
specific anticipated usage. For example, electrical equipment that
has been tested as a
Group IIB would normally be required for
ethylene-based chemical atmospheres but
may also have been
tested for hydrogenbased chemicals. This
permits the equipment
to be marked with a IIB Photo 9. Westerberg Apparatus
marking instead of hav- Experimental Safe Gap (MESG)
ing to be subjected to
the more rigid tests for acetylene, as would normally
be required if a IIC equipment listing is used.
Listing, Marking, and Documentation
Listing of zone equipment is a requirement in 505.20.
Equipment listed for use in a Zone 0 location can be
used in a Zone 1 or Zone 2 location for the same gas or
vapor. Listed Zone 1 equipment can be used in a Zone
2 location for the same gas or vapor. Electrical equipment may be marked with both Division 1 or Division
2 marking and Zone 1 or Zone 2 for the same gas atmosphere. Where marking division equipment for the
zone designation, the marking should include the Class
I, Zone 1 or Zone 2 designation, the applicable gas
classification group, and the temperature class (T Code)
of the equipment.
Zone equipment must be marked to indicate the
class and the particular zone for which it has been listed.
Zone equipment marked for use in the United States
will have an “AEx” marking to indicate that the equipment has been listed in accordance with United States
nationally recognized testing and listing standards.
Many European and other countries permit
manufacturer’s self-certification of electrical equipment
for suitability of the equipment for environmental conditions, wire bending spaces, electrical insulation, in
other words, the ordinary electrical equipment requirements. These countries do, however, require the equipment used in a zone location be third party certified
for the hazardous location into which the equipment
will be installed. The “AEx” mark and the testing lab
mark should indicate to all persons involved in the installation that the electrical equipment has been tested
and listed by a third party certification organization
based on U.S. standards for the particular protection
techniques plus the aforementioned ordinary electri92
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cal equipment requirements
for shock and fire hazards.
Temperature
Classification of Zone
Equipment
Electrical equipment used in
a zone location must be
marked with the operating
temperature of the equipment using a reference ambient temperature of 40°C
(104°F). This ambient temperature provides a starting
point at which to test and as
a reference to the operating
for Determining Maximum
temperature of the equipment. The temperature class
must be marked on the equipment in the form of a
“T” code. “T1” is for equipment operating at equal
to or less than 450°C; “T2,” for equal to or less than
300°C; “T3,” for equal to or less than 200°C; “T4,”
for equal to or less than 135°C; “T5,” for equal to or
less than 100°C; and “T6,” for equal to or less than
85°C. Electrical equipment designed for use in an
ambient temperature of between -20°C and +40°C
do not require any additional temperature marking,
other than the “T” code.
Equipment designed be used in an ambient temperature below -20°C or above +40°C must have the
ambient temperature range marked on the equipment.
This is necessary since the ambient temperature at
which it has been designed may affect the operating
temperature of the equipment. These extremely low
or high ambient temperatures may also affect the
amounts of gases or vapors present. Low temperatures
may also produce higher explosion pressures in flameproof or explosionproof equipment.
Equipment suitable for use in an ambient temperature exceeding 40°C (104°F) must be marked with both
the maximum ambient temperature in which it may be
operated and the operating range of the equipment at
that elevated temperature. Extremely high ambient temperatures may elevate the gas or vapor close to the
autoignition temperature of the gas or vapor. It is critical that the autoignition temperature of a gas or vapor
is not exceeded by contact with extremely hot electrical equipment.
Electrical equipment that is not of the heat producing type or that operates at no greater than 100°C
(212°F) is not required to have a marked temperature
range or a marked operating temperature. Many hazardous chemicals have a much higher autoignition temperature than 100°C, but even if dealing with a chemiwww.iaei.org
cal that has a low autoignition temperature, the electrical equipment is just one of many items that could
be located in the area and be a concern for causing
possible ignition of the material.
Conclusion
By providing the user with an optional method for area
classification and an expanded number of equipment
options, the NEC has become a more global and progressive document. It has also expanded the ability of
manufacturers to enter into the global market in a more
competitive way.
Ed Briesch is a senior staff engineer with Underwriters
Laboratories and has been involved with Hazardous Locations equipment and installations for over 30 years. He is a
principle member of NEC CMP-14 and the NFPA Technical
Committee on Electrical Equipment in Chemical Atmospheres. He is also a member of several IEC, ASTM, IEEE and
ISA technical committees related to explosion protected
equipment. Ed has a BSChE from the Illinois Institute of Technology and is a Registered Professional Engineer in the State
of Illinois.
Mark C. Ode, staff engineering associate at the Underwriters Laboratories
Inc. in the Research Triangle Park, North
Carolina Regulatory Services Department, was a senior electrical specialist
for the National Fire Protection Association. He is a former staff liaison and
secretary to the NFPA Electrical Equipment in Chemical Atmospheres Committee. He was the executive secretary for the NFPA Electrical
Section and editor of the Electrical Section news bulletin,
Current Flashes. Prior to joining NFPA, Mr. Ode worked for
over 27 years as a licensed electrician and a licensed electrical contractor. He is a licensed electrician in the State of
Massachusetts. He has taught the National Electrical Code
throughout the United States. He also taught the NEC for
10 years at Maricopa County Community College in Phoenix, Arizona. He is certified for electrical inspection for
one and two family dwellings, general electrical installations, and for electrical plan review by the International
Association of Electrical Inspectors (IAEI), and is certified
as a general electrical inspector by the International Conference of Building Officials (ICBO).
He is certified as an electrical instructor for the States of
Wyoming, New Jersey, North Carolina, Michigan, and Massachusetts, as well as others. He was Vice-President of Field
Operations for an engineering firm for two years and was
President of Ode Electric for 15 years. He is an electrical
code columnist for Electrical Contractor magazine. He was
a principle member of NEC Panel 20 for the 1990 NEC and
an alternate member of NEC Panel 3 for the 2002 NEC. He is
a principle member of CMP 4, an alternate member of CMP
1, and an alternate on the NEC Technical Correlating Committee for the 2005 NEC. He has been a member of IAEI since
September 15, 1975.
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