The Evaluation of
Steel Conduit and
EMT as Equipment
Grounding Conductors
Program Outline
• NEC Requirements
– RMC, IMC and EMT
– Copper and Aluminum Conductors
•
•
•
•
•
•
Georgia Institute of Technology Research
GEMI Analysis Software
Sizing the EGC
Comparison of NEC and GEMI Software
Summary of Results
Computer Exercises
Grounding & Bonding is
Paramount to a good
installation
Protection against
•Lightning & Voltage Surges
•Short Circuits
•Ground Faults
•Limit Fire & Shock Hazards
NEC Compliance
NEC 110.10
“The overcurrent protective devices, the
total impedance, the component
(equipment) short-circuit current rating,
and other characteristics of the circuit to
be protected shall be selected and
coordinated to permit the circuitprotective devices used to clear a fault to
do so without extensive damage to the
electrical component (equipment) of the
circuit”. . .
NEC 110.10
“This fault shall be assumed to be either
between two or more of the circuit
conductors or between any circuit
conductor and the (equipment)grounding
conductor or enclosing metal raceway.
Listed products applied in
accordance with their listing shall be
considered to meet the requirements
of this section.”
2008 NEC®
• Verification of an adequate and safe
effective fault return path (EGC) is a
requirement.
• All metal parts that may become a
part of this path must be bonded
(connected) together.
300.10
• Metal raceways, cable armor, and other
metal enclosures for conductors shall be
mechanically joined together into a
continuous electrical conductor and shall
be connected to all boxes, fittings, and
cabinets so as to provide effective
electrical continuity. Unless specifically
permitted elsewhere in this Code, raceways and
cable assemblies shall be mechanically secured to
boxes, fittings, cabinets, and other enclosures.
300.12
• Mechanical Continuity — Raceways and
Cables.
• Metallic or nonmetallic raceways, cable
armors, and cable sheaths shall be
continuous between cabinets, boxes,
fittings, or other enclosures or outlets.
NEC Article 250 is Unique
• 250.4(A) & (B) Contains the General
Performance Requirements for
Grounding and Bonding.
• The remainder of Article 250 contains
prescriptive methods to comply with
these performance requirements.
NEC Article 250 Organization
I
II
III
IV
V
General
Circuit and System Grounding
Grounding Electrode System and Grounding
Electrode Conductors
Enclosure, Raceway and Service Cable Grounding
Bonding
VI
Equipment Grounding and Equipment Grounding
Conductors
VII
VIII
IX
X
Methods of Equipment Grounding
Direct Current Systems
Instruments, Meters, and Relays
Grounding of Systems and Circuits of 1kV and Over
NEC 250.118 “Types of
Equipment Grounding
Conductors”
• “The equipment grounding conductor
run with or enclosing the circuit
conductors shall be one or more or a
combination of the following:”
NEC Article 250.118
(1) A copper, aluminum or copperclad aluminum conductor
(2) Rigid metal conduit
(3) Intermediate metal conduit
(4) Electrical metallic tubing
PLUS . . .
10 Other wiring methods
NEC 250.4(A)(5)
• “Effective Ground-Fault Current Path. Electrical
equipment and wiring and other electrically conductive
material likely to become energized shall be installed in
a manner that creates a low-impedance circuit
facilitating the operation of the overcurrent device or
ground detector for high-impedance grounded systems.”
• “It shall be capable of safely carrying the maximum
ground-fault current likely to be imposed on it from any
point on the wiring system where a ground fault may
occur to the electrical supply source.”
NEC 250.4(B)(2)
• “Bonding of Electrical Equipment. “Non-currentcarrying conductive materials enclosing electrical
conductors or equipment, or forming part of such
equipment shall be connected together and to the
supply system grounded equipment in a manner that
creates a low-impedance path for ground-fault current
that is capable of carrying the maximum fault current
likely to be imposed on it.”
Paths to Ground
Grounded
Neutral
Grounded
(Neutral)
Metallic
Raceway
(EGC)
Grounded
Conduit
Equipment
Grounding
Conductor
Equipment Grounding
Conductor
(EGC)
Grounding
Electrode
Conductor (GEC)
Grounding
Conductor
Ground Electrodes
Grounding
Electrodes
NEC 250.24(A)(5)
• “Load-Side Grounding Connections. A grounded
conductor shall not be connected to normally noncurrent-carrying metal parts of equipment, to equipment
grounding conductor(s), or be reconnected to ground on
the load side of the service disconnecting means except
as otherwise permitted in this article.”
250.122
Size of Equipment Grounding
Conductors
• (A) General. Copper, aluminum, or copper-clad
aluminum equipment grounding conductors of
the wire type shall not be smaller than shown in
Table 250.122
• Where a cable tray, a raceway, or a cable armor
or sheath is used as the equipment grounding
conductor, as provided in 250.118 and
250.134(A), it shall comply with 250.4(A)(5) or
(B)(4).
Table 250.122
Table 250.122 Minimum Size Equipment Grounding
Conductors for Grounding Raceway and Equipment
Rating or setting of
Automatic Overcurrent
Device in Circuit Ahead
of Equipment, Conduit,
etc., Not Exceeding
(Amperes)
Size (AWG or kemil)
Copper
Aluminum or
Copper-Clad
Aluminum*
15
20
30
14
12
10
12
10
8
40
60
100
10
10
8
8
8
6
200
300
400
6
4
3
4
2
1
500
600
800
2
1
1/0
1/0
2/0
3/0
1000
1200
1600
2/0
3/0
4/0
4/0
250
350
2000
2500
3000
4000
5000
6000
250
350
400
500
700
800
400
600
600
800
1200
1200
Note: Where necessary to comply with 250.4(A)(5) or (B)(4), the equipment grounding
conductor shall be sized larger than given in this table.
System Grounding Connections
250.24(A)(5)
Courtesy of IAEI
Development of GEMI Program
• Validation Needed for Existing Data
 Soares Book on Grounding (Tables)
 Safe lengths for copper and aluminum
conductors
 Safe lengths for
 Rigid metal conduit
 Intermediate metal conduit
 Electrical metallic tubing
 R.H. “Dick” Kaufman studies
What is “GEMI”?
Grounding and ElectroMagnetic Interference
Analysis Software
• Phase I: Grounding
• Phase II: EMF/EMI
GEMI
Phase One
Performance of Grounding Paths
This research is the first
update in forty years on the
impedance and permeability of
steel conduit.
GEMI Background
• 1992
Grounding Study Initiated at Georgia Institute of
Technology
• 1994
Phase I completed (Grounding Study)
“Modeling and Testing of Steel EMT,
IMC and Rigid Conduit”
• 1997
Phase II completed “Modeling and
Evaluation of Conduit Systems for
Harmonics and Electromagnetic
Fields”
• 1999
GEMI software – Windows version
• 2002-04
GEMI updates
GEMI Phase I
Key Issues Examined
•
Are steel Rigid, IMC and EMT effective
equipment grounding conductors?
•
What length of run can be safely installed?
•
Does supplementary EGC lengthen the
run?
About the Project
• In-Laboratory Resistance &
Permeability Tests
• Computer Modeling
• Full Scale Field Testing
• Written Report
• User Software Released
• IEEE Paper
Illustration of Kearney Laboratories’
Facility
Transformer
Kearney Laboratory transformer and
control room in McCook, IL
Kearney Lab testing facility
Test blocks and leads to Transformer
Full Scale Test Set-up
7"
Figure 5.2
Illustration of the
layout of ten runs
of Steel Conduit.
Power conductors
were enclosed but
not shown. Total
length of each
conduit run is 256
feet. Wood blocks
were used to space
the steel.
1/2" EMT
3/4" EMT
3/4" IMC
3/4" GRC
2" EMT
2" IMC
2" GRC
3" EMT
3" IMC
3" GRC
Fittings Used in the Testing
• Tests at both 120 & 277 volts to ground
were run using zinc die-cast and steel
set-screw and compression couplings
with no significant difference in the
overall impedance.
Grounding Concerns
Load
(a)
Load
(b)
Load
(c)
Load
(d)
(a) Single Phase Circuit without Ground Conductor
(b) Single Phase Circuit with Ground Conductor
(c) Three Phase Circuit without Neutral
(d) Three Phase Circuit with Neutral
NEC Validation
• Validates 250.118
• Safe Lengths for
Rigid Steel Conduit
Intermediate Metal Conduit
Electrical Metallic Tubing
• Validates Table 250.122
• Safe Lengths for
Copper & Aluminum conductors
GEMI Opening Screen
Single Circuit Analysis
Maximum System Length
40 Volt Arc - 400% of Protective OC Device
• The software predicts maximum safe
circuit lengths.
• Rigid Steel Conduit, IMC, and EMT
• Copper and Aluminum Conductors
• Line to ground faults
• Line to line or line to neutral faults
Note: All other paths must be considered to establish the
limiting condition. Each additional ground return path
reduces the overall impedance.
Steel Conduit or Tubing
Parameters
Select Parameters
• Select type of conduit or tubing
• Size
• Temperature
• Copper or Aluminum Conductors
• Type and size
• Temperature
• Fault Clearing Current
• Based on the overcurrent device
• Check MFG Trip curves (4 – 5 x the OC rating)
• Circuit Voltage to ground
• Arc Voltage
• 40 Volts unless you have different information
• A bolted fault is unlikely
Click on “Update”
289.88
404.80
Arcing Fault Compare
Allowable Safe Lengths
Fault Currents Are Low
141.15
Grounding Concerns
Load
(a)
Load
(b)
Load
(c)
Load
(d)
Summary of Key Findings
• Steel EMT, IMC and Rigid conduit meet NEC
requirements for EGCs.
• Comparably sized steel EMT, IMC and Rigid conduit
allow flow of higher fault current than aluminum and
copper EGCs.
• Steel EMT, IMC and Rigid conduit provide low
impedance path to ground.
• Facilitate operation of OC devices in runs not
exceeding maximum allowable lengths
detailed in report
Summary of Key Findings
• Supplementary equipment grounding conductors,
when participating in the fault circuit, reduce the
overall impedance. . .
• Do not add to safety in phase to neutral fault
• May increase the allowable length of steel
conduit, dependant on size and system
design
• Steel conduit is not the limiting factor in a
conductor to neutral fault
Start
Program
Analysis of Specific Systems
Utility
Substation
3-Phase Feeder
1-Phase
Lateral
C
A
Load
B
Load
Examples of Maximum Length Equipment Grounding Conductor (Steel EMT, IMC, GRC, and Copper, Copperclad or Aluminum Wire)
Computed As A Safe Return Fault Path To Overcurrent Device
Based on 1994 Georgia Tech Software (GEMI windows 1.1)
With an Arc Voltage of 40 and 4 IP at 25° C Ambient
120 Volts to Ground
(1)
(1)
(1)
Overcurrent
400%
Circuit
EMT, IMC Equipment Length of Length of Length of
Copper
Aluminum
Device
(4IP)
Conductor
GRC
Grounding EMT Run IMC Run GRC Run Grounding or Copperclad
Rating
Overcurrent
Size
Trade
Conductor Computed Computed Computed Conductor
Grounding
Amperes
Device
AWG-kcmil
Size
Size
Maximum Maximum Maximum
Conductor
(75° C)
Rating
Copper
Copper
Max
Max
Amperes
or
or
Run
Run
Aluminum
Aluminum (In Feet)
(In Feet)
(In Feet)
(In Feet)
(In Feet)
20
80
12
1/2
--395
398
384
----20
80
12
--12
------300
--20
80
10 AL
--10 AL
--------293
30
120
10
1/2
----383
------30
120
10
3/4
--404
399
386
----30
120
10
--10
------319
--30
120
8 AL
--8 AL
--------310
40
160
8
3/4
----414
------40
160
8
1
--447
431
418
----40
160
8
--10
------294
--40
160
8 AL
--8 AL
--------232
60
240
6
1
--404
400
382
----60
240
6
--10
------228
--60
240
4 AL
--8 AL
--------221
100
400
3
1 1/4
--402
397
373
----100
400
3
--8
------229
--100
400
1 AL
--6 AL
--------222
200
800
3/0
2
--390
389
363
----200
800
3/0
--6
------201
--200
800
250 AL
--4 AL
--------195
(1) Per NEC Table 250-95.
Applicable to non-metallic conduit runs.
* NEC Wire Fill Table Permits Smaller Conduit Size
Note: Software is not limited to above examples
Examples of Maximum Length Equipment Grounding Conductor (Steel EMT, IMC, GRC, and Copper, Copperclad or Aluminum Wire)
Computed As A Safe Return Fault Path To Overcurrent Device
Based on 1994 Georgia Tech Software (GEMI 2.3)
With an Arc Voltage of 40 and 4 IP at 25° C Ambient
277 Volts to Ground
(1)
(1)
(1)
Overcurrent
400%
Circuit
EMT, IMC
Equipment
Length of
Length of
Length of
Copper
Aluminum
Device
(4IP)
Conductor
GRC
Grounding
EMT Run
IMC Run
GRC Run
Grounding
or Copperclad
Rating
Overcurrent
Size
Trade
Conductor
Computed
Computed
Computed
Conductor
Grounding
Amperes
Device
AWG-kcmil
Size
Size
Maximum
Maximum
Maximum
Conductor
(75° C)
Rating
Copper
Copper
Max
Max
Amperes
or
or
Run
Run
Aluminum
Aluminum
(In Feet)
(In Feet)
(In Feet)
(In Feet)
(In Feet)
20
80
12
1/2
---
1170
1179
1140
---
---
20
80
12
---
12
---
---
---
890
---
20
80
10 AL
---
10 AL
---
---
---
---
870
30
120
10
1/2
---
---
1135
---
---
---
30
120
10
3/4
---
1199
1182
1143
---
---
30
120
10
---
10
---
---
---
946
---
30
120
8 AL
---
8 AL
---
---
---
---
920
40
160
8
3/4
---
---
1228
---
---
---
40
160
8
1
---
1326
1276
1239
---
---
40
160
8
---
10
---
---
---
871
---
40
160
8 AL
---
8 AL
---
---
---
---
690
60
240
6
1
---
1197
1186
1131
---
---
60
240
6
---
10
---
---
---
676
---
60
240
4 AL
---
8 AL
---
---
---
---
657
100
400
3
1 1/4
---
1192
1176
1107
---
---
100
400
3
---
8
---
---
---
680
---
100
400
1 AL
---
6 AL
---
---
---
---
659
200
800
3/0
2
---
1157
1155
1077
---
---
200
800
3/0
---
6
---
---
---
598
---
200
800
250 AL
---
4 AL
---
---
---
---
578
(1) Per NEC Table 250-95.
Applicable to non-metallic conduit runs.
* NEC Wire Fill Table Permits Smaller Conduit Size
Note: Software is not limited to above examples
Questions
www. http://steeltubeinstitute.org/steel-conduit/
Thank You
This Software, Supporting Reports, and other material related to this presentation
available on the website; www.steelconduit.org.