Ambient Temperature
Correction Factor Task Group
Ambient Temperature Correction
Factor Task Group
 Maintainer Installer
 Larry Ayer, IEC, Chairman
 Stan Folz – NECA Arizona
 Carmon Colvin, IEC, Alabama
 Labor
 Jim Dollard, IBEW, Co-Chair
 IAEI
 Donny Cook, IAEI – Alabama
 Patrick Richardson, IAEI Tamarack
Florida
 Manufacturers
 Alan Manche, NEMA
 Research and Testing
 Bill Fiske, Intertek
 Dave Dini, UL
 Tim Shedd, Professor Univ of
Wisc Madison
 William Black, Professor
Georgia Tech
William Black, Phd
William Z. Black received his BS and MS in Mechanical Engineering from the University
of Illinois in 1963 and 1964, respectively, and his PhD in Mechanical Engineering from
Purdue University in 1967. Since taking his doctorate, he has been at the George W.
Woodruff School of mechanical Engineering at the Georgia Institute of Technology,
where he is presently Regent's Professor and the Georgia Power Distinguished
Professor of mechanical Engineering. He has directed a number of EPRI projects
relating to ampacity of underground cables and overhead conductors. He is on several
IEEE ampacity committees and is a member of CIGRE Committee 22.12 on the thermal
behavior of overhead lines. He is a registered Professional Engineer in Georgia.
Member, IEEE/ICC Committee 3-1 Ampacity Tables
Member, IEEE/ICC Committee 12-44 Soil Thermal Stability
Member, IEEE Standard 442-1981 WG
Member, IEEE Standard on Soil Thermal Resistivity Working Group
Member, ICC/IEEE Standard 835-1994 Working Group
Member, IEEE Standard. 738-1993 Working Group
Member, IEEE/ICC Transient Ampacity Task Force
Member, Emergency Ratings of Overhead Equipment Task Force
Member, IEEE Thermal Aspects of Bare Conductors and Accessories Working Group
Member, IEEE/ICC, Working Group C24, Temperature Monitoring of Cable Systems
Chairman, IEEE/ICC C34D Committee on Mitigating Manhole Explosions
Tim Shedd, Phd
Direct applications of this work are spray cooling of high heat flux electronics, boiling and
condensation in smooth and enhanced tubes, and the development of cleaner, more efficient
small engines through a better understanding of carburetor behavior. We are approaching this
through the use of unique experimental flow loops and flow visualization techniques. Long, clear
test sections are used to study a range of fluids and flow conditions. New optical measurement
techniques, such as Thin Film PIV, are being developed to quantify flow behavior. Results from
these measurements will be fed into efforts to develop accurate, flexible and computationally
efficient models for use both by university researchers and system designers in industry. Though
he has several areas of interest, Tim's current focus is on identifying the primary mechanisms
responsible for two-phase heat and momentum transfer in thin films. While this may sound a
little esoteric, these conditions exist in literally millions of appliances and commercial products
world wide. A better understanding of the behavior of vapor-liquid systems can lead to improved
efficiencies, less waste materials (refrigerants and heat exchangers), and greater affordability of
products.
Task Group Approach
 Reviewed Historical Information
 Conference Call – invited all concerned parties to express their
views.
 Discussed if any known failures if they had occurred.
 Reviewed UL/CDA and IAEI papers
 Developed Heat Transfer Model with UW-Madison
 Developed Public input for CMP-6
Historical
1889-Kennelly
• 1894 Insurance Co. set
at 50%
• 1896 Insurance Co.
revised to 60%
• 50C Code Grade Rubber
Year
1889
1894
1896 NEC
AWG
Kennelly
50%
60%
14
12
10
8
6
5
4
3
2
1
0
00
000
0000
250
300
350
400
500
600
25
33
46
58
78
90
104
120
144
172
206
246
298
360
12.5
16.5
23
29
39
45
52
60
72
86
103
123
149
180
15
20
28
35
47
54
62
72
86
103
124
148
179
216
1923
15
20
25
35
50
55
70
80
90
100
125
150
200
225
Year
1940-Present
•Used basic Heat
Transfer Equation to
determine ampacity
1940
50C
Rubber Insul
50C
Rubber Insul
20
12 193820 Rosch
15
15
20
20
20
26
10
25
25
25
25
35
8
35
35
35
35
48
14
6
5
3
2
1
0
•Ampacity for
Conductors in conduit
1935
Single
Conductor in
Free Air
4
•Ampacity for
Conductors in free air
1925
3 conductors
in
conduit
AWG
Rosch
1923
00
000
50C
Rubber
Insul
15
50C
50C
Rubber Rubber
Insul
Insul
15
50
50
50
45
• Used
basic
Heat
Transfer
55
55
55
52
Equation
to
determine
70
70
70
60
ampacity
80
80
80
69
90
90
90
80
100
100for Conductors
100
91
• Ampacity
in
125
105
free
air 125 125
65
76
87
101
118
136
160
150
150
150
120
185
175
175
175
138
215
• Ampacity
in
200
200for Conductors
200
0000
225
225
225
160
conduit
248
250
250
250
250
177
280
300
275
275
275
198
310
350
300
300
300
216
350
400
325
325
325
233
380
500
400
400
400
265
430
600
450
450
450
293
480
1938-1940
𝑽𝒐𝒍𝒕𝒂𝒈𝒆
𝑰=
𝑹𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆
I Current Flow
0V
120V
Resistance of copper
conductor
Q Heat Flow
Q=
∆ 𝑻𝒆𝒎𝒑𝒆𝒓𝒂𝒕𝒖𝒓𝒆
𝑻𝒉𝒆𝒓𝒎𝒂𝒍 𝑹𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆
50
30C
Thermal Resistance
Heat Transfer of Cable
Heat Transfer within Conduit
90
R1
Insulation
Resistance
30
R2
Air Resistance
Inside Conduit
R3
Conduit
Resistance
R4
Conduit to Air
Resistance
Heat Transfer
 Conduction through
Insulation
 Natural Convection
outside conduit
x Radiation in
x Radiation out
x Forced convection
outside (wind)
x Forced convection inside
(wind, chimney effect)
x Natural Convection inside
conduit
Ampacities of Three Single Insulated Conductors,
SIZE
60C
AWG
MCM
14
12
10
8
6
4
3
2
1
0
0
0
0
250
300
350
400
500
Rated 0-2000 Volts, IN Conduit in Free Air Based on
Ambient Air Temperature of 40C
75C
90C
TYPE RH,
TYPE SA,
RHW, RUH,
TYPES RUW,
AVB, FEP,
THW, THWN,
T, TW, UF
FEPB, THHN,
XHHW, USE,
RHH, XHHW
ZW
18
23
29
36
50
65
76
87
104
119
135
160
184
210
232
254
274
314
Copper
22
28
37
48
64
83
98
112
134
153
175
207
238
271
300
328
354
407
25
32
42
55
75
97
114
130
156
179
204
242
278
317
351
384
475
477
1984-1987
 Proposals to NEC




Neher-McGrath Method 1956
Corrected Rosch – 1938
Considered to be more accurate
Included in 1984 NEC for adoption in
1987
 Most parts rejected in 1987 due
to termination concerns
 Retained for medium voltage
 Moved to Annex B for low voltage
Proposal 6-41 (1984)
1. The NEC is very conservative in its ratings of bare and covered conductors
(line wire).
2. The NEC does not employ a technique to account for the effect of sun and
wind.
3. The NEC does not correctly account for the difference in ampacity of bare and
covered line wire.
4. The NEC ratings for not more than three conductors in a raceway can cause
both the inspector and the user to make significant errors because:
 They do not provide for the variables of load factor and earth thermal
resistivity in underground applications.
 There is no derating factor that will get one to the most common earth
ambient - 20°C.
 For most direct burial applications the NEC will waste money because it is
too conservative.
 For conduit-in-air applications, the NEC ratings are too conservative.
Proposal 6-41 1984
 COFFEY (UL Representative) : I am voting against the Panel
recommendation to accept this proposal even though I
agree it is technically correct. My negative vote is based
on: (i) its far-reaching impact on equipment and
installations covered by many other parts of the Code and,
(2) the need for coordination with those parts of the Code
that are effected by changes in the ampacity rating of
conductors. I recommend that a study be made to assess
the overall impact of these changes and to identify any
needed modifications to other provisions of the Code.
Numerical Model of Wire
Heating
Timothy A. Shedd
29 September 2014
Univ of Wisc-Madison Report
 When conduit is in contact with roof surface the conductor
temperature is highly dependent on the roof surface temp.
 When the roof surface is 77 deg C, the conductor temp rise
above ambient is approximately 33C above ambient.
 When roof surface is 42C, conductor temperature rise above
ambient is 7.2C.
 When conduit is raised off the roof, conductor temperature
is approximately 22.8C above the ambient.
 Numbers obtained from model are in-line with numbers
from UL fact-finding report.
Roof
Wiring systems
mounted directly
on roof
Add 33C Celsius
Figure 8: EMT Conduit with Roof Surface at 350 K (77 °C, 170 °F), 30 Degree Contact Angle
Wiring systems
raised off roof
Add 22C Celsius
Figure 9: EMT Conduit Raised off of Roof Surface
Roof
Roof
Convection
Reflected Solar
Radiation
Solar
Radiation
Rooftop
Conduction
Roof
Convection
Solar
Radiation
Reflected Solar
Radiation
Case 4: 3 No. 12 AWG in ¾” EMT
¾” EMT raceway
O.D. 0.92 in =23.4 mm
ID = 0.824 in = 21 mm
Wall = 0.049 in = 1.25 mm
Galvanized steel
k_s = 51 W/m-K
emissivity = 0.83
absorptivity = 0.7
Assumptions in model
•
•
•
•
•
•
•
•
•
•
•
Tamb = 41 °C (105.5 °F)
No forced air movement external to conduit (only natural convection)
No axial air movement internal to conduit
Absorption coefficient α = 0.7 (from NREL database)
Emission coefficient ε = 0.83 (from NREL database, where ε = 0.88;
adjusted downward to match UL study data; Pessimistic adjustment)
Natural convection coefficient = 6 W/m2K
Resistance between wire and conduit = 0.5 K-m/W (from finite element
simulation)
Solar radiation 1050 W/m2 (UL results only use data for insolation
between 1000 and 1100 W/m2)
I = 0 A (for comparison with UL data)
Temperature-variable model of wire resistivity used
Radiation only through upper half of conduit (both absorption and
emission; net radiative exchange with roof assumed negligible)
Results – Compare to UL
measurements
Twire,mod = 63.3 °C; ΔTamb = 22.5 °C (40.4 °F)
Results – I2R losses included
• I = 20 A (per wire)
– Twire,mod = 75.6 °C; ΔTamb = 34.7 °C (62.5 °F)
• I = 25 A (per wire)
– Twire,mod = 82.7 °C; ΔTamb = 41.9 °C (75.4 °F)
Case 15: 3 500 kcmil in 4” EMT
4” EMT raceway
O.D. 4.5 in =114.3 mm
ID = 4.334 in = 110.1 mm
Wall = 0.083 in = 2.11 mm
Galvanized steel
k_s = 51 W/m-K
emissivity = 0.83
absorptivity = 0.7
Results – Compare to UL
measurements
Twire,mod = 61.6 °C; ΔTamb = 20.7 °C (37.3 °F)
emissivity increased to 0.88 (NREL value)
Results – I2R losses included
• I = 430 A (per wire)
– Twire,mod = 80.6 °C; ΔTamb = 39.7 °C (71.5 °F)
• I = 380 A (per wire)
– Twire,mod = 76.2 °C; ΔTamb = 35.4 °C (63.7 °F)
UL / CDA Report infers rooftop issue is linear
Example
o 41 degree C ambient in Nevada
o 33 degree C ambient Temp Rise
in Conduit due to Radiation
o 50 degree C rise due to fully
loaded conductor.
124 degree C rise Total
UNLV Report
• All conduits tested were raised off roof 8 inches. Did not compare with
conduits on roof to test for affects of roof conduction.
• Circuit had 13.3 amps. Well short of NEC allowable limits.
With 8 in
Without
Rooftop Adder
12 AWG Cu. 90°C Ampacity
Ambient Temp Correction
Final ampacity with rooftop
temp deration
30
0.65
30
0.82
19.5
24.6
UNLV Report
Each of the wiring methods experienced a temperature rise that exceeded the
ambient temperature. In the case of the energized conductors, which were the
minimum allowable size for the continuous load carried, the maximum
temperature experienced was 69° C, approximately 77% the temperature rating of
the conductor insulation (i.e., 90° C). In the case of the non-energized conductors,
the maximum temperature experienced was 60° C, approximately 67% the rated
temperature of the conductor insulation.
Since this is an experimental setup and not a working installation, the measured
temperatures are likely higher than a real-world installation due to the complete
exposure of the entire conduit length including origination points. Real-world
installations usually terminate on a rooftop, but originate in lower ambient
temperature locations such as in an electrical room or on the side of a building.
Findings
 Heat Transfer is complex.
 CDA / UL Report do not take into account electrical loading in conduit
 CDA / UL Report do not take into account how conduits are terminated.
 CDA / UL Report assume that Heat Transfer outdoors is linear when it is not.
 If conduits are not elevated above roof conductor temperature can be
elevated above 90C due to added conductive heat transfer from roof.
 1000 W/m2 solar radiation. 1000 W/m2 is based maximum solar radiation
during a one or two hours a day, during one or two months out of a year.
 When considering full loading of conductors, conductors inside conduits
raised off roof will be below the 90C threshold.