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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
Ambient Temperature Correction Factor Task Group
William Black, PhdWilliam 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 GroupMember, ICC/IEEE Standard 835-1994 Working GroupMember, IEEE Standard. 738-1993 Working GroupMember, IEEE/ICC Transient Ampacity Task ForceMember, Emergency Ratings of Overhead Equipment Task ForceMember, IEEE Thermal Aspects of Bare Conductors and Accessories Working GroupMember, IEEE/ICC, Working Group C24, Temperature Monitoring of Cable Systems Chairman, IEEE/ICC C34D Committee on Mitigating Manhole Explosions
Tim Shedd, PhdDirect 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.
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
Task Group Approach
1889-Kennelly
• 1894 Insurance Co. set at 50%
• 1896 Insurance Co. revised to 60%
• 50C Code Grade Rubber
Year 1889 18941896 NEC 1923
AWG Kennelly 50% 60%
14 25 12.5 15 1512 33 16.5 20 2010 46 23 28 258 58 29 35 356 78 39 47 505 90 45 54 554 104 52 62 703 120 60 72 802 144 72 86 901 172 86 103 1000 206 103 124 125
00 246 123 148 150000 298 149 179 200
0000 360 180 216 225250300350400500600
Historical
Rosch•Used basic Heat Transfer Equation to determine ampacity
•Ampacity for Conductors in free air
•Ampacity for Conductors in conduit
1940-Present
1938 Rosch
• Used basic Heat Transfer Equation to determine ampacity
• Ampacity for Conductors in free air
• Ampacity for Conductors in conduit
Year 1923 1925 1935 1940
AWG50C
Rubber Insul
50C Rubber
Insul
50C Rubber
Insul
3 conductors in
conduit
Single Conductor in
Free Air
50C Rubber Insul
50C Rubber Insul
14 15 15 15 15 20
12 20 20 20 20 26
10 25 25 25 25 35
8 35 35 35 35 48
6 50 50 50 45 65
5 55 55 55 52 76
4 70 70 70 60 87
3 80 80 80 69 101
2 90 90 90 80 118
1 100 100 100 91 136
0 125 125 125 105 160
00 150 150 150 120 185
000 175 175 175 138 215
200 200 200
0000 225 225 225 160 248
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
50 30C
Q Heat Flow
120V 0V
I Current Flow
Resistance of copper conductor
Thermal Resistance
1938-1940
𝑰=𝑽𝒐𝒍𝒕𝒂𝒈𝒆
𝑹𝒆𝒔𝒊𝒔𝒕𝒂𝒏𝒄𝒆
Q
Heat Transfer within Conduit
90
R1Insulation Resistance
R2Air Resistance Inside Conduit
R3Conduit
Resistance
R4Conduit to Air
Resistance
30
Heat Transfer
Conduction through Insulation
Natural Convection outside conduit
x Radiation inx Radiation outx Forced convection
outside (wind)x Forced convection
inside (wind, chimney effect)
x Natural Convection inside conduit
SIZEAmpacities of Three Single Insulated Conductors,
Rated 0-2000 Volts, IN Conduit in Free Air Based on Ambient Air Temperature of 40C
AWG MCM
60C 75C 90C
TYPES RUW, T, TW, UF
TYPE RH, RHW,
RUH, THW, THWN, XHHW,
USE, ZW
TYPE SA, AVB, FEP,
FEPB, THHN, RHH,
XHHW
Copper14 18 22 25 12 23 28 32 10 29 37 42 8 36 48 55 6 50 64 75 4 65 83 97 3 76 98 114 2 87 112 130 1 104 134 156 0 119 153 179 0 135 175 204 0 160 207 242 0 184 238 278
250 210 271 317 300 232 300 351 350 254 328 384 400 274 354 475 500 314 407 477
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
1984-1987
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.
Proposal 6-41 1984
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.
Wiring systems mounted
directly on roof
Add 33C Celsius
Wiring systems raised off roof
Add 22C Celsius
Roof
Roof
Rooftop Conduction
Reflected Solar Radiation
Solar Radiation
Convection
Roof
Reflected Solar Radiation
Convection
Roof
Solar Radiation
Case 4: 3 No. 12 AWG in ¾” EMT
¾” EMT racewayO.D. 0.92 in =23.4 mmID = 0.824 in = 21 mmWall = 0.049 in = 1.25 mmGalvanized steelk_s = 51 W/m-Kemissivity = 0.83absorptivity = 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 – 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 racewayO.D. 4.5 in =114.3 mmID = 4.334 in = 110.1 mmWall = 0.083 in = 2.11 mmGalvanized steelk_s = 51 W/m-Kemissivity = 0.83absorptivity = 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)
Exampleo 41 degree C ambient in
Nevadao 33 degree C ambient
Temp Rise in Conduit due to Radiation
o 50 degree C rise due to fully loaded conductor.
UL / CDA Report infers rooftop issue is linear
124 degree C rise Total
UNLV Report
With 8 in Rooftop Adder
Without
12 AWG Cu. 90°C Ampacity 30 30Ambient Temp Correction 0.65 0.82Final ampacity with rooftop temp deration 19.5 24.6
• 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.
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.