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

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 of Cable

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

Numerical Model of Wire Heating

Timothy A. Shedd29 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.

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 – 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 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.