1. Ambient Temperature Correction Factor Task Group

2.  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

3. 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

4. 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.

5.  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

6. 1889-Kennelly 1894 Insurance Co. set at 50% 1896 Insurance Co. revised to 60% 50C Code Grade Rubber Year188918941896 NEC1923 AWGKennelly50% 60% 142512.515 123316.520 1046232825 8582935 678394750 590455455 4104526270 3120607280 2144728690 117286103100 0206103124125 00246123148150 000298149179200 0000360180216225 250 300 350 400 500 600 Historical

7. 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 Year1923192519351940 AWG 50C Rubber Insul 3 conductors in conduit Single Conductor in Free Air 50C Rubber Insul 1415 20 1220 26 1025 35 8 48 650 4565 555 5276 470 6087 380 69101 290 80118 1100 91136 0125 105160 00150 120185 000175 138215 200 0000225 160248 250 177280 300275 198310 350300 216350 400325 233380 500400 265430 600450 293480

8. 5030C Q Heat Flow 120V 0V I Current Flow Resistance of copper conductor Thermal Resistance 1938-1940

9. Heat Transfer of Cable

10. Heat Transfer within Conduit 90 R1 Insulation Resistance R2 Air Resistance Inside Conduit R3 Conduit Resistance R4 Conduit to Air Resistance 30

11. Heat Transfer Conduction through Insulation Natural Convection outside conduit xRadiation in xRadiation out xForced convection outside (wind) xForced convection inside (wind, chimney effect) xNatural Convection inside conduit

12. SIZE Ampacities of Three Single Insulated Conductors, Rated 0-2000 Volts, IN Conduit in Free Air Based on Ambient Air Temperature of 40C AWG MCM 60C75C90C TYPES RUW, T, TW, UF TYPE RH, RHW, RUH, THW, THWN, XHHW, USE, ZW TYPE SA, AVB, FEP, FEPB, THHN, RHH, XHHW Copper 14 182225 12 232832 10 293742 8 364855 6 506475 4 658397 3 7698114 2 87112130 1 104134156 0 119153179 0 135175204 0 160207242 0 184238278 250 210271317 300 232300351 350 254328384 400 274354475 500 314407477  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

13. 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)

14.  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

15. Numerical Model of Wire Heating Timothy A. Shedd 29 September 2014

16. 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.

17. Wiring systems mounted directly on roof Add 33C Celsius Wiring systems raised off roof Add 22C Celsius Roof

18. Rooftop Conduction Reflected Solar Radiation Solar Radiation Convection Roof Reflected Solar Radiation Convection Roof Solar Radiation

20. 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

21. Assumptions in model T amb = 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)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/m 2 K Resistance between wire and conduit = 0.5 K-m/W (from finite element simulation) Solar radiation 1050 W/m 2 (UL results only use data for insolation between 1000 and 1100 W/m 2 ) 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)

24. Results – Compare to UL measurements T wire,mod = 63.3 °C; ΔT amb = 22.5 °C (40.4 °F)

25. Results – I 2 R losses included I = 20 A (per wire) – T wire,mod = 75.6 °C; ΔT amb = 34.7 °C (62.5 °F) I = 25 A (per wire) – T wire,mod = 82.7 °C; ΔT amb = 41.9 °C (75.4 °F)

26. 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

27. Results – Compare to UL measurements T wire,mod = 61.6 °C; ΔT amb = 20.7 °C (37.3 °F) emissivity increased to 0.88 (NREL value)

28. Results – I 2 R losses included I = 430 A (per wire) – T wire,mod = 80.6 °C; ΔT amb = 39.7 °C (71.5 °F) I = 380 A (per wire) – T wire,mod = 76.2 °C; ΔT amb = 35.4 °C (63.7 °F)

29. 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. UL / CDA Report infers rooftop issue is linear 124 degree C rise Total

30. UNLV Report With 8 in Rooftop Adder Without 12 AWG Cu. 90°C Ampacity30 Ambient Temp Correction0.650.82 Final ampacity with rooftop temp deration19.524.6 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.

31. 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.

32. 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/m 2 solar radiation. 1000 W/m 2 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.