effect of rooftop exposure (in direct sunlight) on conduit ambient temperatures connected home...
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Effect of Rooftop ExposureEffect of Rooftop Exposure(in Direct Sunlight)(in Direct Sunlight)
On Conduit Ambient On Conduit Ambient TemperaturesTemperatures
Connected Home CouncilConnected Home Council
October 27, 2011October 27, 2011
Presented byPresented by
David BrenderDavid Brender
Copper Development Association Inc.Copper Development Association Inc.
© 2011 CDA Inc.
Effect of Rooftop Exposure On Conduit Ambient Temperatures
Correct application of ambient temperature correction in NEC
Conductors in insulation
Original goal: How does sunlight affect electrical conductors in conduits exposed to direct sunlight?
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When electrical conductors operate in high ambient temperature environments…
Copper Development Association’sCopper Development Association’sAmbient Temperature Research ProgramAmbient Temperature Research Program
Why Las Vegas?
In Summer, Las Vegas has similar solar radiation to other Southwestern and Western states.
In Spring and Fall, Las Vegas solar radiation matches Summer solar intensities in most of the U.S.
Las Vegas has high number of ‘clear days’ to conduct research
Solar Radiation
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SUNDAY JULY 16, 2006
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NEC Covers Ambient Temperature Correction
NEC 310.10
Temperature Limitation of Conductors. No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved.
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Table 310.16
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Temperature correction is required,but usually ignored
Data did not exist to allow:
Engineers and contractors to choose an appropriate temperature for correction.
Inspectors to effectively enforce 310.10 .
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Experiments to determine the extent of heating in copper wires in areas exposed to solar radiation…
ElevatedAmbientTemperatureStudy
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Residential and Commercial Buildings
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Rooftops and exterior walls
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May 20, 2003
8:00 9:00 10:00 11:00 12:00 1:00 2:00 3:00 4:00 5:00 6:00
AM Time of Day PM
70 F
90 F
110 F
130 F
150 F
170 F
Tem
pera
ture
Temperatures of 12 AWG Wires in EMTCompared to Outdoor Temperature
Tarred to roof
On Surface
On Struts
Outdoor Temp.
Rooftop temps considerably higher than outdoor temps…
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Types of Conduit/Cable
EMT, RNC, MC
Distance above roof On roof5/8 inch1-1/2 inch3.5 inch6 inch12 inch36 inch
Roof Colors Black mineral rolled roof surface White mineral rolled roof surface White elastomeric paint roof surface
UnloadedRange of:
Outdoor temperatures Solar radiation intensities
Solar Research
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Black mineral rolled Black mineral rolled roof surfaceroof surface
White mineral rolled White mineral rolled roof surfaceroof surface
White elastomeric White elastomeric paint roof surfacepaint roof surface
20042004
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On Roof
Strut (5/8”)
3.5 inch12-inch 1.5 inch6-inch36-inch
2004
(85 Conduits/Cables)© 2011 CDA Inc.
Effect of Rooftop Exposure On Conduit Ambient Temperatures
Research Findings
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119.3 F119.3 F
160.5 F160.5 F
125.7 F125.7 F
Roof Temperature 9/28/04 13:15Roof Temperature 9/28/04 13:15
Outdoor Temp = 92 F© 2011 CDA Inc.
Temperature Differential vs. Outside Ambient (92F)
On DeckOn Deck38 F38 F
3.5” above3.5” above19.4 F19.4 F
6” above6” above16.6 F16.6 F
36” above36” above12.7 F12.7 F
RoofRoof68 F68 F
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Recommended Ambient Temperature Adjustments
June, July and August averagedWhite and dark roofs averagedASHRAE 2% design temperatures
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Distance Above RoofºC ºF
On roof, up to and including 1.6 cm. above roof 33 60Above ½ inch, up to and including 9 cm. above roof 22 40Above 3-½ inches, up to and including 30 cm. above roof 17 30
Above 12 inches, up to and including 91 cm. above roof 14 25
Temperature
Table 6. Recommended Adjustments (Additions) to Outdoor Temperatures to Determine Ambient Temperatures Inside Conduits
Table 310.15(B)(2)(c)
Mandatory in 2008 and 2011 NEC
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UL – MC cable
Studied all types of raceway and cables allowed on rooftop
Type MC cable was chosen to represent metal clad and metal sheathed cables and conduits (e.g. Type MC, MI, LFMC, etc.)
Sizes 12 AWG, 1/0 AWG, and 500 kcmil were chosen to represent cables of the smaller, medium, and larger sizes
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UL - Cables
Type SE and TC cable was also chosen in the 1/0 AWG size to represent jacketed cables (e.g. Type SE,TC, MV, UF, etc.)
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All conduit types
Type RMC and EMT were chosen to represent circular metal raceways in ½, 1 ½, 4” RMC,
¾, 1 ½, 4” EMTType PVC was chosen to represent nonmetallic
conduits and raceways in ¾, 1 ½, 4” sizesMetal raceway was chosen in the 4x4 inch and 8x8
inch sizes to represent smaller and larger raceways in general
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UL Fact Finding August 2011
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UL Fact Finding August 2011
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Solar Irradiance
Procedure
For each test sample, the rise above outdoor ambient temperature for the three thermocouple
temperatures were averaged for each five-minute increment to record the temperature rise above outdoor ambient
For analysis of results purposes, 10 days were chosen during this time period where solar irradiance exceeded 1000 W/m2 for a prolonged period of time
3 thermocouple readings were averaged
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Procedure
For each of the ten chosen days, the maximum, minimum, 90th, 75th, and 50th percentile temperatures were calculated for each test sample at each elevation for the period of time that day when the solar irradiance was greater than or equal to 1000 W/m2.
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Results
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Results
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Results
“For all wiring systems mounted directly on the roof, the maximum temperature rise above outdoor ambient with a 95% confidence interval would statistically be 62.2° F. For all wiring systems mounted at a distance above the roof (1/2 inch and greater) the maximum temperature rise above outdoor ambient with a 95% confidence interval would statistically be 51.4° F.”
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Remember:
Temperature derating is a “CORRECTION FACTOR”Correction factors AND adjustment factors BOTH apply
More than 3 current-carrying conductors in a conduit requires further multiplier
Section 110.14 (c) still applies
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Tools
Web: http://www.copper.org
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