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Under The Hot Sun: Ambient Temperature Adjustments on Rooftops

By Mark C. Ode | Oct 15, 2016
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Ambient temperature correction adjustments relating to conduits exposed to sunlight on rooftops were inserted into the 2008 National Electrical Code (NEC) as new subsection 310.15(B)(2)(c) with accompanying Table 310.15(B)(2)(c). The subsection’s purpose was to provide a temperature adder for any conduit installed on a building’s rooftop.


In the 2011 NEC section and table titles, “conduits” changed to “circular raceways.” In the 2014 NEC, the section and the table titles changed to “Raceways and Cables Exposed to Sunlight on Rooftops,” and a new exception was added. At the 2017 NEC comment meeting in San Diego this past January, the proposed text and table again changed for the 2017 NEC, if any appeals through the Standards Council are not upheld.


Let’s look at the origin of this text and table for the 2008 NEC and trace the changes through the 2017 NEC to determine if and how they affect the installation and use of wiring methods in direct sunlight on building rooftops. Direct sunlight adds heat to the outside of a raceway or a cable, but are there mitigating factors that may occur in actual installations that help reduce the effects of heat on the raceway or cable and, ultimately, the heat transfer to the internal conductors or cables?


In the 2008 NEC, the text and the table applied only to conduits with the text: “Where conductors or cables are installed in conduits exposed to direct sunlight on or above rooftops, the adjustments shown in Table 310.15(B)(2)(c) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.16 and Table 310.18.”


A fine print note stated, “one source for the average temperature in various locations is the ASHRAE Handbook—Fundamentals.”


The table provided temperature adders of 33°C or 60°F for distances of 0–1⁄2 inch from the roof to the bottom of the conduit, of 22°C or 40°F for distances of above 1⁄2–31⁄2 inches, of 17°C or 30°F for distances of more than 31⁄2–12 inches, and of 14°C or 25°F for distances above 12 inches of the roof. The result of this temperature adder was to either increase the size of the conductors or cables in the conduit installed on rooftops or rule out any conduit installations on rooftops due to the increased cost of larger conduits and conductors and cables.


During the summer in many parts of the United States, a temperature of 90°F with the temperature adder of 40°F for a conduit installed on a roof above 31⁄2inches would result in an ambient temperature of 130°F. Where a THWN-2 copper conductor was installed in the conduit, the ambient temperature correction would be 0.96, based on the 90°C column in Table 310.16; however, with the added ambient temperature, the new correction factor would be 0.76, requiring a much larger conductor or cable. 


In the 2011 NEC, the title changed, and this temperature-correction factor applied to any circular raceway. In the 2014 NEC, the titles again changed to apply to all raceways and cables. However, the ambient temperature adders and distances within the table remained as originally submitted in the 2008 NEC.


An exception was also added that states, “Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment” as mandated for other conductors and cables in 310.15(B)(3)(c). Based on manufacturer’s test data, moisture-resistant thermoset (cross-link poly) does not react the same in higher heat values as thermoplastic insulation where the thermoplastic starts to deteriorate.


In the 2017 NEC, the text reads: “Where raceways or cables are exposed to direct sunlight on or above rooftops, raceways or cables shall be installed a minimum distance above the roof to the bottom of the raceway or cable of 7⁄8 of an inch. Where the distance above the roof to the bottom of the raceway or cable is less than 7⁄8 of an inch, a temperature adder of 33°C or 60°F shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the temperature correction factors in Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b).”


The existing exception for XHHW-2 remains unchanged. The change in 310.15(B)(3)(c) was the result of studies of actual installations as well as data provided by a Correlating Committee Task Group study.

About The Author

ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected]

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