In the 2005 National Electrical Code (NEC), 310.10, fine print note No. 2 contained text warning that “conductors installed in conduit exposed to direct sunlight in close proximity to rooftops have been shown, under certain conditions, to experience a temperature rise of 17°C (30°F) above ambient temperature on which the ampacity is based.” In the 2008 NEC, the ambient temperature rise for conduit installed exposed on a rooftop was moved from a fine print note to mandatory text in a new 310.15(B)(2)(c) with an accompanying Table 310.15(B)(2)(c). In the 2011 NEC, the temperature rise text and table in 310.15(B)(2)(c) were moved to 310.15(B)(3)(c) and Table 310.15(B)(3)(c) with a title change so the temperature rise applied an adder to the existing ambient temperature to circular raceways installed exposed on a rooftop.
Removing “conduit” and replacing it with “circular raceways” applied the temperature adder to any circular raceway, such as electrical metallic tubing (EMT), rigid metal conduit (RMC), intermediate metal conduit and other similar raceways. In addition, a proposal for the 2011 NEC recommended adding “cables” into the temperature rise adder, but it was rejected because the term “cables” was too general and only one type (MC cable) and size was tested. Panel members requested a more comprehensive research project to determine the effects of sunlight on various conduits, Type MC cables and other general cables that might be installed on a rooftop.
For the 2014 NEC, a proposal was submitted to change 310.15(B)(3)(c) and Table 310.15(B)(3)(c), based on a fact finding report on ambient temperature adjustments for raceway and cable systems exposed to sunlight on rooftops that was performed at an outdoor laboratory site in Las Vegas. That city was chosen because of its high frequency of sunny days with high solar radiation intensity (solar irradiance). Solar irradiance—radiation from the sun—is measured as power density in watts (W) per square meter. Time of day; time of year; atmospheric conditions, such as clouds, haze, rain and smog; and the geographical location all are factors that can affect the level of solar irradiance of the sun at the test’s point of measurement. The maximum solar irradiance reaching the Earth’s atmosphere can be up to 1,400W per square meter; however, levels of 1,000 to 1,200W per square meter are very typical of sunny days during summer months in the United States. Obviously, any clouds or other atmospheric conditions can lower these values substantially. The test site, with full sun, no clouds and a maximum ambient temperature of 110ºF, had a measured solar irradiance of greater than 1,000W per square meter for a two- to three-hour period over many days during June, July and August 2011 when the data was measured.
The tests were conducted on Type MC cable, which represented metal-clad and metal-sheathed cables; Type MC and mineral insulated (MI) cable; and liquidtight flexible metal conduit and similar conduits. Conductors in sizes of 12 AWG, 1/0 AWG, and 500 kcmil were used within the cables or conduits to represent small, medium and large cables. Service entrance (SE) and tray cable (TC) types were used in the 1/0 AWG sizes in the tests to represent jacketed cables. In addition, trade size RMC and EMT were tested for smaller sizes. For medium and larger sizes, 1 and 4 trade sizes were used. PVC conduit was tested in the trade sizes of , 1, and 4. For noncircular raceways (metal wireways and gutters), 4-inch-by-4-inch and 8-inch-by-8-inch sizes were used. Each of these raceways was installed on a roof in direct sunlight at distances of , 3, and 12 inches above the roof to represent worst-case applications. For the -inch-above-the-roof test, the roof was painted black to account for the additional heat from the roof material’s solar heating and the transfer of that heat back into the raceways. For raceways mounted at greater distances from the roof, the roof surface was white asphalt to account for solar reflection from the roof back into the raceways being tested. Using the various size raceways and conductors in the test provided comprehensive research into the effect of solar irradiance and heat on raceways and, ultimately, on the insulated conductors in the raceways.
The results have provided a proposal for the 2014 NEC with modifications to Table 310.15(B)(3)(c), where the table now applies to raceways and cables, with temperature adders of 60ºF for raceways and cables mounted on the roof, and 50°F for raceways and cables mounted any dimension above the roof.
ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and firstname.lastname@example.org.