Many installations have exposed raceways and electrical equipment of all kinds on rooftops, walls and other direct sunlight applications, since this has been a common installation method for many years. However, the practice of installing exposed conduits in direct sunlight will add heat directly to the conduit, the conductors inside the conduit and the electrical equipment connected to the conduit.
The 2008 National Electrical Code (NEC) has taken a major step in recognizing the added heat for installations where the conduit is on the roof of a building. Conductors within the conduit, exposed to this added heat, now must be derated substantially due to the direct heat exposure of the conductors through the conduit. Section 310.15(B)(2)(c) and an accompanying Table 310.15(B)(2)(c) were added to the 2008 NEC, covering ambient temperature adjustment for conduits exposed to sunlight on or above rooftops.
It is interesting, however, that this new text only covers conduit. The new section and table do not cover conductors installed in wireways or similar wiring methods, only conductors installed in conduit.
Table 310.15(B)(2)(c) requires a temperature adder of 33°C or 60°F to the normal ambient temperature of the area where the conduit is installed up to ½ inch above the roof, 22°C or 40°F for distances above ½ inch up to 3½ inches, 17°C or 30°F for distances above 3½ inches to 12 inches, and 14°C or 25°F for distances above 12 inches up to 36 inches high. The higher the conduit is elevated above the roof, the lower the temperature adder would be required, since less heat will be radiated from the roof into the conduit, and there will be more air circulation around the conduit.
Also very interesting is that the table’s title covers the ambient temperature adjustment for the conduit and not for the conductors in the conduit. The title should be changed for the 2011 NEC to apply to an ambient temperature adjustment for the conductors in conduits exposed to sunlight on or above rooftops and not an adjustment for the conduits.
This information is not new to the 2008 NEC, since the original concept was added in the 2005 NEC to Section 310.10 as a new fine print note (FPN), providing information that conductors installed in conduit exposed to direct sunlight in proximity to the rooftop may have a temperature that is 17°C (30°F) higher than the normal ambient temperature surrounding the conduit. Since FPNs are for informational purposes and not enforceable text, the 2005 NEC text was simply a warning to the installer that an elevated temperature may exist, and the installer should account for the higher temperature during installation.
Neither the data supplied for the 2005 NEC FPN nor data provided for the mandatory text added in the 2008 NEC had historical information of insulation failures in the field from this elevated temperature. If there had been insulation failures due to related high temperatures in exposed conduits, Las Vegas, Phoenix and similar high-heat areas would surely have experienced problems. However, there did not seem to be any data available about insulation failures.
There may be many reasons why the anticipated higher heat did not seem to lead to insulation failure. The angle of the sun with respect to the raceway location on the roof; prevailing wind that may provide a cooling effect; atmospheric conditions, such as humidity and clouds; roofing color; roofing materials; and internal air circulation within the raceway are just some of the conditions that may affect the ultimate temperature of the conductors.
The new text in 310.15(B)(2)(c) will result in much larger conductors where conduit is installed in direct sunlight on a rooftop. For example, if a conduit containing conductors supplying a 65-ampere continuous load (already calculated at 125 percent) is installed on a rooftop where the ambient temperature is 104°F using 2-by-4-inch redwood support blocks to support the conduit, the temperature adder would be an additional 40°F. Where normally a 4 AWG THWN copper conductor would be permissible (85 amps in the 75°C column of Table 310.16 with a 0.88 ambient temperature correction factor), the new correction factor would be 0.33, requiring a 3/0 copper conductor (200 amperes × 0.33 = 66 amperes). This conductor size difference would certainly cause problems with wire terminations and enclosure sizing for a 100-ampere disconnect. The change in the 2008 NEC makes installing conduits in direct sunlight on rooftops prohibitive.
ODE is a staff engineering associate at Underwriters Laboratories Inc. in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at firstname.lastname@example.org.