/images/PowerCableUpdate.jpg/images/southwire_100x70.jpgRooftop Installations? Watch Out for SunshineRooftopInstallationsWatchOutForSunshine.htmIf you’re putting cables in conduit on rooftops, there’s something you should know about lovely, sunny summer days: Heat from the roof can raise conductor temperatures enough to affect ampacity ratings. The NEC is now taking note of that.
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Rooftop Installations? Watch Out for Sunshine

A new FPN in the 2005 NEC raises a warning about solar heating and operating temperatures.

If you’re putting cables in conduit on rooftops, there’s something you should know about lovely, sunny summer days: Heat from the roof can raise conductor temperatures enough to affect ampacity ratings. The NEC is now taking note of that.

In the 2005 National Electrical Code, Section 310.10 carries a new Fine Print Note (FPN). FPN No. 2 says, “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 17ºC (30ºF) above ambient temperature on which the ampacity is based.”

An additional design factor on rooftops

The impact of the new FPN in 310.10 is that designers may need to de-rate the ampacity of conductors in roof-top conduits by an additional temperature increment. Section 310.10 is titled “Temperature Limitations of Conductors,” and it requires that: “No conductor shall be used in such a manner that its operating temperature exceeds that designated for the type of insulated conductor involved.”

An existing FPN – now FPN No. 1 – lists the determining factors in conductor operating temperature, and tables in Article 310 and Annex B of the NEC provide ampacity correction factors for various ambient temperatures. “The new FPN No. 2 adds information that system designers should take into account when using the ambient temperature correction factors for roof-top installations,” says Dave Mercier, director, codes and standards for Southwire’s Electrical Division.

Where did this information come from?

The source of the temperature rise numbers was a 2003 study by the Copper Development Association at a Las Vegas research site. The study, which ran from early May through mid-October, compared temperature profiles of three types of conduit when exposed to sunlight and elevated ambient temperatures. The three conduit types were Electrical Metallic Tubing (EMT), Intermediate Metal Conduit (IMC) and Rigid Nonmetallic Conduit (RNC). Conduit runs of each type were either in contact with the roof or raised about 5/8”, and they were. Here are some key points:

  • There were only small differences among the average temperature profiles for the three types of conduit.
  • For the entire period from May to October, the conductors in the conduit runs saw temperatures that averaged 17ºC (30ºF) above ambient air temperature. This is the figure the NEC cites.
  • During the peak period of July/August, average temperature increases ranged from 40ºF to 58ºF, depending on how the conduit was mounted to the roof surface.
  • The maximum increase in temperature above ambient seen was 83ºF.

On rooftops, normal safety factors may be stretched thin

“In most applications, the normal NEC ampacity calculations include generous safety factors,” Mercier says. “But where a roof-top conductor is fully loaded for an extended period in the heat of the sun, worst-case conditions could add up to a problem. It pays to make allowance for that sunlight heating effect.”

Mercier adds, “Southwire was pleased to participate in the Copper Development Association study. This kind of engineering involvement is part of Southwire’s commitment to the industry and the standards process.”