Did you know that conduits on the rooftops of commercial buildings can run more than 70°F hotter than surrounding temperatures? For example, depending on the height of the conduit above the surface of the roof, when the outdoor temperature reaches 95°F, the temperatures inside the conduit may exceed 165°F — even with no current passing through the conductors.

The 2008 NEC now recognizes that air inside conduits on rooftops can be appreciably hotter than the surrounding air — and that ampacity ratings of numerous wire sizes and types must be adjusted to account for these elevated temperatures.

“Although the Code has required derating for ambient temperatures for many years, we were curious to know how hot conduit gets inside because that's the ambient to which the insulation is subjected,” says David Brender, national program manager for electrical markets for the Copper Development Association (CDA), New York, the organization that conducted a number of the experiments that ultimately led to the changes in the 2008 NEC. “This was not an attempt to change the derating in the Code, but rather an attempt to establish what the temperatures is in the conduit to then perform the derating,” he explains.

Two new guides released by the CDA address the issue of sizing the wires and cables inside conduits on rooftops in accordance with the 2008 NEC. Offered free of charge, the CDA says the publications make it easy to estimate temperatures inside electrical conduits at various heights above the rooftop for hundreds of cities across the United States and make the necessary ampacity adjustments required by the Code.

“These guides are designed to help prevent the failure of equipment on rooftops,” says Brender. “They will enable electricians, electrical inspectors, and design engineers to apply the Code as it was written.”

Hard copies of the booklet, “Derating: Outdoor Temperatures for Various U.S. Cities and Temperatures Inside Conduits on Rooftops Exposed to Direct Sunlight,” are available upon request from the CDA. In addition, PDF versions of the booklet can be downloaded from the CDA Web site. The booklet includes outdoor temperature data for more than 700 U.S. cities as well as temperatures inside conduits at four distances about the rooftop for each city. For reference, the maximum dry-bulb temperature for that city is also shown.

The slide tool, titled “Rooftop Ampacity Adjustments,” allows users to determine the adjusted ampacity for various copper wire sizes and types, based on estimates of the temperature inside the conduit. Hard copies of the slide tool are offered at no charge from CDA upon request. However, no PDF version of the slide tool is available.

Both guides can be ordered online at the publications section of the CDA Web site, www.copper.org, or by calling 888-480-WIRE.