Copper Development Association Offers Free Guides to Ampacity Adjustments for Wire and Cable Derating for Commercial Buildings

October 26, 2007


Slide Tool Calculates Adjustments for Rooftop Conduits in Sunlight

NEW YORK, NY— The Copper Development Association announces two new guides for sizing the wires and cables inside conduits on rooftops in accordance with the 2008 National Electrical Code (NEC). Available free from CDA, these two publications make it easy to estimate temperatures inside electrical conduits at various heights above the rooftop for hundreds of cities across the US and make the necessary ampacity adjustments required by the Code.

Conduits on rooftops of commercial buildings can be more than 70 degrees Fahrenheit (Δ 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 temperature inside the conduit may exceed 165°F even with no current passing through the conductors. The 2008 NEC now recognizes that the air inside conduits on rooftops can be significantly hotter than the surrounding air. Ampacity ratings of various wire sizes and types must be adjusted to account for these elevated temperatures.

The Copper Development Association (CDA) has created a booklet and a slide tool to assist inspectors, engineers and contractors in making ampacity adjustments for wire and cable in rooftop conduits:

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

The slide tool is titled Rooftop Ampacity Adjustments. It allows one to easily 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 available free of charge upon request from CDA. There is no PDF version of the slide tool.

According to David Brender, national program manager for electrical markets, "These tools are the first of their kind. They will enable electricians, electrical inspectors, and design engineers to do their jobs more efficiently and adhere to the Code. These invaluable tools eliminate tedious manual calculations without sacrificing precision."

Both publications are offered at no-cost and may be ordered online at the publications section of the CDA Web site, Multiple copies are available by request.