Code Change Summary: The previous measurement of 7/8 inch (which triggers the rooftop temperature adder) has been changed to ¾ inch.
During the 2017 NEC® development process, Table 310.15(B)(3)(c) from the 2014 NEC® which provided ambient temperature adjustment factors for raceways or cables exposed to sunlight on or above rooftops was deleted and replaced with a simple measurement of 7/8 inch. The 7/8-inch measurement was likely chosen to represent typical Unistrut dimensions.
The previous NEC® table (first introduced in 2008 after a study showed significant heating of wiring methods installed close to a roof surface where direct sunlight is present) provided numeric values to be added to the normal ambient temperature based on how close a raceway or cable was to the hot roof surface. The further away the wiring method was from the rooftop, the smaller the temperature adder was.
The concept was this: A wiring method exposed to sunlight and very close to the roof surface will be hotter than a wiring method with a bit of distance from the rooftop. The closer the wiring method is to the roof surface; the more indirect heat is reflected back to the wiring method from the roof surface. As the wiring method gets hotter, wiring in the raceway or cable cannot continuously carry the same ampacity and must be derated.
The issue of ambient temperature and correction factor adders for raceways and cable assemblies installed above rooftops has been seriously debated by code panel 6 for several cycles. In the 2017 NEC®, a code change proposal from a special task group was accepted to delete the table.
The task group was assigned to review the facts surrounding the initial testing for rooftop wiring methods that resulted in the table being added to the NEC® in 2008. The task group found no substantial data that warranted the need for a table with several adjustment factors and agreed to delete the table but retain the 60°F temperature adder which would apply to raceways and cables installed closer than 7/8 inch to the roof surface.
From the 2017 NEC® to the 2020 NEC® rule was this: Where raceways or cables are exposed to direct sunlight on or above rooftops, raceways or cables shall be installed a minimum distance above the roof to the bottom of the raceway or cable of 7∕8 inch. Where the distance above the roof to the bottom of the raceway is less than 7∕8 inch, a temperature adder of 60°F shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b) for the 2017 NEC®, or Tables 310.15(B)(1) or 310.15(B)(2) for the 2020 NEC®.
The 7/8-inch measurement in the above code language was likely selected to represent typical unistrut dimensions since the initial test results showed the most significant heating occurred when the raceway or cable was installed closer than ½ inch to the roof surface.
In the 2023 NEC®, the 7/8-inch measurement was changed to ¾ inch. Different manufacturers of strut create their product with different dimensions. Some strut members are ¾ inch which would still result in more than ½ inch between the bottom of the raceway or cable and the roof surface and provide more options to achieve the minimum spacing before the temperature adder kicks in.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2023 NEC® edition of NFPA 70.
2020 Code Language:
310.15(B)(2) Rooftop. For raceways or cables exposed to direct sunlight on or above rooftops where the distance above the roof to the bottom of the raceway or cable is less than 7⁄8 inch, a temperature adder of 33°C (60°F) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(1) or Table 310.15(B)(2).
2023 Code Language:
310.15(B)(2) Rooftop. For raceways or cables exposed to direct sunlight on or above rooftops where the distance above the roof to the bottom of the raceway or cable is less than 3⁄4 inch, a temperature adder of 33°C (60°F) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(1)(1) or Table 310.15(B)(1)(2).
Based on the 2023 NEC, which of the following is true for a raceway exposed to direct sunlight on or above the rooftop?
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