Table 310.15(B)(3)(c) Raceways and Cables Exposed to Sunlight on Rooftops.
Code Change Summary: Table 310.15(B)(3)(c) was deleted.
For several code cycles, NEC® Table 310.15(B)(3)(c) provided ambient temperature adjustment factors for raceways or cables exposed to sunlight on or above rooftops. The table 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 special task group found no substantial data that warranted the need for the table.
Even though the overall table was deleted, one of the previous temperature adders still remains within the revised text of the code section. Now, the rule is simply 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 in. Where the distance above the roof to the bottom of the raceway is less than 7∕8 in., 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).
Below is a preview of Article 310. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC® edition of NFPA 70.
2014 Code Language:
310.15(B)(3)(c) Raceways and Cables Exposed to Sunlight on Rooftops. Where raceways or cables are exposed to direct sunlight on or above rooftops, the adjustments shown in Table 310.15(B)(3)(c) 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). Exception: Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment.
Exception: Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment.
2017 Code Language:
310.15(B)(3)(c) Raceways and Cables Exposed to Sunlight on Rooftops. 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 23 mm (7∕8 in.). Where the distance above the roof to the bottom of the raceway is less than 23 mm (7∕8 in.), 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)(2)(a) or Table 310.15(B)(2)(b).
Exception: Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment.