Code Change Summary: The previous ambient temperature and rooftop correction factor tables have been renumbered.

In the 2020 NEC^®, the relocation of the medium voltage conductor requirements from Article 310 to new Article 311 created a domino effect resulting in the renumbering of tables and code sections remaining in Article 310.

• Section 310.15(B)(3)(c) pertaining to raceways and cables exposed to sunlight on rooftops is now Section 310.15(B)(2) and the title was shortened to “Rooftop”.
• Table 310.15(B)(2)(a) Ambient Temperature Correction Factors Based on 30°C (86°F) is now Table 310.15(B)(1).
• Table 310.15(B)(2)(b) Ambient Temperature Correction Factors Based on 40°C (104°F) is now Table 310.15(B)(2).
• Table 310.15(B)(3)(a) Adjustment Factors for More Than Three Current-Carrying Conductors is now Table 310.15(C)(1).

The above code sections and tables are pretty much the same other than a few minor changes to provide clarity and usability.

A specific change occurred in the title and text to previous Section 310.15(B)(3)(c) pertaining to raceways and cables exposed to sunlight on rooftops. The new section is 310.15(B)(2) and the title was shortened to “Rooftop”.

The language was cleaned up in 310.15(B)(2) since the previous language first provided a mandatory rule to keep raceways and cables at least 7/8 inch above the roof and then provided a way around the rule by requiring a temperature adder of 33°C (60°F) to be added to the ambient temperature when determining the overall outdoor ambient temperature for the purpose of applying correction factors.

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.

In the 2020 NEC^®, any raceways or cables exposed to direct sunlight on or above rooftops that cannot be installed at least 7/8 inch above the roof to the bottom of the raceway or cable, require a temperature adder of 33°C (60°F) to be added to the outdoor temperature to determine the applicable ambient temperature for applying the correction factors in Table 310.15(B)(1) or Table 310.15(B)(2). The requirement does not apply if type XHHW-2 insulated conductors are used.

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 2020 NEC^® edition of NFPA 70.