Rooftop Conduit Sunlight Ambient Adder (NEC 310.15(B)(2))
Applies the NEC 310.15(B)(2) rooftop sunlight adder, 33 C (60 F) where a raceway is less than 7/8 in above the roof.
Example
You enter
- Measured outdoor ambient (deg-F) 95
- Base ampacity (A, 90 C column) 55
You get
- Design ambient 155
- 90 C correction factor 0.58
- Corrected ampacity (A) 31.9
Details, formula, and sources
to a measured ambient and then the 90 C-column temperature correction (a #8 at 55 A in a 95 F ambient on the roof -> 155 F design ambient -> ~0.58 -> ~32 A). Raising the raceway onto standoffs at or above 7/8 in removes the adder and recovers most of the ampacity. The AHJ-adopted edition governs.
adder = 60 F (33 C) where height above roof < 7/8 in (0.875 in), else 0; design_ambient = measured + adder; corrected = base_ampacity x (90 C-column correction factor at the design ambient).
Raceways and cables exposed to sunlight on or above rooftops, NEC 2020/2023 310.15(B)(2), with the 310.15(B)(1) 90 C-column ambient correction, by name.
NEC is free to read at nfpa.org/freeaccess. The 2017 NEC reduced the older tiered table to this single 33 C adder applied below 7/8 in; standoffs at or above 7/8 in remove it.
Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
Field names used by the API: measured_ambient_f, base_ampacity_a, design_ambient_f, correction, corrected_a
- Sunlight adder 33 C (60 F) added to the outdoor ambient where the raceway is less than 7/8 in above the roofNEC 310.15(B)(2)
- Correction basis the 90 C-column temperature-correction factor applied to the corrected ambientNEC Table 310.15(B)(1)