310.15(B)(3)(c) Raceways and Cables Exposed to Sunlight on or Above Rooftops.
Code Change Summary: The title to section 310.15(B)(3)(c) has been modified to make it clear that the section now applies to more than just circular raceways on or above a roof. Also, a new exception was added to exempt type XHHW-2 conductors from the temperature adders required in Table 310.15(B)(3)(c).
In the 2011 NEC®, section 310.15(B)(3)(c) only applied to circular raceways exposed to sunlight on or above the rooftop. Now, in the 2014 NEC®, this section applies to any raceway or cable exposed to sunlight on or above the rooftop.
Table 310.15(B)(3)(c) provides temperature adders to be added to the ambient temperature correction factors in Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b). These temperature adders are based on how close a rooftop wiring method is located to the hot roof surface. The further away the wiring method is from the hot roof surface, the lower the temperature adder is.
Example: PVC coated, sunlight resistant MC cable containing THWN (75°C conductors) installed 4 in. above the rooftop in a location where the ambient temperature is 100°F.
- First, Table 310.15(B)(3)(c) requires a temperature adder of 30°F to be added to the 100°F ambient temperature for a total of 130°F ambient temperature correction factor.
- Next, use the 75°C column of Table 310.15(B)(2)(a), and the 130°F overall ambient to obtain a .67 correction factor.
- Basically, this MC cable can now only carry 67% of its normal value because of its close proximity to the hot roof surface.
Another 2014 NEC® change in this section exempts type XHHW-2 conductors from the Table 310.15(B)(3)(c) temperature adders.
If using type XHHW-2 conductors for the same example above, the additional 30°F temperature adder is not required. XHHW-2 conductors have thermoset conductor insulation which performs far superior to thermoplastic conductor insulation found on many other conductors.
The performance of type XHHW-2 conductors is such that additional temperature adders are not necessary. The additional temperature from being installed in close proximity to the rooftop does not adversely affect the conductor’s overall performance.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).
2011 Code language:
310.15(B)(3)(c) Circular Raceways Exposed to Sunlight on Rooftops.
Where conductors or cables are installed in circular raceways 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).
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.
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310.15(B)(3)(c) Raceways and Cables Exposed to Sunlight on or Above Rooftops.
Below is a Real Question from our Electrical Continuing Education Courses for Electrical License Renewal:One of the following conductor types is NOT subject to additional rooftop temperature adders. Which of the following conductors is NOT subject to additional rooftop temperature adders in Table 310.15(B)(3)(c)?
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