Understanding Section 310.15(B)(3)(c)

In the 2017 National Electrical Code®, raceways or cables installed on rooftops are now mandated to be installed 23 mm (⅞ in.) above the rooftop to avoid a temperature adder of 33°C (60°F). The Previous Table 310.15(B)(3)(c) found in the 2014 National Electrical Code® [NEC®] was deleted.


2017 National Electrical Code® Change - Table 310.15(B)(3)(c)


Outdoor conditions where cables or raceways are installed on rooftops that are also exposed to the direct rays of the sun, additional care has to be taken regarding the enclosed electrical conductors. These electrical conductors and their vital insulation are subject to excessive heating that must be taken into account.


The ambient heating these raceways and cables are exposed too will increase the temperature inside of the raceway or cable. Where raceways or cable are installed at less than (⅞ in.) from the rooftop surface or even in directly contact with the rooftop surface, additional reflective heating from the rooftop surface will continue to increase the temperature of the annular space within the raceway or cable system.


The 2017 National Electrical Code® simplified these requirements over the more complex rule found in the 2014 National Electrical Code® by reducing the previous four temperature “adder” levels to a single “adder” value to make this rule easier to apply in the field. It was also determined by various testing groups that the required ambient corrections already mandated by 310.15(B)(2)(a) are sufficient when raceways or cables are installed above (⅞ in.).


Understanding National Electrical Code Section 310.15(B)(3)(c)

Note: Under the exception to this rule; Using a product like Encore Wire’s Type XHHW-2 insulated electrical conductors in raceways or cables installed less than (⅞ in.) from the rooftop surface or even in direct contact with the rooftop are permitted to ignore the additional temperature “adder” described in the general rules of 310.15(B)(3)(c).


However, the installer still has to take the ambient temperatures into consideration as normal when sizing the electrical conductors for their ultimate condition of use.

Summary - The 2017 National Electrical Code® deleted Table 310.15(B)(3)(c) and replaced with new text added within section 310.15(B)(3)(c). This new verbiage now requires a temperature adder of 33°C (60°F) only when a raceway or cable is installed less than 23 mm (⅞ in.) above or directly on a rooftop.


Paul W. Abernathy, CMI, CMECP®


National Electrical Code® and NEC® are registered trademarks of the National Fire Protection Association. The references to National Electrical Code® and NEC® are used in this article for informational purposes only

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