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  1. #1
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    6 10 Rooftop Temperature Adder


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    Mike,

    We do make XHHW-2 in 6 AWG and it is widely available. The cost of XHHW-2 over THHN/THWN-2 is negligible in terms of using this exception. It was quite interesting to sit in on CMP 6 during the 2014 NEC ROC process and listen to the debate that created the exception. While it is true Thermoset insulation does appear to have it's thermal advantages over thermoplastic, the passing and subsequent inclusion of this exception was self serving at best.

    If you are interested in the history of this exception feel free to contact me and I will gladly explain how the debate went........
    *All code responses are based on the 2017 National Electrical Code®[NEC®]

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    Rooftop Update

    UPDATE- Here is what was accepted by the 2nd Draft Meeting.

    (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∕8in.). Where the distance above the roof to the bottom of the raceway is less than 23 mm (7∕8in.), 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 XHHW2 insulated conductors shall not be subject to this ampacity adjustment.


    Informational Note: One source for the ambient temperatures in various locations is the ASHRAE Handbook — Fundamentals.
    *All code responses are based on the 2017 National Electrical Code®[NEC®]

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    So, you all figure there is no problem above 7/8" anymore...

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    Quote Originally Posted by shortcircuit2 View Post
    So, you all figure there is no problem above 7/8" anymore...
    I am not sure there ever was a real world problem....the substantiations for the original rule did not include any reports of failures of the conductors in rooftop conduits...they only included testing results that showed elevated temperatures.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

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    Quote Originally Posted by don_resqcapt19 View Post
    I am not sure there ever was a real world problem....the substantiations for the original rule did not include any reports of failures of the conductors in rooftop conduits...they only included testing results that showed elevated temperatures.
    But if there is evidence of elevated temperatures, one would need to still apply the correction factors per table 310.15(B)(2)(a) or 310.15(B)(2)(b)

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    Quote Originally Posted by shortcircuit2 View Post
    So, you all figure there is no problem above 7/8" anymore...
    Who is YOU ALL...I am not on that panel...
    *All code responses are based on the 2017 National Electrical Code®[NEC®]

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