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Thread: 6 10 Rooftop Temperature Adder

  1. #1
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    6 10 Rooftop Temperature Adder


  2. #2
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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]

  3. #3
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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]

  4. #4
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    So, you all figure there is no problem above 7/8" anymore...

  5. #5
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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
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

  6. #6
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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]

  7. #7
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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)

  8. #8
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    Yes to "Shorts" statement.......

    In reality I sat through this debate from the beginning and listened to both sides of the debate. Since I represent both CU and AL I wont comment on that aspect of the issue. However, while no evidence was presented in terms of REAL failures we do however know and can prove the ambients within the raceways or cables will be elevated. Elevated enough to breach the thresholds of the insulation to which this rule is trying to protect....well as you can see this one has flip flopped no less than twice in two cycles.

    As for the 7/8" I will say this.....it was a compromise from the folks who wanted to remove the "adder" and the folks that wanted to keep the "adder". No additional data was submitted. In fact, the average from the two closest values in the table were evaluated and there was only a slight variance so they compromised on the 7/8". In reality the folks that wanted the "adder" removed provided a smoke screen in my opinion....I would never install raceways directly to the rooftop surface (some might I guess but thats me), as I would use mounting blocks or other support system which would inherently place my raceway more than 7/8" from the surface which in effect makes the rule pointless....and the "adder" hatters knew this and effectually won the battle in my opinion.
    *All code responses are based on the 2017 National Electrical Code[NEC]

  9. #9
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    Quote Originally Posted by MasterTheNEC View Post
    Yes to "Shorts" statement.......

    In reality I sat through this debate from the beginning and listened to both sides of the debate. Since I represent both CU and AL I wont comment on that aspect of the issue. However, while no evidence was presented in terms of REAL failures we do however know and can prove the ambients within the raceways or cables will be elevated. Elevated enough to breach the thresholds of the insulation to which this rule is trying to protect....well as you can see this one has flip flopped no less than twice in two cycles.
    Maybe that just means the temperature ratings of the conductor insulation are much more conservative than necessary....that seams to be what they said about XHHW-2.

    As for the 7/8" I will say this.....it was a compromise from the folks who wanted to remove the "adder" and the folks that wanted to keep the "adder". No additional data was submitted. In fact, the average from the two closest values in the table were evaluated and there was only a slight variance so they compromised on the 7/8". In reality the folks that wanted the "adder" removed provided a smoke screen in my opinion....I would never install raceways directly to the rooftop surface (some might I guess but thats me), as I would use mounting blocks or other support system which would inherently place my raceway more than 7/8" from the surface which in effect makes the rule pointless....and the "adder" hatters knew this and effectually won the battle in my opinion.
    And I thought the 7/8" was picked because it is a standard strut channel size
    Don, Illinois
    Ego is the anesthesia that deadens the pain of stupidity. Dr. Rick Rigsby
    (All code citations are 2017 unless otherwise noted)

  10. #10
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    The table gave the industry guidance for location temperature on rooftops. If the table was too restrictive, adjustments should have been made. Without it, there is no guidance for designer and there will be dissension between installer and AHJ.

    I disagree with the change they all made.

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