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Thread: Temperature Ratings

  1. #1
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    Temperature Ratings

    Hi,

    This is a two-part question concerning PV wire de-rating:

    Q1) Let's say we're considering the wire run from the junction box attached to the rails, to the inverter which is mounted upright on the side of the house. Most of the wire is run through the attic in LFMC. There's a tiny portion of the conduit run in EMT from the j-box straight-down to the where it penetrates the roof. Let's say it's only a 5" distance.

    Would the Rooftop Added Temperature Per Table 310.15(B)(3)(c) require me to include a temperature adder because of that 5" distance, or can I just bypass this since it's such a small part of the run, with most of it is in the attic?


    Q2) I have often been confused on why THWN-2 is used in many PV installations, even though we are required to adhere to the 75 degree Celsius temperature terminal rating. Wouldn't it make more sense to just use 75-degree rated wire if we can't exceed the ampacity of the 75C column? Maybe I'm missing something.

    Thanks in advance!

    Regards,
    Andy

  2. #2
    Join Date
    May 2011
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    4,194
    Quote Originally Posted by andrew.tkelly View Post
    Hi,

    This is a two-part question concerning PV wire de-rating:

    Q1) Let's say we're considering the wire run from the junction box attached to the rails, to the inverter which is mounted upright on the side of the house. Most of the wire is run through the attic in LFMC. There's a tiny portion of the conduit run in EMT from the j-box straight-down to the where it penetrates the roof. Let's say it's only a 5" distance.

    Would the Rooftop Added Temperature Per Table 310.15(B)(3)(c) require me to include a temperature adder because of that 5" distance, or can I just bypass this since it's such a small part of the run, with most of it is in the attic?
    Not in my opinion, but it strikes me as quite the matter for interpretation.


    Q2) I have often been confused on why THWN-2 is used in many PV installations, even though we are required to adhere to the 75 degree Celsius temperature terminal rating. Wouldn't it make more sense to just use 75-degree rated wire if we can't exceed the ampacity of the 75C column? Maybe I'm missing something.
    Yes you are missing that there are two different requirements and you have to meet the more stringent of the two. The terminal temperature rating requirement means that you have to look at the 75C column and find the minimum wire size there without doing derating calculations. The other requirement (derating) is to take temperature and current carrying conductor ratings and find the minimum size there. In PV rooftop calculations, the latter calc often rules because of rooftop temperatures. Using 75C wire instead of 90C wire often means you'd have to use the next size up.

    May be easier to understand with an example...

    15A circuit current, plus continuous use factor = 18.75A circuit current requirement.

    1st requirement (terminals): Per the 75 column, you could use minimum 14awg.

    2nd requirement (derating):
    Say you have two circuits (.8 factor) and need a temperature derating factor of .65.
    18.75/0.8/0/65 = 36.05A needed in the table.
    With 90C wire that's 10awg, with 75C wire that's 8awg.

    Many times it won't make a difference, but many times it will.

  3. #3
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    Mar 2015
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    Thanks, jaggedben. I have a much better understanding of what is required by Code. I just wanted to request one last bit of follow-up clarification:

    At the of your response to Q2, you wrote that with a 90C wire, the required size would be 10awg; with the 75C wire, the required size would be 8awg. Can I choose either of these and still be within code? For example, if I chose a THWN-2 #10 for the circuit.

    Andy

  4. #4
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    Quote Originally Posted by andrew.tkelly View Post
    Thanks, jaggedben. I have a much better understanding of what is required by Code. I just wanted to request one last bit of follow-up clarification:

    At the of your response to Q2, you wrote that with a 90C wire, the required size would be 10awg; with the 75C wire, the required size would be 8awg. Can I choose either of these and still be within code? For example, if I chose a THWN-2 #10 for the circuit.

    Andy
    You always use the larger of the two conductor sizes when the derated 90 degree ampacity calculation and the 75 degree continuous use calculation result in different selections.

  5. #5
    Join Date
    May 2011
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    Quote Originally Posted by andrew.tkelly View Post
    ...

    At the of your response to Q2, you wrote that with a 90C wire, the required size would be 10awg; with the 75C wire, the required size would be 8awg. Can I choose either of these and still be within code?
    Yes. I doubt that choosing 8awg 75C wire (and possibly needing larger conduit) would ever save you money over 10awg 90C wire. However, you might want to use 8awg to lower voltage drop, in which case you could spec THWN on plans instead of THWN-2.

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