Is there a good method or rule of thumb for determining ambient air temperature for wire runs through non-climate controlled areas in residential applications (e.g. attics -- I've read literature from the Copper Development Association that attics can get anywhere from 30 degrees to 50 degrees hotter than outdoor air temperature)? There seem to be so many different factors that it would be difficult to get right unless the area is climate-controlled.

The NEC does not seem to give any guidance on how to determine ambient air temperature for use in applying ambient air ampacity correction factors (e.g. Table 310.15(B)(2)(a)):

NEC:2011 Table 310.15(B)(2)(a)

  • Maybe I missed it, but it doesn't seem that the article you linked to cites the actual study that they are referencing. I'd like to read the study myself, to see what the parameters were.
    – Tester101
    Jun 4, 2015 at 12:52
  • If you had the time, you could do an experiment. Put a temperature sensor on the roof, another in the attic, another about halfway down in the insulation in the attic (where wiring might be run), and a final one outside in the shade (to measure outdoor ambient). Collect data for a year (or ten), and then graph all the data to see how the temperatures relate to each other. Tracking weather conditions during the experiment might also be interesting. Don't forget to apply for a government grant, to fund the study.
    – Tester101
    Jun 4, 2015 at 12:56
  • I cannot find the exact article the CDA uses as its source, but Figure 3 from this study shows attic temperatures up to ~43 degrees F higher than ambient using (the study does not seem to use anything near good statistics, but it at least shows temperatures differences that high are possible): fsec.ucf.edu/en/publications/pdf/FSEC-PF-337-98.pdf Jun 4, 2015 at 21:02

2 Answers 2


Size it for the highest expected temps in the area (geographically and part of building - huge differences there). If on the border line, then size to the conservative side and use bigger conductors or more runs.

In my area, the high temps would be 105'F If we add another 50'F for an attic (worst case per your numbers) that would place me in the 155'F+ bracket. However, a crawl space would likely just be based on the 105'F or less.

  • Why "50'F for an attic"? Where does that value come from?
    – Tester101
    Jun 4, 2015 at 11:52
  • His reference above.
    – Ben
    Jun 4, 2015 at 12:28
  • @Ben While this is certainly an answer that accounts for the absolute worst case, I would tend to think very few follow this method given the associated cost increase for even relatively mild climates when you bump up to 150-158 degrees F: For typical 12 AWG wiring for 20 A lines, you would need to upsize all attic runs to 10 AWG for wire with 90 degree C rated insulation (0.58 ampacity adjustment factor) and size 6 AWG for wire with 75 degree C rated insulation (0.33 ampacity adjustment factor). While I want safe wiring, I also do not want to over-design if unnecessary. Jun 4, 2015 at 12:45
  • Sure - every person would have to make their own educated guess on temperatures they will experience - Maine and Arizona will have very diff numbers as will every attic depending on roof materials, ventilation etc. if you can't get that gov grant tester was suggesting, then you'll have to make your own best guess. You can always add ventilation if you think it is more cost effective than increasing wire size.
    – Ben
    Jun 4, 2015 at 13:41

There is no formal guidance or rule of thumb for determining ambient air temperature for wire runs through non-climate controlled areas in residential applications.

Ultimately, since it is open to interpretation, check with your local inspector on what they will accept.

If the non-environmentally controlled space is going to be close to or hotter than outside temperature (e.g. an attic) a good place to start is sizing the wire to account for maximum ambient outdoor temperatures recorded in your area (ASHRAE temperature tables are a good reference).

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