William,
One way to think about this is that temperature correction is used based on the location of the ambient temperature you are concerned about. Mostly, we are worried about the very high temperature of conductors in a sunlit raceway on a roof. This gives us very high temperatures and we use the 90C table and 90C temperature correction factors. In no case can we have an ampacity that is greater than the 75C table at 30C since the terminal is rated that way. It does not matter what kind of insulation we have on the wirethe wire cant get hotter than 75C at the terminal. What the code is not very good at is telling us what to do with a combiner box on a rooftop with 75C terminals. What is the ambient temperature of a 75C terminal in a rooftop combiner box? Kinda, really, pretty hot, I guess Thats about what we have to go on. If the box is white it might be less. Generally, the current on the string conductors is so small that it is not an issue. However, for an inverter at ground level, the ambient temperature to use for the terminals is probably around 40Cnothing like a rooftop conduit. At the end of the day, the short answer is that you use 90C table with 90C temperature correction factors and the temperature adders for rooftop conduit. That will generally be more aggressive than a 75C terminal at 40C with the 75C temperature correction factors. You are welcome to do the calculation, however. Does this make sense? You cant double-dip and use 75C temperature correction factors on the roof where there are no terminals. That would make it impossible to install PV since these temperatures can occasionally exceed 75C. Bill. From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of William Miller Sent: Wednesday, January 22, 2014 4:32 PM To: RE-wrenches Subject: [RE-wrenches] Temperature Compensation calcs for wire Friends: I try to be rigorous in application of NEC codes to everything I do, including wire sizing. I understand that even though I am using conductors rated at 90°C, the breakers I use have terminals rated at 75°C so when deriving the values for ampacity for a given gauge from the tables, I have to use the 75°C column. What is not clear, however, is which column I use when applying temperature derating. Table 315(B)(2)(b) has a column for 75 and a column for 90. I am using 90° wire. The values for 90° are much more generous than the 75° values and I would like to use them. Which is correct? As always, thanks to everyone on this list for all of the help and advice. William Lic 773985 millersolar.com <http://www.millersolar.com/> 805-438-5600 --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com
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