Temperatures in string sizing and cable derates

[Andrew445]

I have seen many opinions on this subject, and I am just looking for a general consensus.

Firstly: Which temperature do you use when calculating the amount of modules allowed in a string? Typically I use the equation with the module temperature coefficient and I grab my maximum temperature from the ASHRAE location nearest the site and use the extreme annual mean minimum dry-bulb temperature.

Second: When derating cables run in the middle of the array, what temperature is used? I have typically gone with the extreme annual dry-bulb maximum for n=20 years or 50 years. I have also been advised to add a 10 degree Celcius adder to this temp. to account for the heating of the modules themselves affecting the cable temp.

I'll be glad to hear any thoughts!

 

[ggunn]

I have seen many opinions on this subject, and I am just looking for a general consensus.

Firstly: Which temperature do you use when calculating the amount of modules allowed in a string? Typically I use the equation with the module temperature coefficient and I grab my maximum temperature from the ASHRAE location nearest the site and use the extreme annual mean minimum dry-bulb temperature.

Second: When derating cables run in the middle of the array, what temperature is used? I have typically gone with the extreme annual dry-bulb maximum for n=20 years or 50 years. I have also been advised to add a 10 degree Celcius adder to this temp. to account for the heating of the modules themselves affecting the cable temp.

I'll be glad to hear any thoughts!

Usually I look at the ASHRAE tables along with climate data (record low and highest monthly average high) from weather.com and make a judgement call . Then to the high temperature I add 20-35 degrees C depending on the type of mounting system - best case is pole mount and worst is flush rooftop - to account for air circulation or lack of it. I am always more conservative with the cold temperature because the penalty for overvoltage is much worse than for undervoltage.

For conductor temp in the array, I use the same high temp as for the modules.