In 2008, the National Electrical Code (NEC) added a second paragraph to 690.7(A) stating, “When open-circuit voltage temperature coefficients are supplied in the instructions for listed PV modules, they shall be used to calculate the maximum PV system voltage as required by 110.3(B) instead of using Table 690.7.” This addition was made because Table 690.7 is very conservative and the temperature coefficient will provide a more accurate voltage increase.
Listed below is the maximum voltage calculation with open-circuit voltage temperature coefficients. As daunting as it may seem it’s quite easy once you’ve done it a few times. Let’s take a look at how it works:
Inverter maximum input voltage with the temperature coefficient percentage of the VOC calculation:
(STC temp – low temp) x temp coefficient % VOC x VOC + VOC = VMax
Inverter max voltage / VMax = Maximum modules per series string
Record-low temperature: -10ºC
Temperature coefficient of (VOC): – (0.30) %/ºC
Module open circuit voltage (VOC): 39.4 V
Inverter maximum input voltage: 600V
The STC temperature is 25ºC. This temperature needs to be deducted from the array location’s record-low temperature of -10 degrees as follows:
25 – (-10) = 35º difference.
Multiply the 35º difference by the temperature coefficient of VOC (I’ve used the positive value for an easier calculation, though you get the same result) then multiply by the module’s VOC:
35 x 0.0030 = 0.105
0.105 x 39.4V = 4.137V
This is how many volts each module will increase due to record-low temperatures. Add the voltage increase to the Module VOC. Then divide the inverter maximum input voltage by that number. This will give you the maximum number of modules that can be wired in a series string per that inverter and specific location.
4.137 V + 39.4V = 43.537 VMax
600V / 43.537 = 13.7 (round down to a whole number)
The maximum number of modules in this series string is 13. A series string of 14 could potentially produce more than 600V during record-low temperatures.
Lastly, the quantity of modules wired in series multiplied by the VMax equals your maximum system voltage.
13 x 43.54 V = 566 Maximum System Voltage
Voilà, we’ve determined the max PV voltage for our example system and are able to ensure a proper system design without fear of over-voltage for the inverter.
Ronnie Raxter (guest author)
Design Application Engineer