PV modules, inverters, disconnects, wiring, and overcurrent protection devices are rated to handle only so much voltage. Equipment used for residential and commercial PV systems in the United States is rated up to 600 VDC, so it is important to make sure a PV array is configured so that this 600-volt rating is not exceeded.
In cold, sunny conditions, array voltage will increase—you’ll need to account for this when designing your system so the voltage stays below the limit. That involves some math, and knowing the lowest expected ambient temperature at your site.
NEC Article 690.7 dictates that if the PV module manufacturer provides a temperature coefficient of open-circuit voltage (TkVoc), it must be used in the calculation. This coefficient tells us how much a module’s voltage will increase per °C below the standard test condition (STC) of 25°C. The temperature coefficient will be listed in volts per °C; millivolts (mV) per °C; or as a percentage per °C. Most module manufacturers provide this data in their module specification sheets.
If a module has a TkVoc of -0.120 V per ºC this means that, for each °C below 25°C, the module voltage will increase by 0.120 volts. If you have a module with a TkVoc given in % per ºC, multiply this TkVoc by the module’s open-circuit voltage (Voc). A module with a Voc of 36.9 volts and a TkVoc of -0.36% per ºC will have a 1.333 voltage increase for each degree below 25°C.
0.0036 × 36.9 V = 0.133 V
Once we have this calculation, we must determine the lowest expected ambient temperature. The 2011 NEC points us to “Extreme Annual Mean Minimum Design Dry Bulb Temperature” found in the ASHRAE Handbook—Fundamentals. If you don’t have access to that handbook, this “extreme min” temperature data is now listed for many locations at www.solarabcs.org/permitting/map/.
Let’s assume our array uses modules with a Voc = 36.9 volts and a TkVoc = -0.36% per ºC, and is located in Albany, New York. The extreme minimum temperature for this location is listed as -23°C. This is 48°C lower than the STC temperature.
-23ºC - 25ºC = -48ºC
Using this value, along with the TkVoc, results in a module voltage increase of 6.38 volts (48ºC × 0.133 V = 6.38 V). That means our maximum module Voc is now 43.28 V.
36.9 V + 6.38 V = 43.28 V
Now that the module voltage has been “adjusted,” multiple it by the number of modules in series to determine the maximum system voltage. If our array consists of 12 of these modules in series, the resulting maximum system voltage is 519.4 volts, which is under the 600-volt limit. However, if we had 14 of these modules in series, the 600-volt limit could be exceeded (43.28 V × 14 = 605.9 V) given this location and these modules.