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Hi king_of_neworleans,
Sorry for the late post, but it does not seem to me that your question about maximum string voltage was sufficiently answered.
Please see the steps below and attached spreadsheet for proper PV module string sizing.
1. Enter data from module datasheet. Primary inputs for this exercise are open circuit voltage (Voc), the Voc temperature coefficient, and the Normal Operating Cell Temperature (NOCT). Typical field practice is to wire the series strings as long as you conveniently can. This reduces the 'homerun' cables (cost and losses) from the strings to the next connection point (for one string, this can connect directly into the charge controller inputs. For >= 2 strings, you will likely require a combiner box enclosure with overcurrent protection if you have ; overcurrent protection can also be accomplished with inline fusing). Re: conversation about dangerous levels of voltage, we normally put 600Vdc max on houses in the US and 1000Vdc in Europe and in US commercial and utility solar projects. Most off-grid charge controllers are limited to 150Voc for reference. I charge a 48V battery with a ~130Voc PV array on my pontoon boat.
2. Enter max and min voltage from charge controller or inverter datasheet.
3. Establish the module voltage operating range. You can pull ASHRAE data or historical local weather data with an online search. If your system is in operation year round, use the lowest recorded temperature for that location. If you are travelling, you should consider the lowest temperature place where your system will operate.
4. Calculate the min and max operating cell temperatures. Equations are listed in the spreadsheet. Max voltage will occur at coldest temperature, typically at system 'wake-up' in the morning. Notice that the lowest cell temp is equal to the lowest ambient temp. However the highest cell temp will exceed the highest ambient temp by 20-30 deg C.
5. Module-level Voc(max) and Vm (min) will calculate from this according to the voltage temperature coefficient.
6. The maximum and minimum string length will calculate - for max, you divide the max input of the CC by the Max Module Voc and round down. Ideally you want to stay within the MPPT window of the charge controller - so strings that are too short can also be a problem.
Let me know if you have any questions and good luck with your project.
Matt
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