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How to determine the Wiring loss parameter ? https://forum.pvsyst.com/viewtopic.php?f=29&t=48 
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Author:  André Mermoud [ Tue Nov 06, 2012 10:57 am ] 
Post subject:  How to determine the Wiring loss parameter ? 
The wiring loss is computed using the equivalent Wiring Resistance Rw of the whole array as basic parameter (calculation R * I² at each hour). This resistance is calculated "as seen" from the inverter input, i.e.putting all strings, connected to all MPPT inputs of a subarray, in parallel. This parameter may be determined in 3 different ways:  At an early stage, you can specify a generic loss as a percentage at STC conditions (default value 1.5%).  Then you can calculate this resistance of the array's wiring by yourself according to the effectve wiring properties of your system.  You have a tool for the evaluation of this resistivity (button "Detailed losses"). This tool asks for the average wire lenghts, allows to determine the wire sections at each stage, for a given loss target at STC. It also evaluates the copper mass, and eventually the wiring cost of your plant. You can define here intermediate junction boxes between strings and inverter inputs. NB: Wiring losses behave quadratically with the current (R·I²). The loss at 500 W/m² will be half the loss at 1000 W/m² (in %) or a quarter (in [W]). Therefore the yearly energetic losses may only be evaluated on an hourly basis during the simulation, and is depending on the power distribution. The energetic loss appearing in the arrow loss diagram is usually far below your STC definition (in %). The Yearly/STC loss ratio is usually around 60% for the middleEurope systems. 
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