SOLAR PV MODULE WIRE DESIGN EXAMPLE

Example: Crystalline silicon module 120 watt at STC (standard testing conditions)
• Voc=21.0V, Isc = 7.2A, Vmp = 17.1V, Imp = 7.0A, Pmax =120 watt.
• Voltage temperature coefficient = -0.5%/°C
• Module Dimensions 660mmx1420mm
• Weight 11kgs.
Circuit Current: It is also called Continuous Current is 125% of short circuit current. In our module specifications, Isc is 7.2A, so Circuit Current or System Current is 7.2x1.25= 9.00 A(A= Ampere). Solar PV module is current limitted device so it's current will not increase beyond 9.00A.
Over Current Device Rating: These devices are fuses or breaker used to save the module and module wiring. This designed 156% of short circuit current or 1.25% of continuous current. In our example it is 1.56xIsc=1.56x7.2= 11.23A. This current limit is only applicable if this device operates below 40°C ambient temperature .
Cable sizing: It's ampacity is designed 156% of short circuit current at 30°C conductor temperature. In our example it is 11.23A
Cable Derating: In actual conditions the module is operated above 30°C to ambient temperature. If the ambient temperature is 48°C, than the module cell temperature will be =48+30=78°C (If module is placed on the roof top) & if 3" to 6" space is provided between the module and roof for air circulation than the cell temperature will be 20°C above the ambient temperature, =48+20=68°C. The cable will work at temperature so always select the 90°C wet rated cable. Selection of cable based on the location
1. Free air: For free air USE-2 single conductor cable 90°C wet- rated & sunlight resistant) is used for exposed applications. 2.
2. Type TC Multi conductor cable for exposed conditions with THWN-2, or XHHW-2, or RHW-2 or equivalent 90°C wet- rated conductors in the cable.
3. Race way or through conduit: Conductors type THWN-2, or XHHW-2, or RHW-2, or equivalent 90°C wet- rated conductors in high rated conduit (Conduit rated for a minimum of 75°C wet conditions.)
From our example four PV modules are connected to make 24 volt circuit. (Two modules are connected in series). Two module sets are connected to junction box in parallel. The wires are already supplied with modules. Now the wires are going from junction box to combiner box through conduit.