Cable size calculation between Solar Panel and charge control

To calculate the cable size which starts from or to the photovoltaic panel and connects to the charge controller, we use the calculation program by entering the following data (found in the specifications of the photovoltaic panel): numbers of solar panels; the voltage at the maximum power (Vmp); current at full capacity (Imp); length of the cable which starting from the panels to charge controller (m); voltage drop (dV); system voltage (V).

In our project: there are four panels; the current is 5.95 A; the voltage is 42 V; the length is 10 m; the voltage drop is 0.48V (2% of 24V), system voltage is 24 Volt. The result is 8.9 mm². We will use cables of 10 mm² type FG21

Figure 1 shows the size of the commercial cables:

Figure 1

We arrive at the same result with the formula below:

Where K = resistivity: 0.0175; I = maximum circulating current (Isc), in our case 5.95 x 2 = 11.9 A since the panels are in parallel; L = cable length (10 × 2 = 20 m); dV = voltage drop (estimated at 0.48V = 2% of 24V).

Of course, if the length varies, as a consequence, the section varies. For example, if the distance were 5 meters, the section would be:

Automatic cable size calculation

Voltage drop as a function of cable length

As mentioned before, the voltage drop must be within 2%; let's see if the condition has been met and use this formula:

ΔV = voltage drop in%; Pmax = maximum power of the photovoltaic; K (ρ) = resistivity; Lc = cable length; S = cable section; Vmax = voltage at maximum power.

 ΔV = 1.98% less than 2% so it's okay.

Characteristics of the FG21 cable 

Calculation of the cable size of the battery to the inverter

Figure 1

If I have to calculate the size of the cable (6 meters long including the return) from the battery (24 volts) to the inverter (1000 Watt), and the voltage drop is 2%, from the calculation program below, the cable size will be 9.27 mm² which I will change to 16mm² (type FG21).

 The result is the same if we use the formula:

Note: I = W / V = 1000/24 = 42 A

Calculation of the cable size from the charge controller to the battery

Figure 2

The size of the cables from the regulator to the battery is the same between the photovoltaic and regulator as the current cannot be higher (max 11.9 A), so ten mm² is fine if the distance is the same. If it were less, for example, 4 meters round trip, the size would be 1.7 mm².

The result of the formula is 1.7 mm² which I will bring to 4 mm².