In this knowledge article we discuss the importance of selecting the correct size cable for the job.
The main factors when choosing cable are
- The physical protection properties
- The electrical characteristics
1. Physical Properties
It is very important to use actual solar cable when dealing with solar systems. This is because it is designed to withstand the harsh UV operating environments it is subjected to. The cable should be rated with IP67 connectors in order to reduce any potential of corrosion, especially in a marine environment. It is also best to use MC4 connectors when ever possible. MC4 connectors are standard in the solar industry and if selected correctly will usually have all the correct IP ratings. Lux solar cables are all rated to IP67 or higher and include MC4 connectors to simplify the connection process.
2. Electrical Properties
In order to choose the correct sized cable we must understand Ohms law. Ohms law states that the voltage drop across a cable is proportional to the current flow in the cable. Most good solar cable will specify a resistance per metre or Ω/m. This will vary with the thickness of the cable. Commonly used solar cables are 2.5 mm², 4 mm² and 6 mm². The value of resistance can be calculated using R = ρL/A
- R = Resistance
- ρ = Resistivity of the material, for example, copper
- L = length
- A = cross sectional area, for example, 4 mm²
Alternatively the resistance can be obtained from the cable datasheet. Approximate values for each cable are.
- 2.5 mm² = 8 mΩ/m
- 4 mm² = 5 mΩ/m
- 6 mm² = 3.3 mΩ/m
To calculate the voltage drop just multiple the cable length, the ohms per metre (Ω/m) and the current flow in the cable. For example
The voltage drop on a Lux solar 10 metre solar cable connected to a 100 watt panel will be
Voltage drop = Ω/m x cable length x curent
= 5 mΩ/m x 10 metres x 5.8 Amps
= 0.29 Volts
The voltage produced on a 100 watt panel is 17.6 volts. Therefore this will be reduced to 17.6 - 0.29 volts = 17.3 volts by the time it reaches the solar controller.
At Lux solar our degree qualified engineers have done all the maths for you for every cable we use. Please ask us what length of cable is correct for your application or simply buy one of our predesignated kits.