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Solar Panel Wiring

How to Choose Cable and Wire for 12V solar panels

Cable and wiring for 12V solar power needs to be thicker than AC wiring because the conductors need to carry a larger current for the same power output.
In Australia the AC power supplied to homes is 240 volts and so if you have a microwave oven that uses 1200 watts when running then the AC current flowing through the AC power lead will be about 5 amps.

Now if you use a 12 volt battery to power an inverter in order to use the same microwave then the current flowing from the battery to the inverter would be 100 amps. That is a very high amperage flowing the the cable!
We can verify this with the Power Law Equation which states that electrical power (watts) is equal to the voltage (Volts) multiplied by the current (Amps), so for the 240V AC supply: 1200W = 240V x 5A, but the 12 volt battery power supply to the inverter: 1200W = 12V x 100A.

Therefore the cable from the battery to the inverter needs to be very thick to carry a 100A current without overheating because even though electrical wires are excellent conductors they still resist the flow of current and some of the power is used to overcome this resistance and this process creates heat in the cable.

Even when the cable is thick enough to withstand overheating we still need to consider the power lost by the cable's resistance. This resistance is a property of the cable and is proportional to the cable's length and inversely proportional to its thickness. so a cable will have the same resistance regardless of the volts and amps in the circuit. Manufacturers state the resistance for a particular cable in the specifications as ohms per Km or as ohms per meter.

The power used to overcome resistance is wasted and causes a loss of voltage called the voltage drop. Using a famous electrical equation called Ohms law we can use the specified cable resistance to see how much power is wasted as a drop in voltage.

For the microwave oven let's say the inverter cable needs to be 2 meters long. We do not want more than 5% loss of voltage (voltage drop). 5% of 12 volts is 0.6 volts so the voltage drop from the battery end to other end of the 2 meters long cable where it connects to the inverter should not be more than 0.6 volts. Ohms law states that volts is equal to the current (Amps) multiplied by the resistance (Ohms) and we need to find the resistance for a cable 2 meters long which can carry a current of 100A with a voltage drop of only 0.6 Ohms, so if V = IR then by juggling the equation around we can say R = V/I and in our case R = 0.6V/100A = 0.006 Ohms.

The resistance in the cable is proportional to the length so our 2 meter cable would have twice the resistance of a 1 meter cable, therefore the cable needs to have a resistance of 0.003 Ohms per meter or less and looking at various available low voltage DC cable sizes we find that 8 gauge cable with a wire diameter of 4mm has a specified resistance of 0.0025 ohms/m and as that is less than our maximum allowable resistance of 0.003 ohms/m it would appear to be suitable for this application.

But it is not! The specifications for the cable also state that the cable's maximum current capacity is 50amps, therefore for our application where we are using a short cable the voltage drop is not the prime concern, we must ensure that we select cable rated at more that 100 amps to avoid overheating and a possible fire hazard.