Solar photovoltaic (PV) panels can generate high voltages and electrical currents when used to charge electrical storage batteries. A battery charge controller is essential if the batteries are to be charged correctly, and are to be protected against overcharging. High currents require wires thick enough to conduct the electricity without getting hot and possibly catching fire. But if the wires are oversized, the cable is heavy, expensive and unnecessary. Determining the correct wire gauge for the wiring of a solar PV system is important.
Things You'll Need
- Wire gauge table
Look at the installation manual for the battery charge controller. The specifications for a battery charge controller will give the nominal voltage of the controller--either 12 or 24 volts--and the maximum rated current in amps. Make a note of the voltage and the current. Measure the distance in feet between the charge controller and the batteries.
Look at a copy of the American Wire Gauge (AWG) table for copper wire (see References). The left column shows the AWG gauge. This number specifies the thickness of the wire and how much electrical current it can safely carry. The third column shows "circular mils" (CM). This number refers to the cross-section area of the wire.
Calculate the circular mils that you need. This is given by the formula: CM equals 22.2 multiplied by the current in amps, multiplied by the wire length in feet, divided by the voltage drop along the wire measured in volts. A 3 percent voltage drop is generally used. If you have a 12-volt charge controller, the voltage drop would be 12 multiplied by 3 divided by 100 which equals 0.36 volts. For example, if the current is 20 amps and the wire length is 28 feet, you would calculate CM as 22.2 times 20 times 28 divided by 0.36. The answer is 34,533 circular mils.
Consult the AWG table. This number of circular mils falls between AWG 5 and AWG 4. The value is close to AWG 5 wire, which is less expensive than AWG 4. In this case, choosing AWG 5 copper wire would be a reasonable choice.
Tips & Warnings
- The AWG copper wire table is conservative. Solar energy equipment suppliers tend to recommend lighter cables that are less expensive. It is worth contacting the supplier of your charge controller and asking for advice. If a lighter gauge cable is recommended, consider selecting a wire gauge somewhere between what you have calculated and what is being recommended by the supplier. The thinner the wire, the hotter it will become, and the greater will be the loss of voltage. You can reduce the required thickness if you shorten the length of wire, so install the charge controller as close as possible to the batteries.
- Photo Credit electric cable image by jimcox40 from Fotolia.com
Battery Cable Sizing Guide
Whether you are changing or resizing your car battery or a deep-cycle battery bank for a renewable energy system, it is important...
How to Connect a Solar Panel to a Car Battery
Solar power can provide you with light, utility and entertainment in any location, lending itself particularly well to a rural or "off-the-grid"...
How to Determine a Solar Charge Controller Rating
Using the sun’s energy to generate electricity for home and business use is a great way to lower utility bills and create...
How to Wire a Solar Panel to a Battery to an Inverter
A solar panel uses the photoelectric effect to convert the sun's energy to electricity. This electricity is generated at a voltage characteristic...
How to Wire a Solar Inverter
Solar inverters are an essential component of a home solar panel installation. There are four components to connect together: the solar panels,...
How to Charge a 24 Volt Battery With a 12 Volt Solar Panel
Many solar energy systems use 24 volt batteries. The batteries in these systems are charged through the use of solar panels; however,...
Wiring for Solar Panels
The wiring will connect the solar panels to the charge controller. Learn about wiring for solar panels from a professional in this...