How to Determine the Wire Size From a Solar Controller to Batteries

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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
  • Calculator
  • 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.

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References

  • Photo Credit electric cable image by jimcox40 from Fotolia.com
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