Choosing The Right Wire For Your Solar Array

When planning a solar array for an RV, Cabin, or Home, selecting the proper wire is always an issue.

The type and gauge of wire is an important factor in the efficiency and safety of a solar array. Every application is different. It is wise to do some studying before you spend money on the wrong wire.

There are five basic variables in wire selection for an array.

  • Multi-conductor
  • Single conductor
  • Gauge
  • Insulation
  • Color
Charge Controller Wires

What Is The Best Wire For My Solar Array?

With a little evaluation, these questions can easily be answered.

Determining Factors

Current

Current is the main factor that needs to be assessed when selecting wire. The Short Circuit Current (ISC) rating of panels is specified on the service tag on each panel.

This is the number that will be used to select a wire gauge.
It will also be important to know if panels will be wired in parallel.

Panels in parallel create multiples of individual panel current.

Panels In Parallel

Connecting the Positive wires together and the Negative wires together of two or more panels is termed Wired In Parallel.

This arrangement boosts the current of the array. The result is the current of one-panel times the number of panels connected together. For Example:

  • 250 watt @ 24 volt panel = ISC of 10.4 amps.
  • Two of these panels wired in Parallel will result in:
  • 500 watt @ 24 volt panel string = 20.8 amps.
  • Three of these panels wired in Parallel will result in:
  • 750 watt @ 24 volt panel string = 31.2 amps.

The wire selected for the array must be rated to handle the current of the string arrangement.

Length Of Wire

Wire has resistance. The longer the wire, the greater the resistance. From panel to panel, within the array, the wire provided by the manufacturer is adequate.

Panel wire tends to be 10 gauge multi-conductor solar wire.

From the end of an array to the combiner box, and from the combiner box to the charge controller, the wire gauge becomes a significant factor in the efficiency of the array.

Stability Of Wire

It is odd to think about, but the stability of a wire can help determine the type of conductor selected.

An array on an RV or boat will experience significant movement and vibration. Over time, movement can work harden the copper wire.

This condition can cause the wire to become brittle and fracture. In such situations, multi-strand wire is preferred.

The finer wires withstand movement better than single conductor wires. In the case of a cabin or home, where the wire can be contained and kept immobile single conductor is preferred.

Most wire larger than 10 gauge is only available in a multi-strand wire. Often, larger wire is termed cable.

Exposure

Determine if all wire will be contained within a conduit or exposed to the weather. There are different insulation types for different exposure conditions.

Selecting The Wire

Once the requirements of the array have been assessed, wire selection is fairly straightforward. When considering amp and distance values, it is wise to round up to the next multiple of 5.

For example, if the distance from the array combiner box to the charge controller is 26 feet, round up to 30.

If the array current calculates out to 37 amps, round up to 40.

This will provide a little extra safety factor. Undersized wire can result in damaged conductors and create a potential for fire.

Conductor Size

The conductor is the most important element in wire selection. Here are a few scenarios to illustrate the conductor selection process.

Garage Door Opener and Lights System

  • A 12-volt, 80-watt panel is mounted to the roof of a garage.
  • A charge controller is mounted inside the garage, 12 feet from the panel.
  • A 12-volt Multipurpose Battery is on a shelf 4 feet from the charge controller.

A little math and an internet search will help determine the proper wire gauge.

  • Find the Short Circuit Current (ISC) on the service tag on the back of the panel.
  • The ISC of a 12-volt, 80-watt panel will be around 7 amps. We will round up to 10 amps.
  • We know the distance is 12 feet. We will round up to 15 feet.
  • An on-line wire gauge selector chart or wire wire gauge calculator indicates that a 10 amp circuit that is 15 feet long requires 12 gauge wire.

Since the wire in this scenario will be fixed and free of vibrations, a solid conductor wire can be used.

It would be fine to use a typical house wire-like ROMEX for this application. 12 gauge – 2 conductors + Ground (don’t forget the ground wire!) would be sufficient.

The same gauge wire can be used to connect the charge controller to the battery as it is the same current and only 4 feet away.

Supplemental RV Power System

  • Two 24 volt x 300 watt panels are mounted on top of a RV.
  • The panels are wired in parallel.
  • A charge controller is mounted in the storage area under the living area 8 feet away.
  • Two 12 volt Lithium-Ion batteries are wired in series 4 feet from the Charge Controller.

Use the same process as described above to determine the wire gauge for this system.

  • Find the Short Circuit Current (ISC) on the service tag on the back of the panel.
  • The ISC of a 24-volt, 300-watt panel will be nearly 14 amps. We will round up to 15 amps.
  • Two panels wired in parallel doubles the current produced. ISC of 14 x 2 = 28 amps.
  • We will round 28 amps up to 30 amps.
  • We know the distance is 8 feet. We will round up to 10 feet.
  • An on-line wire gauge selector chart or wire wire gauge calculator indicates that a 30 amp circuit that is 10 feet long requires 10 gauge wire.

Since the wire in this scenario may experience movement and vibrations, a multi-conductor wire should be used.

The same gauge wire can be used to connect the charge controller to the battery as it is the same current and only 4 feet away.

DO NOT use this wire to connect batteries together. Consult battery manufacturer data for proper connections of batteries.

In most cases, 2/0 or 3/0 cables will be required. Some manufacturers have specific connectors for their batteries.

Wire Insulation

The insulation on a wire protects the wire from damage and retains the electric current from getting out and doing damage. Two insulation codes will appear regularly when looking at the wire for domestic applications.

  • THW = Thermoplastic – Heat resistant – Wet
  • THHN = Thermoplastic – High Heat Resistant – Nylon coated

Both of these will be adequate for your array. They do require conduits to protect them from the elements.

Products like ROMEX package multiple wires in a convenient sheath. In some jurisdictions, such wire does not need to be in conduit.

Refer to your local code for the application of such products. Other, supplemental insulation configurations are available.

The purpose of insulation types will be printed on the surface of the wire. These include, but are not limited to:

  • “UV Resistant” for sun exposure. Works well for roof-mounted arrays.
  • “Direct Burial” for burial runs. Ideal for pole-mounted arrays.

Color of Wires

The color of wire insulation is mainly a safety feature. In the case of DC, electricity color is used to indicate polarity.

The black wire is used for the Negative (-) side of a circuit. Red is used for the Positive (+) side. In AC wiring, Black is used for the Hot side.

White is used for the Common side. Green or bare wire is ground in all cases.

Review and Reference

The wiring of a PV array and associated components can be an intimidating process. This is why electricians are paid well. It is not just some wires connecting stuff together.

There is a lot of science and history behind every element of an electrical system. Bravo to you if you decide to undertake such a project.

You will learn a lot, and the sense of accomplishment at a job well done makes the effort worthwhile. If you decide it is beyond your skill level, believe me, nobody will think less of you.

It’s not as easy as it looks! Good Luck. Be Safe. Have Fun.

References

Solar Panel Setup FAQ

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