Solar wire exposed: types and sizes

Although we often treat words “wire” and “cable” as synonyms, there is a clear distinction between the two. A solar wire is a single conductor whereas an insulated cable contains two or more wires in itself. The wires thus are a part of cables but they are also used independently to carry electricity in commercial and residential buildings. Wires are cheaper than cables but their lifespan is shorter. They also aren’t suitable for industrial applications that require the heavy-duty strength of a cable.

Manufacturers don’t put wires and cables in solar kits and along with panels for all the good reasons. The right wire depends on the properties of your system, environmental conditions and the needed length. Since the supplier doesn’t know your situation, he leaves the task to you or your installer.

So what type of wire is used for solar panels? They vary in their conductor material, insulation and their structure.

Two materials have different qualities that make each of them popular. Here they are:

• Conductivity. Copper by nature is a much more conductive material than aluminum. As a result, a thinner copper wire can carry through just as much current as a thicker aluminum one.
• Flexibility. Copper wires resist bending and flexing better whereas aluminum gets weaker. Besides, aluminum is less resistant to high and low temperatures.
• Maintenance. Aluminum wires require maintenance. Connections often suffer from corrosion and thus you have to clean them or use antioxidants to protect them.
• Cost. Aluminum wires are much cheaper though. Their low upfront cost is their main advantage.

In short, copper is stronger and more conductive, and aluminum is cheaper. There is also a copper-clad aluminum wire that is available on the market. It’s cheaper than full copper and it doesn't require a lot of maintenance. Most people still find copper to be much more reliable and quality — and wiring is not part of a system where you should cut corners.

Some wire jackets are resistant to fire, heat and ultraviolet, while others are better at dealing with moisture. Here are the main types of insulation that you can use:

• THHN is a good choice for indoor solar wiring. This is the most popular type of insulation.
• THW, THWN and TW are okay in wet outdoor applications.
• UF and USE are good for moist or underground applications.
• PV Wire, USE-2 and RHW-2 cables are used for connecting solar panels together. They are UV and moisture resistant.

Solar cable is the preferred choice for PV systems. The difference between a standard one and a PV cable is the insulation. A normal one is PVC-insulated, and the solar cable has an XPLE jacket. This makes PV cable just as sturdy as a solar panel itself. Standard DC cable lasts for 8-10 years, while the lifespan of a PV cable is over 25 years.

A stranded wire is made out of, well, metal strands. It’s generally a bit larger in diameter, but it’s more conductive and more flexible. Solid wire is cheaper but it is typically available only in small gauges.

Generally, stranded is the preferred type of wire for solar panels, especially in mobile systems, such as for RVs and boats. Solid wire is good only in certain situations, for example, when you need a long wire for low current without much bending. Keep in mind that it keeps the shape when you bend it.

The fire department may ask you to use colored wires so that conductors are easy to identify. Color codes apply both to AC and DC systems. The installers recommend marking positive cable with red color and negative one with black. Grounded circuit conductor must be white or gray. Equipment grounding conductors should have a green jacket or be bare. If you forget to pick the right colors, use duct tape to mark the wires.

The size of the wire that you need is determined by current that comes from panels and distance between panels and electrical units. In the US wire sizes are defined by the American Wire Gauge table or AWG. The higher the AWG number, the thinner the wire is. Thin wires are cheaper but their resistance is higher and they conduct less amps.

What happens when you use a small-sized wire in a system where current is high? The wire overheats and may even start a wire. Things usually don’t get to that point, because your installation simply won’t pass the inspection — safety code must be respected.

A helpful metaphor to understand wiring might be the water pipes. Current is the water and the voltage is what pushes the water through the pipes. If there is a lot of water, you need bigger pipes to make it flow faster. If the pipe is too small, it won’t be able to handle the pressure and will burst eventually.

Normally, when you choose a wire, you already know the voltage of your system, amps coming from your array and the lengths of the wire that you need. Pre-made charts and online calculators make the task easy. The exact formulas that they use include figures like the conductivity of copper and aluminum. It gets complicated quickly so we won’t go into too much detail.

No matter what, some energy is inevitably going to be lost in the transmission of power from panels to your equipment due to the resistance of the wire. Generally, the acceptable loss is somewhere between 2%-5%. Here is the table of wire gauges for 12V systems with 2% voltage drop.

Some experts say bluntly: when choosing wires, buy the thickest one that works in your system. Even though it’s an expensive purchase, it’s better than buying wire 2-3 times. First of all, it’s a question of safety. Second, thicker wire gives your system more flexibility. Maybe you won’t add new panels to your system but you might need to rewire some things, possibly add batteries or a new inverter.