Solar Panel Wiring – Basics

Introduction


Solar panel wiring is a fundamental topic that any solar installer should know. It is important to know how wiring and stringing panels will affect the current, voltage across the panels and ultimately the power output of the array so that you can select the inverter with the right power rating and make sure the system operates at its maximum potential.

Figure 1: Wiring of Standalone PV System

Solar Cable


The two common conductor materials used in residential and commercial solar installations are copper and aluminium. Generally, solar cables are required to have resistance against UV light, extreme temperatures, and weather conditions. Solar cables interconnect solar panels and other electrical components of a photovoltaic system. Specific requirements for material used for wiring a solar panel installation are given in national and local electrical codes which regulate electrical installations in an area. The cable could be solid or stranded, where stranded wires consist of many small wires that allow the wire to be flexible. This type is recommended for larger sizes. The current tends to flow on the outside of the wire, thus stranded wires have slightly better conductivity as there is more wire surface.

Figure 2: Solar Cables with MC4 Female and Male Connector on the Left and a Junction Box on the Right.

Importance of a Junction Box


An important feature in the construction of solar panels is the junction box. Attached to the rear of the solar panel, it serves several important functions. The junction box is often an overlooked piece of the solar panel. Usually, pre-installed on the backside of a solar module, installers pay it little mind until connecting panels. A junction box has bypass diodes that keep power flowing in one direction and prevent it from feeding back to the panels. Every string is protected by a diode. The diode is the gateway that allows an endless stream of power. If part of a solar panel is shaded, that string will want to consume power, reversing the flow of electricity. Diodes inside the junction box prevent that from happening.

Connections with the Junction Boxes

With the use of a junction box usually cables like MC4 and MC5 are using to connect to other solar panels of the array. A good junction box prevents or keeps the level of corrosion to a minimum at the leads. While purchasing solar modules it is important to check the IP rating of the junction boxes, as if it is not water-resistant then it could short-circuit and damage the entire series of solar modules attached to it.
Modern solar modules tend to use MC4 connectors because they make wiring your solar array much simpler and faster. The connectors come in both male and female types which are designed to snap together. They meet the requirements of the National Electric Code, they're UL certified, and the preferred connection method of electrical inspectors. Due to the locking mechanism of the MC4 connectors, they will not come unplugged and are well suited for outdoor environments. The connectors can be separated but it requires a special MC4 unlocking tool.

Earthing

To earth, an electrical system is to connect its non-current-carrying components or the body or enclosure of the system to the earth. Solar panel systems are mostly earthed and not grounded. They are near a lightning conductor system. When a transformer less inverter is being used, they are thus less complex in their designs and thus more prone to surges and shocks.

Following components will be needed if earthing is required.



Figure 3: PVC Insulated Earthing Cable

Connections in Series and Parallel

When solar panels are wired in series, the voltage of the panels adds together, but the amperage remains the same. So, if you connect two solar panels with a rated voltage of 50 volts and a rated amperage of 5 amps in series, the voltage of the series would be 100 volts, while the amperage would remain at 5 amps. Putting panels in series makes it so the voltage of the array increases. This is important because a solar power system needs to operate at a certain voltage for the inverter to work properly. So, you connect your solar panels in series to meet the operating voltage window requirements of your inverter. Wiring in parallel allows you to have more solar panels that produce energy without exceeding the operating voltage limits of your inverter. Inverters also have amperage limitations, which you can meet by wiring your solar panels in parallel. If there is a problem with the connection of one panel in a series, the entire circuit fails. Meanwhile, one defective panel or loose wire in a parallel circuit will not impact the production of the rest of the solar panels. In practice, how solar panels are wired today depends on the type of inverter that is being used.

Figure 4: Solar Panels Connection in Series and Parallel

Which Wiring Works Better - Series or Parallel

In theory, parallel wiring is a better option for many electrical applications because it allows for continuous operation of the panels, even if one of the panels is malfunctioning. But it is not always the best choice for all applications. You also might need to meet certain voltage requirements for your inverter to operate. A critical balance of voltage and amperage needs to be achieved for your solar array to perform at its best. So, in most cases, a solar installer will design your solar array with a hybrid of both series and parallel connections.

AFSHIN BAKHTIARI

AE Solar Senior Engineer



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