Photovoltaic (PV) systems are one of the most important renewable energy sources worldwide. Learning the basics of solar panel wiring is one of the most important tools in your repertoire of skills for safety and practical reasons, after all, residential PV installations feature voltages of up to 600V.
There are three wiring types for PV modules: series, parallel, and series-parallel. Learning how to wire solar panels requires learning key concepts, choosing the right inverter, planning the configuration for the system, learning how to do the wiring, and more. In this article we will teach you all of these, saving you weeks if not months of hard studying on the subject.
Key concepts and items required for solar panel wiring
Solar Panel String
The “solar panel string” is the most basic and important concept in solar panel wiring. This is simply several PV modules wired in series or parallel.
Solar panels feature positive and negative terminals. Wiring solar panels in series means wiring the positive terminal of a module to the negative of the following, and so on for the whole string. This wiring type increases the output voltage, which can be measured at the available terminals.
Wiring solar panels in parallel increases the output current, while keeping the voltage constant. The output current is the sum of all currents generated by the modules in the string.
Solar panels wired in parallel also have to meet NEC regulations. This includes conductor size and overcurrent devices. This is calculated by oversizing the Short Circuit Current (Isc) by 125%, considering the number of modules in the system, as specified in the NEC 690.8(A)(1) and NEC 690.8(A)(2).
There is a solar panel wiring combining series and parallel connections, known as series-parallel. This connection wires solar panels in series by connecting positive to negative terminals to increase voltage and connects these strings in parallel. All solar panel strings connected in parallel have to feature the same voltage, and they also have to comply with the NEC 690.7, NEC 690.8(A)(1), and NEC 690.8(A)(2). Modules need to be the same model in all cases in order to provide optimum performance on the system.
Crimping Tool & Solar Connector Assembly Tool
You should learn beforehand about the tools used to wire solar panels. These are the crimping tool and solar connector assembly tool.
The crimping tool is used to crimp the connecting plate of the solar connector to the naked wire. In most cases, this means an MC4, the most popular one in the solar industry. The solar connector assembly tool is used to tighten all pieces of an MC4 connector to the female/male connecting plate. This tool is also used to unlock the connector after it has been plugged in.
Solar Panel Inverter
The solar panel inverter is one of the most important components in a PV system. This component converts DC energy generated by solar panels into AC energy at the right voltage for your appliances. The output is a pure sine wave, featuring a 120V AC voltage (U.S.) or 240V AC (Europe).
Solar Wire Type
Wiring solar panels together can be done with pre-installed wires at the modules, but extending the wiring to the inverter or service panel requires selecting the right wire. For rooftop PV installations, you can use the PV wire, known in Europe as TUV PV Wire or EN 50618 solar cable standard. For ground-mounted PV installations requiring underground installations, you need an Underground Service Entrance (USE-2) cable.
Are you using microinverters or string inverters for your array?
There are two types of inverters used in PV systems: microinverters and string inverters. Both feature MC4 connectors to improve compatibility. In this section, we will explain each of them and their details.
String inverters or centralized inverters are the most common option in PV installations, suitable for solar panels wired in series or series-parallel. Centralized inverters convert DC power for the whole string, which is why they are recommended for PV systems not subjected to partial shading.
Microinverters can be used in PV installations that will or will not be partially shaded, or even those that will be modularly expanded in the future. A microinverter converts DC power for a single module into AC, featuring a 120V AC output, which is why solar arrays featuring microinverters are exclusively connected in parallel.
Planning the best solar array configuration for your PV system
Planning the solar array configuration will help you ensure the right voltage/current output for your PV system. In this section, we explain what these items are and their importance.
Maximum DC Input Voltage
The maximum DC voltage has to be limited for safety reasons, NEC regulations, and to match the technical specifications for a string inverter. The limit for residential PV systems is 600V for NEC regulations, but this can vary depending on the centralized inverter.
Minimum DC Input Voltage
There is a required minimum DC input voltage to start up a string inverter, which is why this is an important planning configuration for PV systems. This number drastically varies according to the selected model and brand.
Maximum DC Input Current
The maximum DC input current is limited by the technical specifications of the inverter. This value is designed after the current-voltage curve (IV-Curve) for a solar cell. This is an important factor to be considered when wiring solar panels as the system DC output should not exceed the maximum input current for the inverter.
Number of MPPT Trackers
MPPT trackers optimize power output for PV systems considering the IV-Curve. Centralized inverters with several MPPT trackers can optimize power output for solar panel strings featuring different specifications from one another, allowing you to wire a more complex solar array to the inverter. If your inverter has two or more MPPT inputs, make sure to take advantage of them properly, especially in scenarios with multiple orientations or shading impact.
Wiring your solar panel array: Step-by-step guide
Up to this point, you learned about the key concepts and planning aspects to consider before wiring solar panels. Now, in this section, we provide you with a step-by-step guide on how to wire solar panels.
Connecting a PV connector to your PV wire
Most solar panels come with pre-installed MC4 connectors, which will allow you to interlock solar panels between them. For the ending points of the system, you may be able to use an MC4 extension cable that generally comes in multiple sizes to interconnect the PV system and the inverter.
However, it is still important to learn how to properly install a PV connector, since in some cases or sections, the system may require you to make the connection yourself. This will probably occur if you do not find an MC4 extension cable with the right length.
The steps to add solar connectors to PV wires are the following:
- Strip the wire.
- Place the connecting plate on it and use the crimping tool.
- Insert the lower components of the connector (terminal cover, strain reliever, and compression sleeve).
- Insert the upper components (safety foil, male/female MC4 connector housing, O-ring).
- Attach all the components together and slightly tighten them by hand.
- Give the final torque to the MC4 connector by using the solar connector assembly tool.
All the steps are illustrated in the image below.
Wiring solar panels in series
Wiring solar panels in series requires connecting the positive terminal of a module to the negative of the next one, increasing the voltage. To do this, follow the next steps:
- Connect the female MC4 plug (negative) to the male MC4 plug (positive).
- Repeat steps 1 and 2 for the rest of the string.
- Connect the male MC4 connector of the first module and the female MC4 connector of the last one to the centralized inverter. Most inverters feature MC4 connectors to make this an easy task.
Wiring solar panels in parallel
Wiring solar panels in parallel is achieved by connecting the negative terminal for two or more modules, while doing the same thing with the positive terminals. The process is the following:
- Take the male MC4 plug (positive) of the modules and plug them into an MC4 combiner.
- Take the female MC4 plug (negative) of the modules and plug them into an MC4 combiner.
You can use a 2-in-1 MC4 combiner for two modules, or bigger ones (4-in-1 combiner, etc.) for more modules. The output of the MC4 combiner will contain the parallel output connection of the solar panels.
Wiring solar panels in series-parallel
A series-parallel connection combines the benefits of wiring solar panels in series vs parallel. To wire solar panels under this configuration, follow the next steps:
- Connect solar panels in series by following the steps in our “wiring solar panels in series” section.
- Connect solar panel strings in parallel by using a connector known as MC4 T-Branch Connector 1 to 2, following steps similar to those in our “wiring solar panels in parallel” section.
The main difference is that you will be connecting two strings and not two modules, using the available MC4 connectors at the beginning and end of the string.
Solar panel wiring: Tips from a professional
Now, it is important to learn some tips to wire solar panels like a professional, below we provide a list of important considerations.
Always wear safety gear
Safety gear is not optional, in PV installations you can be exposed to direct current at voltages of up to 600V, which is extremely dangerous. During PV installations you should wear isolating gloves, electrical safety shoes, a safety jacket, and more. All this gear will keep you safe during the installation.
Proper wiring management is an important consideration for a successful installation. There are typically two important methods to know about when wiring solar panels in series: Leapfrog and Daisy Chain.
Daisy chain is the basic wiring method, connecting one panel to the next one, while Leapfrog jumps a wire over a module to connect to the next one, as shown below. With Daisy Chain you get a regular wiring, while Leap Frog saves money on wire and reduces power losses produced by heat, being the most efficient wiring technique.
Another good wiring management practice is fixing or channeling cables in a natural route by using zip ties and looping wires when they are too long. This will reduce mechanical stress, eliminate hanging wires, and keep PV wires organized and safe. This is a great practice to avoid anyone who is walking on the roof or ground from tripping over a loose wire, damaging the connection or even worse, getting electrocuted.
Double-check every connection
You can never be too safe when wiring solar panels. Double-checking all connections will help you be extra safe, and even eliminate possibilities for electrical hot spots, which could cause serious home accidents.
Verify the output voltage for each string
Measuring the voltage for each solar string is extremely important in regular installations, but even more so in series-parallel installations. Aside from helping you properly install the PV system, it is a great method to detect any solar panel that might have a factory defect or if there is a loose connection.
Slightly oversize your PV system
A good practice is to oversize the PV system slightly above the maximum power output of the inverter. This ensures that in case there is low solar radiation, the system will still be able to generate a power output that is very close to the maximum rating of the inverter.
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