A simple off-grid  solar power system consists of three parts:

• PV source circuit (extends from array to combiner box/junction box)

• PV output/inverter input circuit (extends from combiner box/junction box to inverter)

• Inverter output circuit (extends from inverter to motherboard)

In many electrical guides, the PV output and inverter input circuits are separate parts with a DC Disconnect in between. However, since home grid-tied systems typically have the same electrical characteristics on either side of the disconnect.

PV Source Circuit

Your solar module already has 4 or 5 feet of cable attached, both positive and negative. In most cases, the wire type on the module leads is PV Wire. The two leads should include plugging into the mc4 connector, either male or female, which makes connecting the modules together. Be sure to note the connector type on the module you choose for the array, as you may need to buy more mc4 connectors to connect the circuit.

If you have determined the number of modules to be placed in series and/or parallel. Take 10 modules in parallel as an example. The negative conductor is located at the end of the module row closest to the combiner box. Meanwhile, the positive conductor runs from the other end of the string through the top and into the box. In installer terms, whichever conductor is at the far end (i.e. furthest from the junction box or combiner box) represents a "home run". It can be the negative wire or the positive wire, depending on how you choose to interconnect the modules. Here, the home run hit is positive.

As you can see, in order to complete the source circuit, some extra PV lines must be added to the array. You must also decide how to ground all modules and racks together. There are two options:

1. Run the equipment ground conductor (EGC) from module to module, rack to rack, and all the way to the junction box. The third ground wire, technically called the Equipment Ground Conductor (EGC), is not part of the PV circuit. This is a safety device that connects all metal boxes and other electrical equipment to the ground. Traditionally, a solid AWG 6 solid bare copper wire (not stranded) goes from lug to lug. AWG 6 is very strong, so it can withstand the elements (or rodent bites) without breaking. If you prefer to use AWG 8 or finer PV/USE-2 wire, you must wrap it in a PV wire (or USE-2) jacket. But bare copper is the usual choice.

2. Use ground clips and jumpers connection cable to connect the rack to the module. You can achieve the same result by slipping in the small ground clips (they look like washers) when you clamp the modules to the rails instead of connecting the ground to each module and rail section. You will also insert electrical jumper cables on either side of the splice rails in the rack to keep the equipment ground path uninterrupted. As the picture shows, when you're done, you simply connect the bare copper ECG to one lug per row of connections, and then connect the same wire to the junction box or buss. AWG 6 bare copper ground wire is typically used with lugs to ground each row of modules. When used with jumper wires, you simply ground each row in one place, then connect the bare copper wire to the junction box. This securely grounds the roof array in accordance with NEC standard.

Figure 1: Jumper connection cables

Many installers prefer to use USE-2 instead of PV wire for home run because it is cheaper than PV wire. Both types have the same electrical characteristics in terms of their ratings and NEC. Both types are UV and moisture resistant and rated to 90 degrees . However, the PV wire is double insulated, so it should enjoy a longer service life than USE-2 wire. Now, systems including transformerless inverters also require pv lines. As for choosing a size, since the module manufacturer has already determined the size of the leads (AWG 10 or 12), it can be assumed that the size of the other wires in the PV source circuit has already been done for you.

  Figure 2: PV Wire

When choosing a PV wire or a USE-2 wire, always choose black. You can use black on the positive and negative leads, just add a strip of colored tape on both ends to identify grounded and non-grounded conductors (or positive and negative). Wire sheathing colors other than black contain less carbon and deteriorate faster in sunlight, so you should avoid using them on your roof.

A very important exception to the wiring rules for pv source circuits is the use of arrays of microinverters or newer AC modules. Wiring in these cases is a completely different animal. In particular, you will have 240v AC as the power leaves each microinverter or module. Solar cable suppliers such as Kuka Cable can provide 10-12 AWG PV Wire for rooftop array wiring. If you measure the home run distance from the end of each string to the junction box or combiner box, then you'll know how many feet you need. In most cases, you can connect the module leads closest to the box directly to it. When purchasing PV cables, you can also choose to have MC4 connectors on one or both ends. This will cost more money but will save you time, so consider how your modules and home/proximity will be connected together and to the junction box/combiner box.