Do-It-Yourself Solar Array Wiring
This article serves as an overview for a solar electric system's array wiring. It references a generic example of components that reflects a typical installation. We include this with many customer orders. Remember: wiring up solar panels in the daytime means you are dealing with LIVE electricity. Use all common sense and proper precautions. If you are not comfortable with this, wait until sundown or cover your panels with heavy blankets or cardboard.
Let's begin by looking at the top of a typical sales order. Five types of components are listed: panels, cabling, combiner box, combiner box breakers, and wire fittings for the box.
On the back of each panel there is a Positive (+) and Negative (-) connection cable. The connectors themselves use solar industry standards with the most common types being MC4, and MC4 with other compatibles such as PV4 and H4. The combiner box is an outdoor rated enclosure, intended to be installed outside, within 15 feet of the array. It serves as a junction point to tie together (combine) all solar wiring; Positive (+) through breakers, and Negative (-) through a negative bus bar. Wire fittings serve as wire pass-through-points for each pair of wires into the combiner box and are mentioned because they should be installed during this process.
String Size and Parallel Strings
Besides the components, pay attention to the quantities. You'll see a pattern here. In this example, we have 6 panels, 2 MC4 cables, 2 PV breakers, and 3 wire fittings. These quantities tell us the array will be configured for two parallel strings of 3 panels each. Each string will use 3 panels, one MC4 cable, one breaker, and one wire fitting (with one left over).
Start with the series connections of the first 3 panels. Plug a Positive (+) lead from one panel into the Negative (-) lead of a second panel. Then plug the Positive (+) lead of the second panel into the Negative (-) lead the third panel. That's it. We now have two series connections for this string. Repeat this step with the second set of three panels.
Now you have two series strings of three panels each. There is a leftover Positive (+) lead at one end of each string and a leftover Negative (-) lead at the other end. Caution: DO NOT plug these two leads into each other. It would create a short circuit and is not useful and wrong for the installation. These are the leads that will eventually go to the combiner.
MC4 cables are typically 30' long, and one will be used for each string. Assuming your combiner box will be centrally located with respect to the layout of the panels, cut a single MC4 cable into two 15' pieces. If your combiner box is off-center from the panels, cut the cable at appropriate lengths such as 10' and 20' or 8' and 22', etc.
Now you have one 15' MC4 cable that will match to the Positive (+) lead of your string and one 15' MC4 cable that will match to the Negative (-) lead. Go ahead and connect them, and repeat the process with the second MC4 cable and string. Each string now has two 15'+ leads.
The next step is to wire the strings to the combiner box. This is a great time to use a voltmeter to verify polarity; confirming which leads are positive and which leads are negative. Reversing polarity WILL blow out the diodes and make the module inoperable. This will also void the warranty on the modules.
DO NOT assume that the markings on the MC4 connectors are correct at this point. It's very likely they are not, and this completely normal. Best practice is to trace the polarity back to the junction box, as the bare lead at the end of the cut MC4 cable will match the polarity referenced where it exits the junction box. Most panels will have Positive (+) and/or Negative (-) markings on the back of the panel near or on the junction box. Those markings CAN be trusted; but better still to always use a voltmeter to double check and verify which leads are positive and negative.
Once satisfied with polarity, install the wire fittings in the combiner box. Route the Positive (+) and Negative (-) of the first string through the first wire fitting. Strip a bit of insulation off each lead, and connect the Positive (+) to the first breaker, and the Negative (-) to the negative buss bar. Repeat this step for the second string using the second wire fitting and the second breaker.
Lastly, with the circuit breakers in the Off position, install the metal "comb" that came with the combiner box, on the opposite side of the breakers from the solar wires.
You'll note that the comb and the negative buss bar each host a large wire lug, for connecting heavier gauge transmission wire to run power to your charge controller. Use the last wire fitting as the pass through to bring your transmission wire into the combiner.
That's it, the wiring is done. The voltmeter is useful to verify that each string's voltage matches it's rating. Simply place the positive lead on the wire side of the breaker, and the negative lead on the negative buss bar.
The circuit breakers can be left in the Off position for now, until the rest of the system installation is complete.
- Written by Backwoods Solar Technical Salesman Alan Smith -