We’re ready to finally wrap up our series on the ground solar installation we used during Hurricane Matthew to keep our freezer and refrigerators cold. We’ll provide links to all the previous articles at the end of this post as a convenient reference. In this post, we talk about the wiring, connectors and tools used for connecting all the components. We’re not done talking about solar, though; future articles will address lead-acid battery alternatives and other solar power options beyond our hurricane experience.
PV Wire and THHN
Each practical solar installation should use special PV wire for all the exterior connections. PV wire has a special UV coating that prevents damage when exposed to the sun or the elements. At a minimum, use PV wire between the panel connectors and the combiner box. If the combiner box is mounted outside (as it should usually be near the panels), then conduit should run to the inside components with less-expensive THHN, although PV wire can be used here also. We use a 10 AWG PV wire for all connections to the combiner. The maximum current with even large panels in this application is usually under 10 amps, and the runs are fairly short. Expect to pay about $0.35 to $0.40 per foot for this wire when bought by the foot. If you buy a large roll, you can get it a for a little less per foot, as shown here on Amazon. Try to get this in both red and black to avoid wiring disasters at the combiner as the wires in there will be a big confusing jumble otherwise.
For the remainder of the wiring, we use THHN, which is a durable and fairly inexpensive wire. The best source for THHN appears to be Lowe’s. For the main, long, high-voltage runs from the combiner to the indoor components, we use 6 AWG, which will cost about $169 per 500 foot roll. While both red and black would be best for this main run, it isn’t as essential as the combiner connections to the solar panels. Expect this run to carry about 30 to 40 amps, peak, for three or four PV strings per combiner, respectively.
For internal wiring of the battery array, we also use 6 AWG here as the internal string currents are relatively low and the runs are short. Then, when wiring the battery array to the charger and the inverter, we prefer a 2 AWG or 4 AWG. 2 AWG would be better, but there is a steep price jump. We had intended to use 2 AWG on a recent project, but Lowe’s was out, so we chose 4 AWG instead. We haven’t noticed significant power losses or cable heating on a modestly-sized project (2 215 amp-hour battery strings), but on a larger project, 2 AWG would be essential. In any case, the runs are so short that the amount of required cabling is very small.
One issue with Lowe’s is that all the stores in this area seem to only carry 2 AWG and 4 AWG by the foot in black. Some red electrical tape on each end of the positive side solves this problem nicely.
Solid, Stranded or Fur?
Solid wire is too tough to work with, and puts mechanical stress on the entire system. We prefer to use stranded wire, as this is the best compromise between handling and durability of connections. However, there is also a very flexible stranded cable made for welding applications. Welding cable is made of what appears to be a large bundle of fine copper threads, which resembles a thatch of fur. Making a good crimp connection, which requires a certain amount of strand deformation, with welding cable is very difficult. Either the threads will simply move out of the way, or they will be weakened and break, defeating the point of crimping in the first place. If you have to use this very flexible cable, consider soldering the end first. Even so, the fur will eventually fail at the margin of the solder. Stick with normal 7- or 19-strand unless you have no other choice.
For making connections, the combiner and charger typically have screw terminals. Simply strip the end of the wire (another reason to not use fur), put it in the terminal block, and tighten the screw. At the solar panels, you’ll need to attach mating MC4 connectors, and then use solder lugs everywhere else.
MC4 connectors, as shown above, are the typical connector found on most brands of solar panels. There are a few others, but all of them have similar properties. We focus on the MC4 because it is the most popular. These connectors are fairly inexpensive, we have had great results with this model, which is now about a dollar cheaper for a pack of 20 pairs than when we last bought them. At slightly more than a dollar per pair, it pays to have more of these around than you think you’ll need, especially since it is easy to accidentally connect or disconnect them under load. That little “pop!” means “replace!” in solar panel language.
Most MC4 connectors will accept a variety of sizes, with 10 AWG and 12 AWG being the most common. As mentioned above, 10 AWG is our PV wire recommendation. Be sure to check the wire size before ordering any MC4 connector.
Note the crimp pin and cylinder, the metal portions below the black connectors. The PV wire is stripped, and crimped into the connectors using a special crimping tool, as shown below.
As with the MC4 connectors themselves, this tool is fairly inexpensive, typically around $20 or less. We’ve been happy with this model purchased from Amazon. It is currently about three dollars cheaper than when we bought ours.
You will also want to have a set of MC4 spanner wrenches, as shown below.
These tools not only help with tightening the cable seal at the ends of the MC4 connectors, they are also essential for disconnecting mated pairs using the probes at the end. These tools are also inexpensive, such as this set of two for less than $10. Why two? One to hold the connector (the interior holes), and the other to tighten the seal (the knurled open end). We 3D printed ours from plans on Thingiverse and have been happy with them. There is an alternative design that looks interesting, but we haven’t tried it yet.
A complete set of MC4 tools will then cost less than thirty dollars, and be well worth it.
As the photo above indicates, unless your batteries lack threaded posts, never use the starting cable version of battery connectors. These are prone to loosening, become badly corroded, and are difficult to wire. That said, if your batteries do not have threaded posts (hardly ever the case with marine or deep cycle batteries), the kind of connector above is great for converting to threaded posts.
The preferred battery string connector, which is also used for many chargers, inverters and DC breakers, is to use a solder lug, as shown below.
The three key features of any solder lug are tinned/not-tinned, cable size and post size. We prefer tinned, although bare copper lugs are usually a little cheaper. A tinned lug can be crimped and not soldered for a while, but a bare copper lug can corrode quickly and become useless. Not-tinned is OK, though, if you solder them immediately.
Make sure you get the correct size lug for your wire; you’ll need to keep several varieties on-hand. Also, make sure that the post size is compatible with your batteries. Our 29HM and GC2 batteries both have 5/16″ lugs, as do many of our other off-grid power items. 3/8″ is another typical size. We’ve had great results with this model of solder lugs, sold by Windy Nation in packs of 20. That page on Amazon allows selection of wire and post size. Or you can get bare copper lugs here.
To attach the lugs, you’ll need a beefier version of the MC4 crimping tool, as shown below:
We use the IWISS cable lug crimper, and have had great results so far. As with the MC4 crimper, it can crimp a variety of lug sizes, selected by rotating the tool heads. Some people have had trouble with the retaining pins popping out, but we haven’t had this problem. This item is also about five dollars cheaper now than when we bought ours several months ago.
Crimp or Solder?
The correct answer is to both crimp and solder, although for our hurricane system, we only crimped and noticed no issues at all. Some sources say to solder the wire into the connector and then crimp, but we think this fractures the solder bead, much like a crystalline cold solder joint. We prefer to flux the wire, flux the crimp lug, crimp and then solder. We’ll show how to do this in a future article, as well as how to post-flux a connection if you didn’t have a chance to flux it before crimping. For soldering, forget electronics, think plumbing: big chunky solder wire and a propane torch.
We also crimp-only on the MC4 connectors that mate to the solar panels. Otherwise, solder can wick onto the connector and ruin the physical mating surfaces. The currents here are limited to that produced by a single panel (again, typically under 10 amps) so a soldered connection isn’t that important.
Finally, how did we cut all this wire? No special tool, just a fencing cutter. Get the kind that cross-cuts onto itself with curved blades. Pinching, as with a bolt-cutter or regular wire-cutter, merely pinches the wire flat, making it harder to get into an MC4 or solder lug. Here is an example on Amazon, but you can find these at any agricultural supply store that carries real fencing for only about $30. The important feature is that the hole stays more or less circular as it gets smaller, and long handles (about 18″ or more) makes it easier to control the location and quality of the cut.
This wraps up this ground solar series. As promised, here is a review of all the relevant articles on this site:
We’ve also posted supporting articles about solar panel and battery principles:
We still have more articles to go, especially involving off-grid storage alternatives to lead-acid batteries. Expect more on this topic soon.