What Wire Size and Gauges for Small Solar System

“Wiring” may be one of the less expensive parts of a small solar electrical system, but definitely not the least important. In fact, if you make a mistake in wiring there can be big consequences, all bad, even on a small system. And, they’re easy mistakes to make. There are literally hundreds of different types of wire and associated paraphernalia for all kinds of electrical purposes.

Wiring is addressed both as “wire” and also as “cable”. To eliminate some confusion, for myself anyway, I defined the difference as being wire is smaller/thinner and cable is bigger/fatter. While mostly true, it doesn’t sound very technical. So, the more technical explanation is that “wire” is a single conductor wire while cable is a group of conductors, each wrapped in insulation inside an even bigger insulator.

As mentioned earlier, a solar electric system’s components, including wiring, are all interconnected and interdependent. As the charge controller, for example, must be big enough to handle the energy produced by the solar panels, so must the wiring between them be big enough to carry the energy or current that flows from one to another. This is true for all wiring or cable between every component.

If a cable or wire attaching two components is too thin (too small) or too long the wire/cable can heat up and eventually may catch on fire. So, size matters, a lot. It’s much better to get wiring too big rather than too small. People can be tempted to go with smaller wiring simply to save money. Don’t. Cost is dependent on size, type and length of wire or cable and can run anywhere from say \$4.50 for 25 feet to \$70.00 or more for only 5 feet in this usage area.

Wiring Size and Amps

Wiring or cable size is measured in gauge or referred to as AWG. You’ll see AWG used as an identifier. AWG means American Wire Gauge which is a standardized wire gauge measuring system used since 1857 mostly in the U.S. and Canada for measuring the diameter of conducting wire. You could see, for example, 12 “gauge”, or 12 “AWG” meaning the same thing. Increasing gauge numbers, bigger gauge numbers, indicate decreasing, smaller, wire diameters. A 22 gauge wire is smaller than a 16 gauge wire and a lot smaller than a 8 gauge or 8 AWG heavy wire. This is counter intuitive for most people as bigger usually means, well, bigger, not smaller. “Gauge” can also be spelled “gage”.

Wire is rated according to amps, the number of amps that can safely pass along it. The higher the current (amps), the thicker the wire, the lower the gauge. Let’s say for example, your solar panels, (pretend you have two), put out 5 amps together in total, 2.5 amps each. Let’s further say that the distance between them and the batteries/controller (assuming controller is close by) is 20 feet.

You’d need 13 gauge wire. In which case I’d use 12 gauge wire or lower to account for the fact that my “feet” measurement might be a bit off and I’d rather have bigger wiring not smaller. T.

Wiring Size and Distance

Distance matters. Amps aren’t the only things that determine size of wiring. There’s also the distance of wiring between components to consider. Increased distance requires increasingly heavier wiring. This has to do with something called OHMS. The OHMS law says that when voltage goes down, which it does over distance, the current goes up. Wiring has to be able to accommodate the increase.  To continue with the 5 amp example above, a wiring size chart may indicate that 5 amps over 10 feet need 18 AWG wire, over 15 feet a 16 gauge wire, and over 40 feet a 10 gauge wire.

From another perspective, if you have for instance, a 10 amp usage device running on a 10 amp wire, the wire has no wiggle room to handle a potentially increased current distance might add to it. It would be smart to increase wire size in this kind of situation. An amp rating cannot be exceeded.

Wiring for 12 volt DC circuits is of the multi stranded type, or commonly just called “stranded”. Stranded wire, instead of being one solid piece, like you might find in your house, is divided into many small strands. These strands are encapsulated in an insulating material. Stranded wire is usually used in areas that move or receive more vibration like boats, RV’s, airplanes, and the like. Single strand wire can break or weaken under the same conditions. Stranded wire is somewhat easier to work with.

Big Easy Wiring Mistake – Mixing Positive and Negative Wire

Except for DC devices and components that just plug into a receptacle like the power plug in your car, 12 volt systems have positive connectors and negative connectors. You can’t mix them up! Your battery will have a positive post (or two) and a negative post, as will your inverter, solar panels, charge controller and most every other do-dad you add to it.

For example, the negative wire off the inverter connects to the negative post on the battery, the negative wire off the solar panel connects to the negative connection on the charge controller which connects to the negative post on the battery. Same for the positive wires except, of course, they’re connected to the positive connectors and posts. Solar panels, charge controllers, inverters, etc. come with wiring instructions, well, not always. Follow them carefully if you’ve got them.

Some Common Solar Wiring Connectors for Small Systems

Ring terminals

Wires don’t come with ends; ends to connect them to something else. One common “end” is the ring terminal or insulated ring terminal. The ring terminal is a small metal ring that’s crimped onto the wire you tape with electrical tape or other protective insulator. This ring terminal could be used to fit onto the secondary posts of your battery. It may be used to connect two wires together that you might want to separate later. (You could use a nut and bolt.) These also come in sizes, not only physically, but according to gauge. Everything must fit and be strong enough to handle the electricity running through it including the ring terminal.

Fork terminals

A fork terminal is a terminal similar to the above but in a U shape.

Butt splices

Sometimes a wire is just too short. You can connect one wire to another using a butt splice. It’s just a small connector that allows you to crimp two wires together. Gauge is an issue here as well.