Solar system wiring can look complicated and even a little overwhelming if you aren’t an electrical engineer. The wiring that you choose for your system is just as important as the batteries or the solar panels. With a little knowledge of Direct Current (DC) electrical systems, one can wire a solar system of any size.
I hope you have a basic memory of positive and negative polls in a DC system from grade school science classes. Like Egon said the most important thing to remember is “don’t cross the streams.” Ghost Buster references are the best! So long as you keep positive wires going one way, negative running the other, never letting the two meet, and don’t overload the wiring, you will be fine. Using fuses is also a must just in case you do get a surge on the wire.
Quick Links for this Truck Camper Solar System Series
When it comes to solar system wiring we are really referring to a DC system as opposed to an alternating current (AC) system. In a DC system the closer/shorter the runs the better. DC encounters a lot of impedance in the wire causing huge power losses quickly. This means if the wire is too long you will lose substantial amounts of power over the distance of the cable run.
Choosing Wire & Fuses
Below is a table chart for what is an ideal wire gauge (yellow column) based on current flow over 10′, 15′, & 30′ run lengths. An ideal run meaning that it will have a 3% or less voltage drop on the wire. The red column should be read as the maximum amperage on the line at the specified distance will have a 3% or less loss in voltage/power.
For my solar system wiring, I needed to run a 12′ cable from the roof of my camper through the charge controller and into the battery with a maximum of a 600W (50A) solar system. Therefore the 6AWG cable was my ideal cable choice. As a side note, most solar packages come with 10AWG standard and then they run them up to 30′ in length which is fine so long as they don’t combine the wires to have more than 10A flowing down the line. The name of the game in solar is to not only produce the power but to get it to the battery for storage. If you lose 10% of the power on the way to the battery you have bought too many expensive solar panels and not large enough solar system wiring.
The green section of the chart is for determining maximum fuse sizes based on the wire gauge. Use this to know what size fuse is needed on all of your solar system wiring. Do not exceed what is listed. I recommend working off of the 75°C column and using a smaller fuse based on what your actual power draw will be. As an example, my 6AWG cables could have used a 65Amp fuse but that would have pushed the wiring to the limit. Since my max 600W system only has the potential of 50Amps (not likely to generate the full potential even if I have that size system) I choose to use a 50amp fuses on most of my 6AWG lines. The last thing you need in a camper is an electrical fire so I recommend fusing every line. Better to pop a fuse than to have a fire.
Our Fuses (breakers)
I used Bussmann breakers for our solar system wiring which are more like circuit breakers than fuses. They trip instead of blowing out. This is nice because I can trip certain parts of our system allowing me to see what specific areas are doing, as well as shutting off power flow for maintenance. You can pull fuses as well, but a breaker is easier.
Charge Controller Location
Along with solar system wiring comes the location of equipment. It is important to keep the distance from your solar panels to your batteries short or get thicker gauge wire. But, the most important location piece is the distance from the charge controller to the batteries. The charge controller regulates the voltage holding it constant and varying the amps depending on the battery level. Here the wiring is all about efficiency so make sure to use a large enough gauge cable and make the run as short as possible so that power isn’t lost in this final step to the batteries. This will ensure that your batteries charge up as quickly as possible. For this reason, a few feet of high gauge cable is recommended.
I did less than 3′ of total cable from the charge controller through the positive busbar to the battery. I put a 50A Bussmann breaker in the 6AWG line from the controller to the busbar and then a 100A Bussman breaker on the 2AWG line from the busbar to the batteries.
One of the last decisions to be made for a solar system wiring project is the fuse box. Get one that can accommodate your wiring situation. For us, we had a small system, but I wanted to have most of our electronics on rocker switches for ease of use. This would allow us to completely kill the power draw of some devices; ensuring that no phantom load was being drawn (USB ports/Inverter/wireless amplifier). For other devices like the fridge and furnace I wanted them to go through without a switch so they were always able to be on. To accommodate these decisions we choose a small 6-pole blade fuse box for the static devices and a 6-switch marine style panel with a USB port and a cigarette-lighter port… not used for cigarettes.
One of the things to consider early on in your power calculations is how much AC power (Alternating current at 120V) are you going to need to be able to produce. There are some devices that just won’t run off of DC power even though I converted those units to 12V for the purposes of calculating my total load. An inverter takes 12V power and steps it up to 120V for use on those standard house appliances. This is the 2 or 3 prong connector most Americans are used to seeing in their homes.It is also known as an Edison connector.
When it comes to inverters keep in mind that it takes power to invert power. Usually about 2% of the load. Meaning that if my laptop takes 85W, the power drawn from the battery will be 2% higher. Inverters also constantly use power unless turned off so I put mine on a switch allowing us to turn it on and off as needed.
Our Morningstar 300W Inverter
The size of the inverter you need is based on the needs you have. Ours were very modest so we went with a Morningstar 300W inverter. The key for us is to not use a single device or multiple devices at the same time that exceed a power draw of 300W. We are able to charge both of our laptops simultaneously and still have the power to charge camera batteries. If you want to run a microwave, a griddle or the motherload, an AC unit, you will need a much larger inverter, larger solar system wiring and a much larger system in general.
Other Add-ons to Consider
While taking on the challenge of my solar system wiring I took the opportunity to add 3 additional USB outlet locations around the camper. I also changed out all the bulbs in our camper’s existing lights to LEDs and changed out the entire socket for the “porch light” on the back of the camper. Later, I added a wireless amplifier for our cellular connection.
You can wire the charge controller to the alternator of the vehicle to supply power to the batteries as well, but the safest way to do this is to upgrade the wiring between the alternator and the camper to an appropriate higher gauge cable. I didn’t want to fool with this replacement and decided to stick with a separate system.
You could also use a converter (steps down 120V to 12V) to use shore power to power/charge your system, but this adds complications I didn’t want to or need to figure out so I kept my shore power system separate.
Actual Power Consumption of Our Devices Using the Trimetric Readings:
Now that my solar system wiring is done I can shut off the power flowing in from the solar panels by tripping the breaker on the positive intake wire. This allows me to use the Trimetric Meter to see the actual draw of each device on the batteries. What follows are my actual power loads. The fridge is a little tricky in that the hotter the outside temperature the harder it has to work to maintain the desired temperature inside. So at night, it is more efficient than in the daytime. Click here for more details on the fridge and why we choose this particular model.
Dometic CFX65W Fridge (replacement for the Palomino factory fridge):
4.8A for initial cooling (55min from 80deg F to 12deg F)
Cooling Cycles (80deg weather) 7min of Cooling @ 3.5A (average) & 3min in Standby @ 0A.
The heater is propane but it has a built-in blower that runs on DC.
My computer is old and the internal battery is shot so I think it constantly needs a charge because it can’t really hold a charge. My wife’s computer (Computer 1) is probably a better indicator of power draw.
All 5 LED lighting fixtures (indoor and out) at the same time: 0.5A
Inverter Only Load: 0.3A (included in all calculations for devices using the inverter)
Estimated total daily power usage
Using these numbers on an 80-degree day I estimate my average usage should be about 7A/hour or 84W/hr. Much lower than the 200W/hr I had originally calculated from the electrical specs on my original devices. I attribute almost all of these power savings to the new fridge. To calculate 7A/hour I added all the above numbers together based on the hours I use each item in an average day and then divided that by 24 hrs. So, I have to generate 168A of power every day to keep my batteries near full for our estimated usage. This means that I have to be generating during the 12 hours of daylight on average about 14A/hour. Thankfully, I can run the refrigerator and pretty much everything else off of shore power should we run into to many days of really bad weather.
Keep in mind that days in the winter are shorter than summer days and depending on where you rig winters this could mean very few hours of recharging each day.