In this photo I have the panels tilted toward the middle while I was wiring the array. This is obviously not something I would do normally, but it does show how I can still access the roof and still move around on the roof. Now that the wires are installed I can't tilt all the panels toward the outside of the coach without removing the wires.

Wiring Solar Panels

We have installed eighteen Zamp Obsidian solar panels to recharge our lithium battery. This article explains the wiring of the solar panels so that they act as one panel.

This year, because our system is so much more complex, I am going to go into much more detail about wiring solar panels than I did two years ago. There were several possible ways we could have wired our solar panels. This post is going to look at possible wiring designs and why we chose the one we did. But first I need to give you a big picture overview and review some different electrical background knowledge.

I included the above picture to show how I arranged the panels during the wiring. As you can see, I have tilted all the panels toward the center of the RV. This gives me room to walk around the array while I installed the wiring. The same tilt, from both sides, will be really nice when performing maintenance on the roof.

Wiring Solar Panels — routing the wires.

We had two small solar panels on the roof that were removed before installing our eighteen-panel system. This meant that there was already a wire path from the roof to the storage compartment under the RV. Our solar controllers and batteries are directly under the floor in our storage compartment. Since the previous small system was installed when the RV was still in the factory, I assumed that my new wires could follow the same path.

Wiring solar panels, This is the "before photo" of the roof.
This is the “before photo” of the roof. These two panels were removed and the only thing that remained was the wire leading into the RV.

I left the previous wire sticking out of the roof until after I installed seventeen of the eighteen panels. This wire’s located at the top of the solar panels in the above picture. When I removed the sealant and started exploring the route from the roof to the wall between the television and the bathroom, this is what I found hiding (in my way).

The solar wire is at the top and right side of this picture. I have removed all the sealant from the 1 1/4" hole. The rest of this extra wire I removed (carefully) from the one inch hole. Later I discovered that this extra wire wasn't connected to anything.
The solar wire is at the top and right side of this picture. I have removed all the sealant from the one-inch hole. I removed this extra wire (carefully) from the one-inch hole. Later I discovered that this extra wire wasn’t connected to anything.

Wiring Solar Panels — inside the RV

At this point, I was unsure that I would be able to route the wire through the existing hole. I really didn’t want to put any holes new holes in the roof and until now, I was successful. The entire racking system is glued to the fiberglass without even one screw.

The rat nest in the above picture wasn’t the only wire I could see through the one-inch hole. I had doubts about using the previous hole after finding this bundle. The next picture is directly below the now open one-inch hole in the roof. The large wire to the left is the wire from the previous solar panel. The other wires do other things in the RV including opening and closing the slides. This picture doesn’t include my new solar wires or the extra wire that I found and pulled up through the hole.

Cabinet above the TV had a false ceiling and this reveled the wire bundle that was under the roof.
Cabinet above the TV had a false ceiling and this reveled the wire bundle that was under the roof.

I knew that I had found a successful wire routing when my probing revealed the false ceiling in the cabinet above the television. Other possible routes that I explored all included cross bracing that at some point and couldn’t be used. From this upper cabinet, I was able to follow the previous wires behind the TV all the way to the bottom cabinet at the bottom of the next picture. From there, it was an easy drop into the storage compartments.

The cabinet in this picture, at the top, left is the same cabinet in the above picture -- open with the ceiling pulled out. From here the wires went behind the TV and then to the back side of the lower cabinet where I found a small hole cut into the back of the cabinet. I made this hole a little bigger and covered it with a piece of plywood that matches the interior of the cabinet. From there the wire drops into the storage compartment with the charge controller.
The cabinet in this picture, at the top, on the right side is the same cabinet in the above picture. From here the wires went behind the TV and then to the backside of the lower cabinet where I found a small hole cut into the back of the cabinet. I made this hole a little bigger and covered it back up with a piece of plywood that matches the interior of the cabinet. From there the wire drops into the storage compartment with the charge controller. Of course, when I was wiring the solar panels the television and soundbar were on the couch, and all the cabinets were empty.

Background Electrical Knowledge Review

I’m guessing that a few of my readers might need a little Electricity 101 review so that I can explain my choices in wiring the solar panels… Really the next section is very basic.

Volts

Volts are a measure of potential energy. The easiest way to think of this is water behind a dam. Volts are a description of how much water could move, if allowed to move, accomplished by opening a valve. Until movement happens, no work can be accomplished.

Current

Current describes movement. Water from a dam is released by opening a valve. In water, the current is measured in volume per minute. The same description applies to electrical current. Current flows when you turn on a switch. This current is measured and recorded as amperes. The word amp is just a shortened word, easier to say than amperes.

Parallel wiring

When wiring multiple solar panels, parallel wiring means that each panel is wired to the controller as if it were the only panel. You would combine the output of each panel into one bigger wire thus increasing the current but not the voltage. If you do not increase the size of the wire, then you will create heat and waste part of the potential energy. (This is the only way to wire a PWM controller, more later.)

Series wiring

When wiring multiple solar panels, series wiring means that each panel is wired one after the other. The voltage of the first panel is added with the voltage of the second panel and in my case, it is then added with the voltage of the third panel. Even though the voltage increases, the amperage on the wire remains the same. The result is that you can transfer the energy through the wire having the lower resistance as if it was only carrying the energy produced by one panel. (Series wiring requires an MPPT controller, again, more later.)

Combining Volts and Amps

To combine the electrical measurement, the result of the combination is that either volts or amps will be combined, not both. If you have two 20 volt sources both having 5 amps they could be combined in series (one after the other) or in parallel (one next to the other).

When wiring the panels in series, the voltage of one source would be added to the voltage of the second source. Two 20 volt sources in series would then be 40 volts. Three 20 volt souces would be combined to make 60 volts and so on. The amperage of the sources would remain the same at 5 amps.

Or you could combine the two sources in parallel (each alongside each other). In parallel wiring, the voltage remains the same but you double the amperage. Thus two 20 volt, 5 amp sources in parallel deliver 20 volts at 10 amps.

Increasing the voltage is like doubling the height of the dam. Twice the pressure. Likewise, if you had a lower pressure water source and opened two valves at the same time, you would achieve double the flow even though the pressure remained the same.

Wiring solar panels Series or Parallel

Resistance

Resistance describes the effect of higher current flow. If you can keep the current the same, resistance does not change, regardless of the voltage. In terms of electricity, resistance creates heat in the wires. Wasted energy or when at an extreme, a fire. One of the keys when wiring the solar panels is to keep from wasting potential energy so resistance needs to be avoided.

Cold panels = higher volts

Solar panels work better when they are cold. After I combined my three panels in series I was now at 60 volts on a standard day. The same three panels, sitting in direct sun, wired the same way at 15 degrees Fahrenheit will deliver about 80 volts. My controller can handle up to 100 volts without danger. Had I wired four panels in series, then my standard day 80 volts would then exceed 100 volts on a cold day and then damage the controller.

Wiring Solar Panels — wire considerations

A very important note pertaining to wires is that all exposed external wires need to be PV-rated (photovoltaic-rated) for external applications. PV wires have a much longer life span when exposed outside. Inside the RV the wires don’t have to be PV wires.

Stranded or solid?

Electricity is conducted along the surface of the wire. In direct current (like solar) it is important to have stranded wire. Stranded wire will conduct direct current electricity with lower resistance than solid conductor wires. When changing to alternating current, the standard for solid wire, the alternating current already has very low resistance to current flow and thus does not need stranded wire.

Gauge (thickness) Lower numbers = thicker

For my system, I am using series of three panels creating 60 volts at 5 amps. I then combine three sets of three panels at the combiner box in parallel. Between the combiner box and the charge controller, I have 60 volts at 15 amps.

The series set could have been at 12-gauge wires still with low resistance. Between the combiner box and the controller, I used 10 gauge wire. I chose 10-gauge wire for the entire system to keep resistance small. This cost a little more than changing wire size from 12 to 10 with a little added benefit. One benefit was that I could buy only one wire size, rather than two different sizes.

Marginally, since each set of panels, is at a different distance to the combiner box, the increased wire diameter in the series strings tends to equalize the voltages at the combiner box.

100 feet of 10-gauge wire and crimp tools
100 feet of 10-gauge wire and crimp tools

Wiring Solar Panels — Safety

There is a risk when wiring the solar panels in series, thus increasing the voltage, that I need to interject. As voltage increases, risk increases. If you remove a cell at 1.5-volts from your flashlight. Even though it holds more energy than a nine-volt battery, there is (nearly) zero chance you could feel the current flow from the 1.5-volt cell when you touch both ends to your body. However, short the terminals on a nine-volt battery, with wet hands, you will feel the tickle of the current.

It is the same when wiring the solar panels, at 20 volts (5 amps) the panels have a low shock hazard. The same panels, when wired in series of three, have a shock hazard. If the panels were wired in series more than three, the shock hazard is very serious.

The National Electrical Code requires that direct current systems greater than volts require installation by a licensed electrician with specialized training in high voltage systems. This is because high voltage direct current electrical systems have a large potential shock hazard.

Warning: When wiring the solar panels in series you better have electrical skills and knowledge equal to the task.

(Also, don’t forget about the effect of shade on the series wired panels, described below.)

Wiring Solar Panels — Connections

At both ends of every wire, you need a connection to complete the circuit. The standard connection in solar panels is the MC-4 connector. There are three good parts to this design as compared to other previous standards. First, improvement to previous connectors is that they lock together. The previous MC-3 connector didn’t lock together.

Wiring solar panels MC-4 connector and inline fuse laying on a paper towel.
MC-4 connector and inline fuse laying on a paper towel.

MC-4 connectors are also a pretty waterproof design and you can take them apart for troubleshooting. Waterproof in this case means weather tolerant for outdoor exposure, not submersible.

Now for a caution. Don’t mix MC-4 connectors from different manufactures. Very slight differences in molding cause them not to work well together. Sometimes they don’t make the connection and other times they will not snap together.

On my previous solar install, I didn’t use MC-4 connectors, rather I made better connections using splices. After that, I waterproofed my connections. One shortcoming of the previous install was that I couldn’t disassemble the connections for troubleshooting.

This time I used MC-4 connectors and each time I have had to disassemble the connectors for troubleshooting, it was the connectors that were causing the problem.

Wiring Solar Panels — Tools

I purchased a special tool to crimp the connectors. I was very satisfied with the connectors– until I started having problems. This isn’t the only crimp tool I have purchased, but it is the fastest and very easy to use. I have started giving the connectors an extra squeeze using a different crimp tool, just to make the connection more solid.

MC-4 crimp tool and cable cutter
MC-4 crimp tool and cable cutter

Zamp Solar panels didn’t come with these connectors but instead had an automotive design, waterproof, ATP connector that wouldn’t work in my eighteen-panel system because they couldn’t support increased wire sizes or my serial panels.

Wiring solar panels Solar ATP connector and inline fuse.
Zamp Solar ATP connector and inline fuse.

Next time I put together a system, I am going to go back a hundred years in installation practice and solder the wires together. Sometimes the old ways are the best. I say next time somewhat tongue in cheek, there won’t be the next time in an RV. However, if we ever settle down in a normal stationary home, I will most certainly have solar and will (most likely) solder the wires together.

Wiring Solar Panels — My Design

Twelve-volts in name only

We chose to stay with a 12-volt battery system for one huge reason — we already had a large 12-volt inverter.

If you are designing an RV electrical system and don’t already have an inverter, then there are very good reasons to choose a different battery configuration. That said, in our very first article on RV solar, I explained that almost nothing really runs on 12-volts. In fact, if our batteries are at 12-volts they are nearly empty.

If you are interested, the previous articles have lots of details about RV Solar electrical systems and when I came across some detail that wasn’t correct, I updated the article, so they are all still current. RV Solar Series: Introduction

Batteries and controllers

Our lithium batteries best charge at 14.4 volts. Our solar panels produce around 20 volts each. One of the controller’s jobs is to step down the voltage from 20 volts to 14.4. There are two different controllers that can accomplish this task. These are the PWM controller (Pulse Width Modulation) and the MPPT controller (Maximum Power Point Tracking).

If I wanted to spend crazy extra money on the wire, I could have wired the solar panels in parallel, each panel straight to the controller, and used (big) PWM controllers. I can’t even imagine how I could pull four very thick wires down from the roof to the batteries. If I did not increase the wire size, then 45 amps could have burned through a thin wire. Even worse, had I not combined the wires on the roof, I would have had to run 36 wires from the roof to the controllers.

MPPT Controllers

MPPT controllers allowed me to pull four moderately thick wires down from the roof rather than four very thick wires. The reason I could do this is that instead of pulling wire for 20 volts I made combinations of three panels act as one panel operating at 60 volts.

There are multiple reasons to choose MPPT controllers, including a little extra power beyond that which a PWM controller could create, but the biggest one would be wire size and cost.

I glossed over the choices between PWM and MPPT in my previous articles discussing solar installation on my previous RV. The most important information is this: “If you have a big system that is to recharge a battery MPPT is the answer. The MPPT is more expensive, but you will save lots of money on wire costs.  If you desire a system to maintain a charge, (but not recharge) PWM is fine.” There I quoted myself… RV Solar Series: Installation

Controller Designs

PWM controllers are pre-programmed to accept panels output at about 20 volts, stepping the voltage down to that which is acceptable to the battery. In a PWM controller, solar output more than the controller needs is discarded. MPPT controllers operate differently in that they will allow much higher voltages and then convert the extra voltage to a higher current. Thus the MPPT controller doesn’t discard the extra output.

This is the biggest reason I chose MPPT controllers was to get higher output with smaller wires and less resistance. This is accomplished by operating at higher voltages. This allows me to run smaller wires without increased resistance.

Lead Acid Batteries are a little different

I have described my lead-acid batteries as a stomach, How we Store Electricity, and then later for my lithium batteries as a bucket. Massive Electric Bucket

The difference is that you can pour water into a bucket as fast as you want (limited by the source). You can’t do that with a stomach. As a lead-acid battery gets full, you have to taper off the charge at a predetermined rate until the battery gets full. In my opinion, this is the number one reason to get lithium batteries for your RV. This makes lithium a perfect match for solar. Here are three articles on that subject. Batteries Lead or Lithium …. Lithium/Lead Acid Final Analysis …. Ten Lithium Battery Myths and Answers

Wiring Solar Panels — Series/Parallel wiring

I have already described how I actually have two different solar arrays. I have nine panels on the driver’s side of the RV and nine panels on the passenger side. Each side of the RV has a separate MPPT controller. With an unobstructed sky, all nine panels on each side should get about the same energy and thus will operate as one much bigger panel. Really, I have three sets of three panels on each side and then all nine become one.

Nine becomes three, and three becomes one

How does nine become three? The answer is that I have wired them to have three sets – of three panels. I accomplished this by wiring the solar panels in sets of three panels in series, thus increasing the voltage to about 60 volts. Each set of three panels then act as one panel. I then connected each set of three panels to two other sets in parallel. It operates as if I have three panels on each side of the RV all in parallel – all having about the same output.

Wiring solar panels. Nine panels in a serial/parallel wiring layout..
This drawing represents either the driver’s side or the passenger side of the RV. Each has nine panels in a serial/parallel wiring layout. Notice also the two different combiner boxes. The negative wires are not in the same combiner box as the positive wires. This eliminates the possibility of a short circuit inside a combiner box. (Plus I have more room to work.)

The output of each set of three panels will be about 60 volts at about 5 amps. The combined output of three sets of panels together in parallel will be about 60 volts at about 15 amps Then at the controller, the voltage will drop to 14.4 volts, trading volts for amps, causes 45-50 amps to feed the battery from each set of panels. I will get about the same results from the nine panels on the other side, and thus can recharge my batteries at something close to 100 amps per hour. Of course, 100 amps per hour is in ideal conditions. I will be overjoyed at 90 amps per hour.

Shade

The problem with wiring the solar panels in series is that if one panel gets shade then the other two panels in the set will be affected as if they were also shaded. If instead, I had wired the solar panels all in parallel, then shade on one panel would only destroy the output of that panel. Wiring in parallel dodges some of the problems with shade.

Plug and Play

In our design, we are able to unplug any group of three panels, or even up to seventeen panels, and still, have a functional system. The possible combinations are one panel, two, three, four, six, seven, nine, ten, twelve, fifteen, or eighteen.

Obviously, this is a mental exercise, not a practical exercise and we want all eighteen to work all the time. The practical aspect of this grouping is associated with the partial current loss associated with partial shading.

Zamp Obsidian Solar Panels are designed to be plug and play, unfortunately, we were not able to use the pre-supplied connectors because of our series/parallel wiring decisions. If you have a small system, the Zamp Obsidian Solar electrical pre-wiring will work great.

Combiner boxes

I have four combiner boxes. Two for the driver’s side array and two for the passenger side array. Each combiner box is where the positive or the negative wires combine at a junction post. Then the combined output of the panels goes to the controller, via a circuit breaker. Inside the combiner box, I could have used a bus bar or a junction post. Both accomplish the same purpose. What you never want to see is a bunch of wire nuts.

Wiring solar panels. Positive combiner box for driver's side panels.
Positive combiner box for driver’s side panels. three of the wires are inputs and one wire is the output. Notice the in-line fuse next to the box. This box is done, All I have to do is screw on the lid.

In my design layout, the positive wires do not enter the same combiner box as the negative wires. Each has a separate combiner box. To me, it is simpler, easier to understand, and much easier to wire if for no other reason than I have room to work. The additional benefit is since the positive wires are in a separate combiner box, and the negative wires are in a different combiner box, I have zero chance of ever creating a short circuit in a common combiner box.

You can see in this picture the combiner boxes and the fuses leading to the positive box. The box in the middle of the picture is the roof penetration box.

Roof Penetration Box

In the middle of the combiner boxes, is a separate box with a hole in the bottom where the wires drop into the roof and then down the inside of the wall. The wires from the combiner box do not junction inside the roof penetration box. Instead, the wires go all the way to the controller uninterrupted except for the circuit breaker on the positive wire.

Wiring solar panels. Combiner boxes and roof penetration box
One of the combiner boxes is just outside the top of the picture located under a solar panel. You can just see the wires going to that box. The roof penetration box is surrounded by the combiner boxes. each combiner box has three input wires and one output wire. When I put the final panel on, covering these combiner boxes, the wires will enter the cable glans in the two boxes that only have three wires.

Once these combiner boxes are finished, they will be sealed. There is no reason to ever access them again except possibly for troubleshooting or to disconnect a series set. If I add new panels, then I will have to open the roof penetration box to run the new wires. I have already provided a cable gland (with a small wire), just for possible future expansion. The purpose of the wire stub is to seal the gland. I could have added an additional gland to the roof penetration box for the other anticipated wire, but this was honestly too much preparation for something I don’t ever expect to need.

Another possibility is that I may use the roof penetration box to route other wires from the roof to the basement for other purposes. Perhaps I may use it to route my cell phone booster antenna wire. Or a different wire for my TV antenna, or perhaps both.

The next post on our Zamp Obsidian solar panel installation will outline the subject of how we wired the solar controllers and how we combined circuit breakers, fuses, and switches to control the system.

Here is a link to the Zamp website explaining the Zamp Obsidian solar panel. Zamp

11 thoughts on “Wiring Solar Panels”

  1. Thanks! I definitely learned a few things by reading about your installation. You have very good descriptions. Since you have 2, 60 volt 15 amp systems (each half of roof), is your total wattage output at 1800 watts? I also get confused when people ask me what my amp hours are on our solar system. We have the Volta lithium system with 6 – 280 watt panels on the roof. So that is about 1.7 KW. Our battery is a 20,000 watt lithium sealed battery and it runs at 57 volts. Therefore I am thinking we have about 350 amphours of power. Thanks for the input!

    1. Chris, people get lost between watts and amp hours, including me. Yes, my panels size out at 1800 watts per hour. The maximum I can squeeze from them is 100 amps per hour.

      My batteries are rated as has been the historical standard in Amp-hours… which is misleading. If they were rated in watt-hours it would be easier to understand.

      I think your math is correct in converting the amp hours to watt-hours. I would say that you have 350 amp-hours of storage and a maximum of 1700 watts per hour of panels. It depends on charge controllers and other efficiencies on how much of the 1700 watts enters the batteries and can be used.

  2. Very informative, I am considering replacing my 2 6 volt lead acid with 2 lithium’s. I am not sure what the output of my existing 2 solar panels is. At best they are old school panels being 17 years old. How do I measure the out put? I’m sure the controller is also old PWM type.
    We purchase an off-road toy this week a Suzuki Samurai.

    1. Assuming how long you camp lithium is a great choice. If you don’t camp without hookups — then anything will work.

      The number one benefit of lithium is how fast they charge — much faster than any lead-acid. Your old PWM controller won’t go a good job with lithium. Output needs to be 14.4 volts

      To test the output of a solar panel, put it in the sun and measure the voltage. Needs to be well above 14 or the panel is a dud.

      To measure the current you need to hook it to a load (both a charge controller (PWM will work here) and a low charge level battery. Then use a DC clamp meter on the wire.

      Scott

  3. Hi Scott. I really enjoy reading your postings on your travels and also the ones regarding what you’re doing with the solar power system in the RV. You do a GREAT job of simplifying/summarizing and explaining what can be a complex topic for many people. We’re still rooted (live in a house), but I’ve recently looked at solar as part of a backup system. I AM a EE geek, so take all this with a grain of salt (NONE if it is a criticism).

    Soooo….

    There may be good reasons for using stranded wire (flexibility, soldering, etc?), but skin effect isn’t one of them. At DC to 60Hz we use in our power systems, skin effect has no effect – it only comes into play at much higher frequencies (RF), where the current IS conducted more toward the surface of the wire. (Look up skin effect and/or skin depth sometime if you’re curious – https://www.everythingrf.com/community/what-is-skin-depth ). At DC the entire cross-section of the wire is used, so solid/stranded doesn’t matter. However, inside the inverter, where the DC is converted back into AC (high power switching transistors and then filtered to get rid of the higher frequency harmonics, leaving just a 60Hz sine wave), it might matter, but that is a problem for the inverter designers!

    I realize that where you have your batteries, you can always keep them above 32F, but it might be good to remind folks, when choosing they’re batteries, that Li-ion batteries are designed to work above 32F. Since we’re ‘rooted’ and can’t move to where it is always warm, we’d have to go with lead-acid, since where we’d have to have the batteries might get pretty cold – we’re at 8000′ in southern Colorado.

    All that being said, great articles and really good information. Enjoy your travels.
    BTW – You and Tami should stop by someday on your meanderings – I used to work with her back in the day at Signetics/Philips in Albuquerque. Good to see she wound up with a good one (you!).

    David

    1. Dave, thanks for clearing things up on the skin effect and harmonics creating that skin effect. That nuance divides us pretend geeks from the real thing… that also is not a critical remark.

      I am humbled by the high level of input I get from my readers.

      Some lithium batteries have built-in heaters that warm the battery before charging. The BMS will cut off discharging before damage is done and prevent charging until they are up to temperature. I did lots of looking into putting some extra heat in the compartment but after several low-temperature, well below freezing nights, it is unneeded.

      Scott

    1. I told a friend that the reason there is a world-wide copper shortage is because of extra unused wire… RVs could be a lot lighter weight without all this unused wire.

    1. I assume that your question is for any solar install where there was no outside (city) electrical system. If so then yes it is almost identical.

      The biggest thing I would change is the battery voltage and the inverter. I would probably also upgrade the charge controller.

      If you were to put in a 48-volt battery (or a combination of four 12 volt batteries in series to create a 48-volt battery) then you could install a 48-volt inverter, and depending on the charge controller, put four times as many solar panels on a single charge controller.

      Without a major electrical remodel and huge expense on a different inverter, I was limited to a 12-volt system.

      Scott

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