RV Solar Specifics

[Shadowcatche]

Wire size is important to prevent voltage drop and there are a couple of ways to deal with this. Use higher voltage panels intended for a 24V or 36V system with an MPPT controller that can use the voltage. Or big wire to prevent the drop. I went the high voltage panel route (36V) and used 10 ga wire.
Here is a voltage drop calculator. https://www.omnicalculator.com/physics/voltage-drop

[rarebear.nm]

There are many ways of doing the downhaul run. Depends on your rig and needs. In my case I installed three 350 watt panels, each producing 44volts and ran them parallel to the basement area with #4 wire. For a roof combiner box I used a Soladeck unit with three fuses, once for each panel and they are combined on the backside of the fuses.

[jcussen]

I am running two 2600 watt strings on my S&B. Each string is series/parallel and I run 240 volts through #10 wire to the two controllers.That is only 10 amps or less, so not a problem. Higher voltages can save on wire cost and installation hassles. Solar controller may cost more than a lower voltage model though.

[spider78612]

Here's the one I use:https://www.calculator.net/voltage-drop-calculator.html If you're connecting your panels together don't forget the furthest ones might run 15 ft, next 10 ft and so on. If you connect them in parallel; you will have 17 or 18 volts at the end depending on your panels but you add the amps together like 5.5 amps from the first panel, 11 amps from the first 2, and 16.5 from all 3. Keep your voltage drop below 2% you'll be good!

[hclarkx]

[QUOTE]

Originally Posted by Brianna

I'm a newbie, so be clear that I am asking, not telling.

While it seems logical that heavy 4ga cables between solar panels on a roof would reduce current loss, Is it sgnificant enough to even bother?

Larger than #10 is only needed if a lot of parallel panels are used. For the series string you outlined earlier, #14 wire would work but have several watts of loss under peak sun (panels running nearly 100%). At half power the losses are 1/4 of that. Actual loss depends on length, of course. Typically #12 wire will get you down to a percent or two of losses under peak sun with fairly long runs (30 feet or more). #10 will get you under a few Watts of loss for even very long runs at peak sun ... and a small fraction of a watt at half power. I ran through examples in another thread a couple of weeks ago. In the end, #10 DC wire is economical and readily available on Amazon in 50' lengths. It's stranded so flexible enough and works in the panel connectors and any solar controller.

More than #10 is needed only for multiple parallel panels and as others have mentioned, run #10 to a junction box and something larger from there to the controller...... though not welding wire ..... the super fine stranding is not needed and it's very difficult to work with. Also, the cheap welding wire found on Amazon has very easily ruptured insulation.

In a typical solar system on a 36--40' motorhome let's say there is 20 feet of cabling between the panels and controller. Are we talking, what..... 3% of loss using 10ga vs 4ga?

Probably less. Assume 60 feet round trip and #10 (0.162 ohms/1000 feet). That's 0.06 ohms. I^2R gives you 7.5 x 7.5 x 0.06 or 3.375 watts. That's at full power. At half power the losses are 1/4 of that ... less than one watt.

Do i have this math figured correctly?

Pretty well.

A hypothetical 680W system (4x170) sending maybe 80% of that at (series) 90 volts open circuit would produce 6.04 amps. Right?

Yes.

So 3% of that is only .2 amps.

Not quite, you want to work in Watts. So of the 680 Watts, you will get about 677W using #10. About 3 Watts loss.

#12 wire is 0.095 ohms/1000 feet. So 50% more losses. In the 680 W case losses would be about 4.5 Watts loss. Just over 1 watt at half power.

I'd use #12 with your 7.5A series string if you can find it. But, #10 DC wire seems to be much more easily sourced.

[hclarkx]

Originally Posted by Brianna

When the tundra melts I need to get up on the rv roof and do some serious measuring.
I was originally thinking I would run PV cables down the refrigerator vent but now I am thinking of adding a new combiner box on the roof directly above the battery compartment. So maybe a maximum of 8 or ten vertical feet of heavy gauge cable straight down to the batteries. Then just use 10ga PV cable between panels on the roof.
Probably time to get one of those fancy new stud finders that show you an image. I'm not afraid to drill a hole in the roof, but I am afraid of hitting existing wires that might be hiding in the 8" of roof styrofoam.

I'm confused. I thought you were going with several panels in series and an MPPT controller. Are you going with parallel panels?

Series is much more efficient than parallel. Lower losses in the wiring. Smaller wire. An MPPT controller that can optimize amphours can get up to 15% more out of a given set of panels. Flexibility for adding more panels in series (if your MPPT controller has voltage range to cover more panels). Etc.

Parallel panels only make sense if some of the panels may be in shade much of the time. Or the panels are not in the same plane (some tilted west, others tilted south, etc.). On an RV, with the panels all more-or-less in one plane, series makes the most sense.

[hclarkx]

Originally Posted by jaearhart

People buy a $70,000 diesel truck for torque and MPG to tow a $60,000 TT that has a 6 kw gas generator that consumes 1.5 gph for boondocking. And buys Solar panels and batteries to be energy conservative for four hrs per recharge on sunny days. I just lost my " Let's go Camping " appeal..

In years of involvement, I've never heard anyone say they were going solar for the reason you suggest. Nobody does that. Solar is for those with an aversion to the noise, smell, and hernia that a generator can cause and would rather be out hiking than tending to a generator that is charging batteries.

[astrocamper]

Originally Posted by hclarkx

In years of involvement, I've never heard anyone say they were going solar for the reason you suggest. Nobody does that. Solar is for those with an aversion to the noise, smell, and hernia that a generator can cause and would rather be out hiking than tending to a generator that is charging batteries.

We did it because we could and enjoy the silent power generation.

In our case it was a $48,000 3/4 gasser pulling a $30,000 Artic Fox 22G. I made many changes to my trailer to make it work off grid like it does plugged in without the hassle of bring a generator, a can of gas and having to refill the gas can after 4 days because I want to run the air conditioner.

The cost is manageable if you provide your own labor.

2200 (6600 surge) watt Spartan Pure Sine Way inverter $700
Four Battle Born 100 amp-hour batteries $3800
Two Grape Solar 300 watt panels $700
Three Grape Solar 160 watt panels $645
One Grape Solar 265 watt panel $315
Six Grape Solar 50 watt panels $300
VE BMV-712 $205
2 x VE SmartSolar MPPT 100/50 $650
2 x VE SmartSolar MPPT 100/30 $450
Cost of major components = $7765
Probably another $1000 or so in all the cables, bus bars, disconnects, breakers, etc...

I installed the system myself and it runs my Coleman-Mach 1 PS 11,000 air conditioner just fine without discharging my batteries as long as the sun is shining. We astronomy camp in with other astronomers in open fields without shade so we can enjoy the stars. Air conditioning is useful in the afternoon but not needed after sundown in the 4500+ elevation and dry Great Basin climate.

The six 50 watt panels are used as a portable array on the ground tilted and turned to face the sun a few times a day. They help keep the solar production up late in the day when the sun is lower. The rest of the panels are on the roof. Only use portable array for honest months when camping several nights in open field. Also could be used when parking the trailer under trees. Six panels stack up in a small space in back of truck bed.

We also have a 160 watt panel on our truck cap to charge a pair of 50 amp-hour batteries used for astronomy gear.

[hclarkx]

X2. I spent $4200 (in stages) plus my labor. This includes a massive Lithium battery that will last four days without sun or 8 days with "mostly cloudy." Run the small generator less than twice per year in 5 months of RV boondocking.

[Brianna]

Hclark,
Sorry for any confusion.
I am definitely planning to wire solar panels in series.
My combiner box idea is to perhaps combine 2 groups of series panels, but more importantly, it will be a 1-1/2" pvc conduit for any future wiring like cell boosters, security cameras, or whatever reason that comes up where you wish there was an easy way to get from roof to basement.
I have sort of decided on the EPEVER 40 amp controller because it has massive screwdown slots to accommodate cable up 4awg going to the batteries, plus the whole unit is a big finned aluminum heat sink for better cooling.
But.....because I'm a winter -bound armchair dreamer, plans could change when I can actually start measuring stuff.

[hclarkx]

[QUOTE=hclarkx;5155886]

Probably less. Assume 60 feet round trip and #10 (0.162 ohms/1000 feet). That's 0.06 ohms. I^2R gives you 7.5 x 7.5 x 0.06 or 3.375 watts. That's at full power. At half power the losses are 1/4 of that ... less than one watt.

Oops. Brain f..t. I said 0.162 ohms ... that's wrong. It's about 1 ohm. But, the 0.06 ohms for 60 feet is correct. So my wattage calculation is good.