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SJ-Chris · 03-28-2021, 12:24 AM

Looking for opinions...

[this is for a 2015 Thor Majestic 28a Class C RV]

I just finished installing a 500w MPPT solar system on one of my RVs....loving it!

https://www.irv2.com/forums/f56/proj...er-529010.html

I have another RV that I'm adding solar to. I have a Windy Nation 200w PWM (20amp) system that I purchased about 8 months ago just sitting around in my garage, so I'll be putting that on my 2nd RV. I also picked up a few more 100w 12v panels for $25 each, so I'll throw up a few more panels.

Disclaimer: I'm mostly putting solar on my RV just for fun and to provide for SOME boondocking capabilities. I don't plan on extended boondocking and just want to have solar for those couple of days a year when I'm camping and don't have power hookups. I also let family members use my RV and I don't ever want the batteries run down. This RV has two Group 31 AGM lead acid batteries (and I might add a 3rd)....200AH now, maybe going to 300AH. My original plan was to put just 200w of solar on the RV and call it done. So....anything over 200w is really just gravy. I don't ever plan on adding more solar (....and even if I did, I would switch to an MPPT controller and route some panels in series thus eliminating most of the voltage drop).

Here's what I plan on doing... I plan on putting up FOUR 100w 12v panels and run them in parallel. Max Voltage is 17.4v. Max current is 5.75amps.

Question: I need to decide on wire thickness from my panels to my solar charge controller. I know from just putting solar on my other identical RV that the length of wire will be ~40 feet. If I look at a Voltage Drop Calculator, and put in 17v, 40', and 20 amps, I can see the following estimated voltage drops for different wire thicknesses:

6 AWG:

Voltage drop: 0.63
Voltage drop percentage: 3.72%
Voltage at the end: 16.37

8 AWG:

Voltage drop: 1.01
Voltage drop percentage: 5.91%
Voltage at the end: 15.99

10 AWG:
Voltage drop: 1.60
Voltage drop percentage: 9.40%
Voltage at the end: 15.4

My question is: Does it really matter how much voltage drop there is as long as it is above say 15v? My (limited) understanding of PWM controllers is that they will simply pass through the current and reduce the voltage to whatever the charging stage needs (ie. 14.4v, 13.6v, etc). So is it correct to assume that anything over ~15v is not needed? If I understand things correctly, I will get the same output from these panels if the solar charge controller sees 15v or 17.4v, right?

I'm not planning on using 10AWG wiring, as a 9.4% drop just doesn't sit well with me. But I would consider 8AWG wiring as a 5.9% drop that still results in a voltage at my charge controller of ~16v seems good enough. Am I missing anything?

Thanks!
Chris

astrocamper · 03-28-2021, 10:03 AM

I think you would be fine with the 8 AWG. Keep the controller to battery distance shorter or use a heavier gauge for that part of the run. PWM basically will keep the voltage at the required level needed for the batteries if the input is reasonably higher than the output.

I use prefer to use Victron Energy MPPT charge controllers with the bluetooth management/monitoring interface.

Tomahawk · 03-28-2021, 05:01 PM

I am not an expert.
I don't think you always get the 17V. For example I have a nominal 24V panel and MPPT controller, a lot of times in the input voltage is less than 24V, I have never written it down but 19V comes to mind. I also think the 8 AWG would be fine but I would worry that a slightly lower initial voltage combined with the voltage drop could make me sad.
Another comment, I ended up buying a 10AWG extension cord, cutting the ends off, and splicing on the MC2 connectors. It was cheaper than I could find the wires alone (and they were already bundled). Buying 10/2 stranded wires (cased like and extension cord) was even more expensive. My system is portable so I wanted the wires bundled.

Pappion · 03-31-2021, 08:28 AM

Take a look at Watts lost. Current * Current * Ohms. 16W peak with 10AWG. You have the watts to waste and the extra panels are good for cloudy days.

PWM works by momentarily shorting the panels to the battery. So, the calculated wire voltage drop is meaningless. The panels internal resistance limits the surge.

Very likely you won't hit max output from the panels. I have two 250W on the ground with different orientations, so I get power all day.

Flange · 03-31-2021, 08:22 PM

My research led me to size for max. 3% from panels to controller, and 1% from controller to batteries.

Handy Bob was a big help.
https://handybobsolar.wordpress.com/...ging-puzzle-2/