Solar Panel Parallel/Series: Electrical Question

[steve2777]

I have a basic understanding of solar wiring, but I've come up with a scenario beyond my knowledge. I have a 2018 Thor Siesta 24ST, and have two 100 watt solar panels on the roof. Because I have a 30 amp MPPT controller, I've wired them in series. I also have two portable / flexible 100 watt panels that I can place up to 50 feet from the coach, which I also wire in series, and have wired a jack in the coach that will allow me to plug them in so that the two 2-panel sets are in parallel with each other, but also with a selector switch that allows me to use only the coach, only the portable, or both banks in parallel with each other.

As long as my two sets were the same wattage, I felt confident that they were, you might say, "balanced", and could be used together. However, I've just removed an old TV antenna from the roof, which has opened up a perfect spot for a third 100 watt panel there.

My question is, if one bank has 300 watts/series and the other has 200 watts/series, will using the two banks at the same time in parallel create a problem? Or would I have to either add a third panel to the portable bank, or go back to everything in parallel, which I don't want to do because of the higher amperage load on the wiring? Also, I know that a 30 amp MPPT controller will handle 400 watts, but what about 500 watts? Probably not 600 watts? It would be close to impossible to have both banks of panels putting out 100% power at the same time, but if it happened, would that be a problem with the 30 amp controller? Thanks.

[petefoss]

I believe it's voltage that would matter. Your three panels in series would be a much higher voltage than two in series. Simplest might be to add a separate controller for the portable panels.

Note no experience here, just using basic electrical knowledge.

[DavidEM]

Having two sets of panels wired in parallel doesn't cause a problem as long as both sets operate at the same voltage.

But you have two sets of two 12V, 100 watt panels and you want to add a third panel to one of the sets. That won't work. If you wire the new panel in series with the other two, you will now have 36 volts nominally. The other set is 24 volts. That won't work with your MPPT controller very well.

Another way to do it is to buy a 24V nominal panel and wire it in parallel with the existing two panels on your roof. That should satisfy your controller but it may not be possible to buy a 24V panel that will fit as they are usually 150 watts or greater and are often more expensive due to shipping costs. Also they may exceed your 30 amp controller's spec.

That spec is how many amps it can deliver to the battery. Batteries usually are charging at about 13 volts, so the maximum wattage to the battery is 13*30=390 or 400 watts nominally. 500 watts of panels will exceed that rating.

You can do it safely only if you never use your roof top panels at the same time you use your portable panels. So if you can fit a 24 volt panel in the space and live with that restriction, then you should be ok.

David

[steve2777]

David: Thanks for the advice! I think I am best advised to stick with what I have, and use the new roof space for a sunporch.

[Chance]

Attached is an article addressing the issue of connecting different types of solar panels together; whether in series and or parallel combinations.

I did not read the entire article, but it does address the main issue that not all solar panels necessarily have the same voltage and or current ratings, even in cases where the rated wattage is the same.

Basically, one would have to look at each panel’s Current-Voltage (IV) curve to estimate what impact connecting dissimilar panels together will have on the entire system.

Attached is a generic IV curve showing that any given panel can operate under many conditions, which allows some flexibility in using dissimilar panels.

https://solarpanelsvenue.com/mixing-solar-panels/

.

[steve2777]

Chance: Thanks !

[ronaldjlee]

Hi Steve
I’ve followed your posts and have a question for you. I have a 2018 Siesta 24SR and am having difficulty in understanding the solar prewires. I’ve received a wiring diagram from Thor showing the prewiring is #10 Orange with a inline 20 amp fuse. Was that your configuration? This line is currently connected to a terminal block that looks like it goes to the batteries? Second issue, where is the other negative wire, and what color? It’s really quite frustrating and difficult with these wires being tagged?

[steve2777]

That's the same + side arrangement I have. The - side just goes to any acceptable ground.

[ronaldjlee]

Thanks Steve
So is the - side on the roof factory grounded? The - side on the controller also just any acceptable ground will be adequate.
Thanks so much

Ron

[steve2777]

The roof panels connect to the solar charge controller, not the ground.

[DavidEM]

I think what ronaldjlee was saying is that he has a single #10 orange wire from the roof to a terminal block but is asking where is the negative/ground wire. AFAIK most motorhomes use the frame as a negative/ground and maybe that is what they expect you to use. Not very good wiring practices IMHO. Frame grounds were abandoned decades ago on cars, although the aluminum frame makes a much better ground than steel.

David

[ronaldjlee]

Thanks Dave
That’s my question, I have a #10 orange connected to a terminal. No matching ground wire( others said there was a white wrapped together with the orange and rolled up connected to nothing) I’m assuming this is connected to to the house batteries?

There is a MCI block on the roof with a female plus and male minus connectors waiting to connect to the panels i don’t see the two wires from that MCI block in the wiring under the bed?

[JoeDS]

Originally Posted by DavidEM

I think what ronaldjlee was saying is that he has a single #10 orange wire from the roof to a terminal block but is asking where is the negative/ground wire. AFAIK most motorhomes use the frame as a negative/ground and maybe that is what they expect you to use. Not very good wiring practices IMHO. Frame grounds were abandoned decades ago on cars, although the aluminum frame makes a much better ground than steel.

David

Don't think frame grounds have been abandoned in cars. Most all cars and trucks attach the "-" terminal to the chassis frame. Most all electrical systems AC or DC "ground" as part of electrical safety fire code. On a car the frame is a great cost and weight saving allowing most components to run one wire and the other attached to "ground".


Same is true on your RV. The batteries , generator, and shore power all have a "ground" (frame) reference. The 120VAC service actually has two! The black or red wires are the "line" voltage. The white wire conmanly called the "neutral" is a "grounded conductor" where as the green/bare wire "ground" is called the "grounding wire".


A little confusing terminology look here for a better description


https://www.castlebri.com/grounding-...ed-vs-neutral/

[The_Breeze]

If you have room for 4, wire 2 in series and join arrays in parallel.

Something I'm working on. Ordered batteries and inverter/charger/transfer switch yesterday.

Hope it helps.