MPPT sizing

[Gould]

Looking for some confirmation that my math is correct. Building a RV system and think I have everything figured out as far as panels, wiring, and MPPT sizing.

I‘m starting with 4 x 200W panels with room to expand to 6 at some point if needed. The RV came pre-wired with 10 AWG from the roof to the garage. Specs on the Renogy panels:
Voc - 24.1 volts
Imp - 9.52 amps

10 AWG wire can handle up to 30 amps. I know you have to account for some loss over distance but honestly not quite sure what the calculation there is. If I configure 2S3P:
24.1v x 2 = 48.2v x 1.2 for temperature compensation (I live in central Alberta so yes, it gets cold!), the maximum voltage is 57.84
9.52 a * 3 = the maximum amperage is 28.56

With this configuration I should be safe with the existing 10 AWG wiring and a 100|50 MPPT. Honestly I was expecting to need a much larger charge controller else I’m missing something?

Now, where I get really confused is the amperage to the batteries from the MPPT does not jive with what the panels are able to produce. 1200 watts of solar could under ideal conditions produce up to 100 amps correct? Would I be leaving too much on the table with this charge controller?

 

[FilterGuy]

If I followed your description correctly, this is what you are planning



I don't know which brand/model controller you are looking at, but when I see 100|50 MPPT, I assume it is the Victron blue-solar or smart-solar.

The 100 is the absolute max input voltage.... you are ok there. The 50 is the max output current it will produce. So in the configuration you want, the 100|50 SCC would be undersized by half. In fact, it is undersized for the initial 4 panel configuration (You need 67 amp output for 4 panels).

 

[FilterGuy]

BTW: When you do the initial 4 panels 2P2S you don't need to fuse them but when you go to 6 panels you will need to fuse them as shown in the diagram in my previous post. Consequently, when you do the initial 4 panels, plan ahead on how you will add the fuses later.

Fuse/breaker sizing and placement guidelines:

 

[Gould]

Lol, just happy you were able to make sense of it all. Your diagram is correct and so is your assumption, I am looking at Victron. I appreciate you clarifying, that’s where my head was going - was hoping I could get away with the smaller of the two at basically half the price.

Now, I hadn’t even considered fusing the panels independently (on the roof I’m assuming). Will have to do so research there.

 

[FilterGuy]

(on the roof I’m assuming)

Technically, closer to the panels is better, but a lot of folks don't put the fuses on the roof of the RV. If you decide not to put the fuses on the roof, the wire from the roof should be sized to handle 1.56 x Isc. That way the wires would handle any type of short that happens.

(If you are currious about the 1.56 multiplier, check out the link in my previous post. It is a paper about solar panel fusing.

 

[Forbisher]

Now, I hadn’t even considered fusing the panels independently (on the roof I’m assuming). Will have to do so research there.

You could wire your 6 panels 3S2P on a 100V max input SCC and will not need fuses

 

[FilterGuy]

You could wire your 6 panels 3S2P on a 100V max input SCC and will not need fuses

True... but 1) the SCC the OP is looking at is still undersized for 1200W of panel and 2) on RVs where shading is hard to control, the more parallel the better.

 

[Gould]

True... but 1) the SCC the OP is looking at is still undersized for 1200W of panel and 2) on RVs where shading is hard to control, the more parallel the better.

This is my main reason for the smaller series runs. I don't have the RV with me so can't take my measurements yet but I'm assuming things like the air conditioner could be a source of shadow.

I'll read through the paper you've referenced, thanks for that.

 

[Forbisher]

True... but 1) the SCC the OP is looking at is still undersized for 1200W of panel and 2) on RVs where shading is hard to control, the more parallel the better.

I was thinking he could use two 100/50's on a 12V system or use a 24V battery.

 

[Gould]

Pretty much sold on 12v and constrained by the existing wiring from the roof. Trying to avoid running an additional drop.

 

[Forbisher]

Pretty much sold on 12v and constrained by the existing wiring from the roof. Trying to avoid running an additional drop.

Ok so you need the 100A MPPT and some fuses for 2S3P when you upgrade from 2S2P

 

[Gould]

Ah yes, the next rabbit hole, combiner boxes! I can’t seem to find a commercially available unit that gives me any confidence so will likely just build my own.

 

[HRTKD]

There are LOTS of combiner boxes out there. Midnight Solar has a very good selection, but most of them are designed for wall mount, not horizontal roof mount.

Here is a roof combiner box from AM Solar. It does not have breakers or fuses in the box.

https://amsolar.com/rv-combiner-box/20-roof

Here's one with breakers from AIMS Power.

Power Inverters, DC To AC Inverters & Solar Panels | AIMS Power

 

[Gould]

Appreciate the advice and am moving forward with the 150|100.

Continuing with the hijack, mind you I'm the OP , thank you for the clarification on the combiner box. I've done a lot of searching over the last couple of days and finding something roof mountable, within reason has been challenging. I've found this one in Canada, a little small but I should be able to make it work otherwise I'll likely build one myself.

Combiner box - commercial
Combiner box - DIY

 

[Hedges]

Instead of QO breakers for 48Voc PV 2s3p, why not go 3s2p with no need for fuse/breaker?

 

[HRTKD]

When I was looking at combiner boxes a few months ago, I was surprised to find a bunch of them on Etsy, the arts and crafts market place. This is just one example:

This item is unavailable - Etsy

Find the perfect handmade gift, vintage & on-trend clothes, unique jewelry, and more… lots more.

www.etsy.com

 

[Gould]

Instead of QO breakers for 48Voc PV 2s3p, why not go 3s2p with no need for fuse/breaker?

Of course, another monkey wrench!

I was under the impression that you want as few in your series configuration as possible because of shading. If one panel sees some shade all others in series are impacted. Have also now learned that you can get MC4 connectors with inline fuses. Maybe that’s an option in a simple gang box.

 

[Gould]

When I was looking at combiner boxes a few months ago, I was surprised to find a bunch of them on Etsy, the arts and crafts market place. This is just one example:

This item is unavailable - Etsy

Find the perfect handmade gift, vintage & on-trend clothes, unique jewelry, and more… lots more.

www.etsy.com

Etsy of all places, interesting.

 

[Hedges]

Of course, another monkey wrench!

I was under the impression that you want as few in your series configuration as possible because of shading. If one panel sees some shade all others in series are impacted. Have also now learned that you can get MC4 connectors with inline fuses. Maybe that’s an option in a simple gang box.

Not really.
If a single string of panels, any (diode bypassed) sections that have shading drop out, but all other sections deliver their power.
If all panels in parallel, any (diode bypassed) section that is shaded reduces that one panel's Vmp so far the entire panel no long contributes.

One guy did a pitch to the company I worked for about per-panel optimizers (development/manufacturing opportunity)
One of my inverters had 9s2p of Sharp 165W panels. I shaded a single panel. I didn't lose 50% (9 out of 18). I didn't lose 11% (2 out of 18). I only lost 6% (1 out of 18)

With long strings and small portion of one string shaded (or small difference in how much shaded per string), Vmp isn't that much different between them, and pulling the longer string down to Vmp of the shorter string is almost as good as separate MPPT for each string.

That does require an MPPT smart enough to not stop at the first local maxima it finds. 8 illuminated parallel with 9 illuminated may blur into a single peak, while 6 illuminated parallel with 9 illuminated may have two peaks which some MPPT would miss.

It also requires a bypass diode that can carry full current. Apparently some panels, manufacturers say shading a panel while others have full sun is improper operation. It burns up the diode, maybe the panel.

Is your RV mount expected to have lots of shading issues? Can be panels be located or tilted so they don't get shaded by A/C units?

 

[Rocketman]

Here is another thing to think about - how difficult would it be to run that extra set of wires from the roof?

The two 100/50’s are cheaper than a 150/100. I think two SCC would out preform a single larger one when some shading issues occur.

Just a thought