Question about mismatched panel setup

[Solar User]

I think max fuse rating is usually rounded up to standard breaker size from 1.56 ISC or to make the panel easier to integrate in a parallel string.

I would just match the max fuse rating unless splitting the difference lets me shrink the wires on that side in a meaningful way. 10A vs 15A would likely both be #14 anyway

Thanks. That is what I was wondering. I plan to use #10 PV wire so it should be plenty for either 10A or 15A.

 

[zanydroid]

Really interested in the wire size discussion here as well as the fusing topic (those are both topics that I had a hard time getting information on until I got the help from this forum). Regarding the wire size, this wire would not be next to a roof - It would be out in the open laying on the ground or possibly running to the inside of a structure (possibly through an attic?) and would be running to an MPPT controller (Victron 150/45 TR). In the example of only using the two panels (300 watt and 100 watt panel), the overall current at max power would be 22.82A so is the concern that 22.82 X 1.56 = 35.60A (which would exceed the 30A current carrying capacity of 10AWG wire)? Or should I be looking at the Isc of the largest panel in the array (18.85A) as that value multiplied by 1.56 would be less than 30A? Just trying to figure it all out. Thanks

The conditions to use free air ampacity table are kind of gnarly and I've given up on using them. Outside of overhead wires I think it's really hard. For instance you could argue that the solar panels are obstructing free air flow ?

You need to add both current since they will both be producing at the same time. In a fault I believe you assume that the faulting panel does not contribute current, which is a different situation.

So the 300W panel here has unusually high current for the size, somebody designed this to have low voltage / high current (and you probably want to do some voltage drop calculation to see how much you're leaving on the table with this current / voltage combination. It might be pretty high). So you would go over #10 limit. Depending on if you believe that solar implicitly has a 35A exemption to the 30A rule (which I think it does). 35A is way over 30A if you believe in the 30A rule, so I would step this up to 50A.

Check my posts on this thread on a splitter that is 30A in / 50A out. If 2P you can do this just with MC4 branch connectors. After the combining you need to use the special MC4 connector along with #8 wire. However, since you need fusing on the smaller one, there is more reason to use a combiner since you need a place to put the fuse (you need to decide whether you prefer inline MC4 fuses or combiner box).

(Not sure if mentioned already but the list price for BougeRV panels is often pretty high. I guess the 300W is sort of a specialty panel, but one could also go for standard 300W panels (which are often super discounted since they're old) and adjust system design. If you have MPPT then the voltage is not a big deal.)

 

[Solar User]

The conditions to use free air ampacity table are kind of gnarly and I've given up on using them. Outside of overhead wires I think it's really hard. For instance you could argue that the solar panels are obstructing free air flow ?

You need to add both current since they will both be producing at the same time. In a fault I believe you assume that the faulting panel does not contribute current, which is a different situation.

So the 300W panel here has unusually high current for the size, somebody designed this to have low voltage / high current (and you probably want to do some voltage drop calculation to see how much you're leaving on the table with this current / voltage combination. It might be pretty high). So you would go over #10 limit. Depending on if you believe that solar implicitly has a 35A exemption to the 30A rule (which I think it does). 35A is way over 30A if you believe in the 30A rule, so I would step this up to 50A.

Check my posts on this thread on a splitter that is 30A in / 50A out. If 2P you can do this just with MC4 branch connectors. After the combining you need to use the special MC4 connector along with #8 wire. However, since you need fusing on the smaller one, there is more reason to use a combiner since you need a place to put the fuse (you need to decide whether you prefer inline MC4 fuses or combiner box).

(Not sure if mentioned already but the list price for BougeRV panels is often pretty high. I guess the 300W is sort of a specialty panel, but one could also go for standard 300W panels (which are often super discounted since they're old) and adjust system design. If you have MPPT then the voltage is not a big deal.)

Thanks for all of the good information. I purchased the Bouge RV 300 watt panel due to the fact that, at the time, there was a sale and I had a discount code so it helped get the price down to a little over $1 a watt (I have tried to get other panels for less than $1 a watt (not too bad for brand new). Interesting side story is that I helped someone else purchase the 100 watt panel from HQST for their system because it was a bargain for a new 10bb panel and we bought two of them - They ended up only delivering one and their customer service took a lot of time getting back to us about the issue. I will definitely check out the splitter you are referencing.

 

[zanydroid]

$1 a watt is bad if purchased in bulk/group by with other locals where you split / pallet.

For singles, it’s pretty painful to do small orders delivered to your door.

I would recommend also gaming out how you plan to expand this. I think you can add a third small panel in parallel and stay under the limit. For series I think you would be leaving some production on the table if you add 2P of either 10A or 13A style panels (for 2*10A your existing panels would bottleneck, for 2*13A the new ones will bottleneck) but it will still be a net gain.

With regard to your size panels, you can probably use the budget priced linked combiners in the other thread, if you replace the fuse or breaker for the 17A panel with a terminal block (IE no fusing). And leave only the smaller panel protected. I don’t know how good the components inside are though, might want to create a thread asking about specific ones. In general I’m not a huge fan of buying solar stuff from Amazon, it needs extra due diligence.

 

[Hedges]

Regarding the wire size, this wire would not be next to a roof - It would be out in the open laying on the ground or possibly running to the inside of a structure (possibly through an attic?) and would be running to an MPPT controller (Victron 150/45 TR).

If in an attic, ignore everything I said about OK to bend/break rules. Obey the NEC. That includes wire ampacity, derating for temperature etc., and PV wires inside building to be in metal conduit.

 

[Hedges]

I purchased the Bouge RV 300 watt panel due to the fact that, at the time, there was a sale and I had a discount code so it helped get the price down to a little over $1 a watt (I have tried to get other panels for less than $1 a watt (not too bad for brand new).

In recent years I have paid $1/W for all hardware of a grid-tie system. (20 years ago I paid $8/W including $5/W for panels)

I just bought new 370W panels for $0.19/W delivered (pallets of 31 panels, about $2000 each) from Inxeption.

Santan Solar has good deals, mostly used some new leftovers.

Check eBay (sort by distance to find some you can pick up) and Craigslist.

 

[zanydroid]

In recent years I have paid $1/W for all hardware of a grid-tie system. (20 years ago I paid $8/W including $5/W for panels)

I just bought new 370W panels for $0.19/W delivered (pallets of 31 panels, about $2000 each) from Inxeption.

Santan Solar has good deals, mostly used some new leftovers.

Check eBay (sort by distance to find some you can pick up) and Craigslist.

I feel like we’re spoiled in California. Feels like people can’t even give panels away sometimes.

I feel a bit salty that I paid 0.70/W for 10kW-DC of new panels last summer due to the NEM2/3 transition rush. I could have planned to do the install in April 2024 after the rainy season, redid the layout with the new form factor panels (though I think it would have been worse), and probably gotten it done for well below .50)

 

[Hedges]

I'm glad I bought what I did in 2019/2020 before the pandemic, before power outages and Ukraine driven gas prices, before I got canned. It made a nice Covid project, and I had battery backup going shortly before the grid went down.
Panels are now 1/2 ~ 2/3 the price, Sunny Islands are 1.5x ~ 2x the price. Lithium batteries can now be purchased for no more up-front cost of lead.

But utility rates have gone up, and even if your system cost with those panels came to $1.50/W (I'd estimate $0.04/kWh over 20 years) you're saving money. Don't need to buy at the bottom.

 

[zanydroid]

Oh I’m doing quite well with the price. It’s more of a theoretical whine. There’s a lot of value in just getting the project done, and when I did the test layout with the wider 56s cell panels it was uglier than 72s I ended up with (the 72s were becoming extinct on the west coast around that time)

 

[Solar User]

If in an attic, ignore everything I said about OK to bend/break rules. Obey the NEC. That includes wire ampacity, derating for temperature etc., and PV wires inside building to be in metal conduit.

Thanks for the advice. I didn’t even think about the metal conduit encasing as that is a good idea (I have a lot to learn). Thinking about the setup we were currently discussing, what if I had another identical 300 watt panel in series (and no other panels) so that the amperage was not increased - Is my thinking correct that you could potentially still get the full 600 watts from the two panels and still be safe using 10AWG wire into a structure? Or do you have to account for the possibility that more current might occur if a short developed in one of the two panels?

 

[Solar User]

The conditions to use free air ampacity table are kind of gnarly and I've given up on using them. Outside of overhead wires I think it's really hard. For instance you could argue that the solar panels are obstructing free air flow ?

You need to add both current since they will both be producing at the same time. In a fault I believe you assume that the faulting panel does not contribute current, which is a different situation.

So the 300W panel here has unusually high current for the size, somebody designed this to have low voltage / high current (and you probably want to do some voltage drop calculation to see how much you're leaving on the table with this current / voltage combination. It might be pretty high). So you would go over #10 limit. Depending on if you believe that solar implicitly has a 35A exemption to the 30A rule (which I think it does). 35A is way over 30A if you believe in the 30A rule, so I would step this up to 50A.

Check my posts on this thread on a splitter that is 30A in / 50A out. If 2P you can do this just with MC4 branch connectors. After the combining you need to use the special MC4 connector along with #8 wire. However, since you need fusing on the smaller one, there is more reason to use a combiner since you need a place to put the fuse (you need to decide whether you prefer inline MC4 fuses or combiner box).

(Not sure if mentioned already but the list price for BougeRV panels is often pretty high. I guess the 300W is sort of a specialty panel, but one could also go for standard 300W panels (which are often super discounted since they're old) and adjust system design. If you have MPPT then the voltage is not a big deal.)

Very interesting that you bring up the high amperage on this panel as it had not even occurred to me while crunching all the numbers that this panel would not even be able to yield full power if used with most of the manufactured solar power stations (usually capped at around 15A). Looking back, I probably should have looked for a panel with higher voltage and lower amperage. Is there a tool available to calculate voltage loss as I was thinking I might need a pretty long run of 10AWG wire? I got used to using the prebuilt systems where you could put 2 or even 3 400+ watt panels in series and run it on 10AWG wire.

 

[zanydroid]

Very interesting that you bring up the high amperage on this panel as it had not even occurred to me while crunching all the numbers that this panel would not even be able to yield full power if used with most of the manufactured solar power stations (usually capped at around 15A). Looking back, I probably should have looked for a panel with higher voltage and lower amperage. Is there a tool available to calculate voltage loss as I was thinking I might need a pretty long run of 10AWG wire? I got used to using the prebuilt systems where you could put 2 or even 3 400+ watt panels in series and run it on 10AWG wire.

You can use standard calculators like this:

Voltage Drop Calculator | Southwire

www.southwire.com

Set it to voltage drop mode, which lets you pick the AWG. Make sure you understand whether it has done the doubling for you on the wire length (you need both out & back for a circuit).

When I enter 50 ft (I think it's one way distance but i'm not sure), and 17V / 17A (close to 300W), I get 10% loss for #10. LOL. 6.7% for #8.

One thing you can maybe look into is the pre-programmed DC optimizers for shifting voltage/current. You can use one to boost/buck panel to any fixed voltage you want. There's a few stores on AliExpress selling them, they're basically reprogrammed SolarEdge optimizers. Like $20-40, which is a decent deal compared to having to schlep around & resell solar panels. Unfortunately I can't find the store link in my saved notes, I found out about it on a thread here though.

 

[Solar User]

You can use standard calculators like this:

Voltage Drop Calculator | Southwire

www.southwire.com

Set it to voltage drop mode, which lets you pick the AWG. Make sure you understand whether it has done the doubling for you on the wire length (you need both out & back for a circuit).

When I enter 50 ft (I think it's one way distance but i'm not sure), and 17V / 17A (close to 300W), I get 10% loss for #10. LOL. 6.7% for #8.

One thing you can maybe look into is the pre-programmed DC optimizers for shifting voltage/current. You can use one to boost/buck panel to any fixed voltage you want. There's a few stores on AliExpress selling them, they're basically reprogrammed SolarEdge optimizers. Like $20-40, which is a decent deal compared to having to schlep around & resell solar panels. Unfortunately I can't find the store link in my saved notes, I found out about it on a thread here though.

That sounds like it could be an option so I need to look into it. Thanks.

 

[Solar User]

Just remember the classic electrical mantra, "In series, volts add while amps stay the same. In parallel, amps add while voltage stays the same."

To add a bit to that, two parallel strings should be close to the same voltage, not varying by more than 5%. Panels in a single series string should match amperage to <5%.

The rollable cart idea has to be look at very critically. Storm winds could easily toss it around and destroy all your panels. One of the neighbors mounted a set of six 275W panels on a automotive transport trailer. Not something light and wimpy, but storm winds filled it completely over, crushing all his panels.

Here's a pic of some of my adjustable ground mounts. I have built several array frame that can handle between three and six large residential panels. I'd recommend copying that rather than something rollable. These arrays that are adjustable for both azimuth and declination have survived storms that have toppled the Oak trees you can see in the background.

@MichaelK. I was showing photos of your setup to someone else and they also liked it. They were asking questions about the specifics that I could not answer. What materials were used and do you by chance have any specific plans you are able to share (or just thoughts on the construction). Nice job.

 

[zanydroid]

That sounds like it could be an option so I need to look into it. Thanks.

I'm having a hell of a time finding the original thread with AliExpress link where I learned about this. Unfortunately Google just finds my recent posts referencing that link. LOL, I spammed myself.

Anyway here is one that I managed to find. Just as an example. It's not the one that I saw earlier in the year.

The idea here is you can tell them what output voltage you want, and they'll pre-program it for you into the optimizer using some magic wand.

The listing here says 37V but if you read carefully through the Chinglish you can contact them for an alternate voltage. 37V will turn that oddball 300W panel into something normal.

EDIT: note that you will have some losses too just from running the optimizer. The big value IMO is in making it a standard current or voltage so that it's easier to match with panels / MPPTs down the road.

 

[Solar User]

I'm having a hell of a time finding the original thread with AliExpress link where I learned about this. Unfortunately Google just finds my recent posts referencing that link. LOL, I spammed myself.

Anyway here is one that I managed to find. Just as an example. It's not the one that I saw earlier in the year.

The idea here is you can tell them what output voltage you want, and they'll pre-program it for you into the optimizer using some magic wand.

The listing here says 37V but if you read carefully through the Chinglish you can contact them for an alternate voltage. 37V will turn that oddball 300W inverter into something normal.

That is an awesome tool. Thanks for finding it. I was actually in the process of looking it up myself and I found pretty much the same thing you did - Your posts must be popular to be at the top of the list -HaHa!

 

[Rednecktek]

Threre's gotta be a programmer on here that can make some sort of program where you can put in the VoC, Vmp and Isc, the number of panels, next column again, and do the math. In theory it should be simple?

 

[Solar User]

I'm having a hell of a time finding the original thread with AliExpress link where I learned about this. Unfortunately Google just finds my recent posts referencing that link. LOL, I spammed myself.

Anyway here is one that I managed to find. Just as an example. It's not the one that I saw earlier in the year.

The idea here is you can tell them what output voltage you want, and they'll pre-program it for you into the optimizer using some magic wand.

The listing here says 37V but if you read carefully through the Chinglish you can contact them for an alternate voltage. 37V will turn that oddball 300W inverter into something normal.

That is an awesome tool. Thanks for finding it. I was actually in the process of looking it up myself and I found pretty much the same thing you did - Your posts must be popular to be at the top of the list -HaHa!

 

[Solar User]

Threre's gotta be a programmer on here that can make some sort of program where you can put in the VoC, Vmp and Isc, the number of panels, next column again, and do the math. In theory it should be simple?

Wouldn’t that be neat. I bet it would see a lot of use.

 

[Hedges]

Thanks for the advice. I didn’t even think about the metal conduit encasing as that is a good idea (I have a lot to learn). Thinking about the setup we were currently discussing, what if I had another identical 300 watt panel in series (and no other panels) so that the amperage was not increased - Is my thinking correct that you could potentially still get the full 600 watts from the two panels and still be safe using 10AWG wire into a structure? Or do you have to account for the possibility that more current might occur if a short developed in one of the two panels?

Panels in series is how we get away with small gauge wire and reduce IR loss.

#1 rule is do not exceed max allowed PV voltage into SCC or inverter.
Multiple panel Voc by number in series, and multiply by 1.16 for conservative adjustment for cold weather. Or get temperature coefficient from data sheet, historical record cold for your location, and calculate the actual increase in voltage.

Same wattage at twice the voltage, half the current, will be 1/4 the loss.

It is great if you can get loss under 1% or under 3%, but that isn't required. A loss of 10% or more at peak output is OK if you decide it saves you money on wire.

Series, all panels have to be same or vary close to same orientation.
Parallel strings can have different orientation.

KISS and don't use optimizers.

If panels are on roof and > 80 Voc, you're probably required to have RSD. Optimizers and module-level monitoring are available at some price above RSD.