Do I need a combiner box for 24V with 20x100W panels in 5S4P?

chic

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Based on Will's recommendation, I got a Growatt SPF 3000TL LVM (24V) and a LiFePower4 24V 200AH battery. From another of his videos, I chose 20x100W Rich Solar panels. They have max open circuit voltage is 22.6 and the controller has a max input of 145V. With the 20% safety factor, that lets me connect 5 panels in series, so would need 4 parallel strings. Does that mean I need a combiner box? What other equipment should I get (e.g. breakers)? Would 2S10P be an option?
 
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rmaddy

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Any time you have 3 or more strings in parallel you need to fuse each string in a combiner box. Make sure you look at the combined current to choose the correct wire size.

What is the max output current for the charge controller? 2000W at 24V is over 83A.

Why choose 100W panels, especially when you want 2000W? Wiring 20 panels is a huge pain. 6 330W panels would have been much simpler. Or 8 250W panels, etc. It would probably be a lot cheaper to get fewer larger panels vs so many small panels.
 

MichaelK

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Three or more parallel strings by code need either fusing or breakers to stop short circuits. It's your choice, but select either breakers or fuses. You rate your breaker/fuse by the amps, not the voltage. If your panels are putting out ~5.5A, then a 10A fuse/breaker is what's appropriate. Personally, I prefer breakers, because I can simply flip a string off if I need to perform any troubleshooting. Never disconnect a DC solar connection while it's under load. DC arcing can get scary.
 

chic

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I didn't see recommendations for the size of the panels, only the total wattage. I questioned the number of strings because of what I read on this forum, so good thing I asked. It looks like 2S10P is better. The manual states maximum amperage is 141 amps, so that should be good (even the recommended units are made in China with confusing instructions). Thanks for the advice!
 

wattmatters

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Wiring 20 panels is a huge pain. 6 330W panels would have been much simpler.
I have 6 x 370W panels in 2S3P configuration wired via a small combiner box. My inverter Voc limit is only 105V. Works a treat. Putting up 6 panels on the garage roof was bad enough. Can't imagine doing 20!
 

GrumpyOldRetiree

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Why choose 100W panels, especially when you want 2000W?
Speaking as a senior citizen DIY'er (and I mean literally "DO IT YOURSELF") 100 Watt panels are still physically manageable. They allow me to still play the game. Also, there are quite a few 100 watt panels available on Amazon that ship "free" via UPS or FedEx ground for Prime members. The bigger panels usually have to be truck shipped for $$$.
 

12VoltInstalls

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The bigger panels usually have to be truck shipped for $$$.
This is true to an extent.

I get your logic on the small panels. I have 3 windy and 3 Rich running nicely right now.

However, just for your information, the cost of bigger panels is often not much more than the cost of 100W commodity panels. When you add the shipping it can still be less than the sum price of 100W panels for wattages in the range of what you are doing.
Alternatively, it’s becoming easier to regionally source panels from an installer- either over-orders, or have them add your panels to their next shipment. While some places are 500% or more markups (their ‘installed’ pricing), others are only 40, 50, or 100% markups and depending on how they buy, the 100% markup ‘retail’ prices are often very attractive, sometimes as little as half of the panels plus shipping costs to your doorstep. You just need to drive to get them.
6 panels is manageable- 20 panels costs extra in a lot of wire, connectors, fuses, time, and sometimes more mounting hardware.

Just thoughts.
 

MichaelK

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I didn't see recommendations for the size of the panels, only the total wattage. I questioned the number of strings because of what I read on this forum, so good thing I asked. It looks like 2S10P is better. The manual states maximum amperage is 141 amps, so that should be good (even the recommended units are made in China with confusing instructions). Thanks for the advice!
I would disagree that 2S10P is better. Far, far too many parallel connections. Your original idea of 5S4P is better. Here's an analysis of your panel strings using Midnight Solar's string calculator. You can access it here. https://www.midnitesolar.com/sizingTool/index.php

What you can see is the panels wired in series, 5 of your panels will put out 90VDC, which will help reduce voltage drop from the array to the controller terminals. Even at zero degrees F you are only reaching 129V, safely below your 145V limit. If you think your winter lows will be lower then zeroF, use the calculator to predict your locale's value.

Only two panels in series would greatly increase your amperage, and greatly decrease voltage, so power loss from voltage drop will be greater. Secondly, your unit's minimum voltage input requirement is 30VDC, and I'm not sure that on a very hot summer day only two panels in series will meet that requirement. Alternatively 4S5P is a better choice than 2S10P.

BTW, where exactly are you getting the 141A value? In the spec sheet I cut and pasted below, it shows a battery charging maximum of 80A. But, assuming your battery might charge at as little as 25VDC, 2000W/25Vcharging = 80A, so you'd be OK.

1636906659843.png
1636907192894.png
 

Rednecktek

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Speaking as a senior citizen DIY'er (and I mean literally "DO IT YOURSELF") 100 Watt panels are still physically manageable.
As someone who just packed 2 dozen full size used panels around the house, I really have an appreciation for the smaller, lighter 100w panels. :)

And yeah, the more voltage your MPPT controller has to play with the more efficient it's going to be. Avoid doing a lot of smaller parallel setups as you won't have the voltage range your SCC wants to work with and you'll need an immense and really, REALLY expensive combiner box to get that many strings connected up.
 

wattmatters

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Speaking as a senior citizen DIY'er (and I mean literally "DO IT YOURSELF") 100 Watt panels are still physically manageable. They allow me to still play the game.
Use what you can use, makes total sense. Good luck with it, have fun and stay safe (y)

I'm retired, and a leg amputee. My Longi 370W panels were second hand, delivered and cost about $100 each off FB marketplace from a local who has a bunch of old stock from PV systems which get removed (lots of system upgrades happen here). I managed to get them up onto my garage roof and installed. Rails were also pre-loved saved from going to scrap.

It certainly wasn't easy for me! But first time installing and you learn a lot, pretty sure next time (if there is one) I'll be a bit more efficient.

That's 2.22kW.

IMG_3031.jpeg

I used a cheapie combiner box:
IMG_3172.jpeg
Only the positives are fused when probably both should be. I do have some inline MP4 connector fuses I could throw on the negative connectors, not sure if it's worth it.
 

fafrd

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Any time you have 3 or more strings in parallel you need to fuse each string in a combiner box. Make sure you look at the combined current to choose the correct wire size.
I believe the rules are that any time you have two or more strings, each will need to be fused and the location of the fuse can be anywhere before the first panel in the string (does not have to be in a combiner box).

I use inline MC4 fuses like this on my 1S3P string:
What is the max output current for the charge controller? 2000W at 24V is over 83A.

Why choose 100W panels, especially when you want 2000W? Wiring 20 panels is a huge pain. 6 330W panels would have been much simpler. Or 8 250W panels, etc. It would probably be a lot cheaper to get fewer larger panels vs so many small panels.
I’ll second this. I was originally going to pull the trigger on used panels from Santan Solar, but after I factored in the cost of racking, which is tough to achieve for less than $40/panel all-in, I realized that that ‘deal’ was not such a great deal, especially once the extra work and effort were factored in.

Add on top of that the unknown level of degradation associated with used panels and the fact that new panels come with a warranted production level for 10 years if not 20, and seeking a good deal on new panels became a no-brainer.

I ended up jumping on new 380W Half-cut panels when they went on sale for $0.39/W.

Sure, the used 250W panels from Santan Solar we’re only $0.24/W (63% premium for the new half-cut panels) but once you add the $40 for racking a panel to each, the difference drops from $0.15/W to less than $0.10/W, or only a 30% premium for fewer larger panels with full factory warranty…
 

fafrd

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Use what you can use, makes total sense. Good luck with it, have fun and stay safe (y)

I'm retired, and a leg amputee. My Longi 370W panels were second hand, delivered and cost about $100 each off FB marketplace from a local who has a bunch of old stock from PV systems which get removed (lots of system upgrades happen here). I managed to get them up onto my garage roof and installed. Rails were also pre-loved saved from going to scrap.

It certainly wasn't easy for me! But first time installing and you learn a lot, pretty sure next time (if there is one) I'll be a bit more efficient.

That's 2.22kW.

View attachment 72260

I used a cheapie combiner box:
View attachment 72261
Only the positives are fused when probably both should be. I do have some inline MP4 connector fuses I could throw on the negative connectors, not sure if it's worth it.
Why would you want to fuse both positive and negative???
 

fafrd

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Speaking as a senior citizen DIY'er (and I mean literally "DO IT YOURSELF") 100 Watt panels are still physically manageable. They allow me to still play the game. Also, there are quite a few 100 watt panels available on Amazon that ship "free" via UPS or FedEx ground for Prime members. The bigger panels usually have to be truck shipped for $$$.
If you looking for new 100W panels, please ignore everything I just posted about new versus used panels.

And shipping cost certainly is a consideration.

I have access to a pickup with an 8’ bed and live 30 minutes from a major solar wholesaler, but if you are more remote and will need to pay shipping, I can understand why a larger number of smaller panels is attractive.
 

rmaddy

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I believe the rules are that any time you have two or more strings
This says three or more, as does countless posts I've seen:

 

fafrd

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This says three or more, as does countless posts I've seen:

2P being safe without needing to be fused is pretty much universally-agreed-upon.

And more than 3P is also universally agreed-upon as requiring a fuse or breaker on each string,

3P may or may not require fusing on each string depending on the precise specifications of the panel.

If you have a 3P array, a short within any panel can draw 2xIsc from the other two (unshorted) strings.

That 2xIsc will add to another Isc within the shorter panel itself for a total current of 3Isc at the short, 2 x Isc through the cables plus another Isc at the short itself.

Panels generally include a maximum fuse rating within the specifications, typically either 15A or 20A.

If your panel has a maximum fuse rating of 15A that translates to the panel manufacturer telling you that no more than 15A should ever flow through the output cable.

If 2Isc is > the maximum fuse rating, you will have a larger current glowing through that panels cables then recommended by the manufacturer and thus, there are no guarantees as far as safety.

So if the maximum fuse rating if a panel is > 2xIsc, a 3P string should almost certainly be safe without requiring fuses on each panel. But if not, it is recommended to fuse each panel/string: https://www.cedgreentech.com/article/when-fuse-when-not-fuse

“Now let’s evaluate a system where there are three strings of modules connected in parallel to determine whether or not fusing would be a requirement. If a fault were to present itself in one string the other two healthy strings could force their currents through the faulted string. This implies that 2.5 times the Isc rating of the modules could be forced through the faulty string. In many cases the integrated OCPD of the faulted string would be exceeded, requiring that there be fuses present on the output of each string to protect the wiring and equipment from damage.’

And in my above analysis I overlooked the 125% factor. Including that, you need a fuse whenever 2.5 x Isc exceeds the maximum fuse rating of the panel (which is most cases).

For 2S strings, the 125% factor does to trigger the need for fuses since that factor is already taken into account when the panel wires were sized (so the wires can safely handle a maximum 125%^2 156% of Isc whether being output from the panel through normal operation of coming into the panel from a parallel string because it has a short:

“Let’s consider a two string system. We know that each string in this condition could produce current that is 1.25 times that of their Isc rating. If a short were to occur the un-faulted string of modules could force its current through the faulted string. In this scenario we know that the faulted string can already handle 1.56 times its Isc rating so it is still protected. No fuse necessary.
 

chocoheadfred

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Speaking as a senior citizen DIY'er (and I mean literally "DO IT YOURSELF") 100 Watt panels are still physically manageable. They allow me to still play the game. Also, there are quite a few 100 watt panels available on Amazon that ship "free" via UPS or FedEx ground for Prime members. The bigger panels usually have to be truck shipped for $$$.
Good point, certainly bigger panels provide much more efficiency yet are much more cumbersome with transportation and installation. This is what grandkids are for though, right! If their hair stands up when it's all connected then you probably need some beefier fuses 🤪
 

Bvillebob

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As another old gimpy guy, another consideration is that some of the newer panels are much lighter than older panels. My new REC 370 watt panels weigh considerably less than my surplus 250's and are only a few inches longer.
 

GrumpyOldRetiree

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As another old gimpy guy, another consideration is that some of the newer panels are much lighter than older panels. My new REC 370 watt panels weigh considerably less than my surplus 250's and are only a few inches longer.
Yes, weight is certainly an issue and I'm sure the panels currently available are lighter then the older panels. The major concern for me was/is "sail area" of the panels. Years ago, when I was re-sheathing my roof I had several close calls with going airborne or letting the plywood go. The 100 watt panels are small enough to let me steer them out of the wind.
 
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