Breaker/fuse sizing questions for solar panels connected series/parallel in 4s2p configuration that are slightly over current Voc? Need help!

[phillipgbrantner]

Hello, Im currently in the process of installing an off grid solar system for a friend in their tiny home here in Hawaii, and as I am very knowledgeable in electrical and solar I am not a certified professional.

I'm currently installing the following,
EG4 48v3000vEHV Inverter/controller charger, 48v LL-S Lithium Battery 100Ah server rack battery,
8x Bluesun bsm460m-72hbd 460w solar panels, 10AWG solar cabel

Specs

Solar controller x1
80a 500vmax input
120-500v Operating voltage

Solar panels x8 4s2p configuration

460w Maximum power (Pmax)
42.4 Operating voltage (vmpp)
10.86 Operating current (Immp)
50.8 Operating current voltage (VOC)
11.5 Short circuit current (ISC)
15a Max fuse size

10AWG Solar Cable 30amp

I plan on installing the panels in 2 strings, 4 panels each string in a 4s2p configuration.

I planned on using branch connectors to to combine the two strings and then into a 125Amp dc disconnect breaker, then directly to the solar controller which would give it 200v and 23amp ISC.

I need to know whether or not I need to install breakers after each series string or any mc4 fuses on each string and what sizes I would be used if any?

Because the combined VOC of the two strings exceeds the max fuse size of the panels.

I'm worried that having 8 panels with a combined VOC of 23 amps could feed back if there were a fault on a panel in either of the strings, and cause a cable to overheat and catch fire.

I'm trying to keep the the system on 2 strings to reduce cable cost and keep the voltage higher because the controller has a startup voltage of 120vdc and running 2 string hybrid has better fault redundancy then 1 string.

Any advice or answers to my questions are welcome.

Thanks,
Phil

Aloha

 

[12VoltInstalls]

I need to know whether or not I need to install breakers after each series string or any mc4 fuses on each string and what sizes I would be used if any?

I would fuse them probably with a combiner box.
Combiner boxes seem expensive for what they are yet they take care of a few different concerns so imho worth the money for what they are.

At the very least a proper DC breaker is a useful disconnect for the panels.

10.86 Operating current (Immp)
50.8 Operating current voltage (VOC)
11.5 Short circuit current (ISC)
15a Max fuse size

Each string is going to be 10.86A or 11.5 if shorted. So you fuse each string for those specs. On the output side of whatever method you parallel with you should fuse to protect the 10ga (30A rated) wire is the boxed answer but I’d likely use a 25A breaker for those.
The 23A loaded circuit really ain’t/cain’t ever trip, though, can it?

worried that having 8 panels with a combined VOC of 23 amps could feed back if there were a fault on a panel in either of the strings

it sorta can’t but the string breakers can’t be less than the string output or it would be so safe it would never work.

 

[phillipgbrantner]

I would fuse them probably with a combiner box.
Combiner boxes seem expensive for what they are yet they take care of a few different concerns so imho worth the money for what they are.

At the very least a proper DC breaker is a useful disconnect for the panels.

Each string is going to be 10.86A or 11.5 if shorted. So you fuse each string for those specs. On the output side of whatever method you parallel with you should fuse to protect the 10ga (30A rated) wire is the boxed answer but I’d likely use a 25A breaker for those.
The 23A loaded circuit really ain’t/cain’t ever trip, though, can it?

it sorta can’t but the string breakers can’t be less than the string output or it would be so safe it would never work.

Thanks for the response and information, especially about using a 25a breaker not a 30a, but I'm still a bit confused and I have listed a few options I was going over in my head, and some questions I still have regarding the fusing/breaker conguration options.

Option 1.
I was thinking about putting a 2 string combiner box with each string on a 15amp breaker then to a shutoff breaker, and then to the controller.

This option would give better redundancy so that if one string has a fault and trips its breaker then the other string would still be providing some power for charging to the controller until the fault is repaired.

Option 2.
The other option was join the 2 strings on the roof with branch connectors then down to a single 25a or 30a breaker then to a shut off breaker, and then to the controller.

But this option has no redundancy and if there were a fault all charging power would be lost, and on top of that if the 25-30 breaker were ever to trip, (not likely) but all 8 panels would still be in a parrallel configuration with no protection whatsoever.

This being the case; the first option certainly may sound best, but that still leaves me with a few questions.
 
Question 1.
Since I'm slightly over VOC on both strings do I need to put mc4 15a fuses on each string say between each of the four sets of panels?

Example
1st string
P1+P2+15amc4fuse+P3+P4+15abreaker
10a+10a+ 10a+10a=40a
When in series remains 10a

2nd string
P5+P6+15amc4fuse+P7+P8+15abreaker
10a+10a+ 10a+10a=40a
When in series remains 10a

Because the combined amps of each string is 40amps when not in series, which is over the 10awg 30a rating and also being slightly 5-6a over VOC, I'm worried that if a panel were to get damaged on a string and trips the 15a breaker then it could then send all 30-40amps and the extra voltage to one of the panels causing over heat and then fire.

This is the part that confuses me most when it comes to putting panels in series and having a fault, because a panel fault I believe breaks the series connection and can draw double the amps to the faulty solar panel from what I understand.

Option 2.
If I were to put mc4 15a fuses on each string between each of the four sets of panels then could I feed that directly to a shut off breaker?

Because if either of the 15a fuses on either string were blow, then that string would just be disabled.

Example
1st string
P1+P2+15amc4fuse+P3+P4+shutoff
10a+10a+ 10a+10a=40a
When in series remains 10a

2nd string
P5+P6+15amc4fuse+P7+P8+shutoff
10a+10a+ 10a+10a=40a
When in series remains 10a

The only problem I can see with this configuration is that if an mc4 15a fuse blows on one of the strings, I would never know because the shutoff breaker would never trip even if it were a 25-30a breaker.

This then leads me back to option 1, putting one 15a breaker on each string but this still leaves the question 1, if I need to put the mc4 15a fuses between the panels?

I'm thinking yes, as you did mention that is normally the boxed answer to protect the 10awg wire that is rated at 30a with a fuse at the end of each string, but I think putting the 15a fuse between the panels might work best to protect them from each other in case of a short because there is 4 panels each at 10a, which is over the 30a wire rating and the 15a max fuse solar panel rating.

Question 3.
Do I need any of this extra protection with it only being 5-6a in current over Voc?

Because it's only 2 strings of panels and they have some sort of redundency technology built in that protects them like one way diodes.

Forgive the confusion I may be over overcomplicating it but I'm still looking for a clearer answer in the matter.

I could aviod all of this confusion running 1 string but I want the extra amps and the redundancy of two strings.

Any further advise helps thanks.

Extra info
I already have a 40pka surge protection device in its own separate sealed box as well, so I'm covered there in case of any lightning shock ect.

 

[12VoltInstalls]

Very organized dissertation.
But in first read I’m seeing a/b/c questions with subsets and alternates and I don’t have time right now to populate the flowchart.