Help in determining what happens when you have solar panels wired in series then paralell? (4S4P) to end volts and amps at bus bar?

[micko]

Please see attached spread sheet.
Solar panels are on the roof of house. 16 by 440w panels each. Can I run 4mm2 cable to the bus bars as they are only carrying 13.86amp, but at the bus bar, its goes up to 55.44 when all four series are added to the bus bar.
So because I am doing a 4S4P, each series is 157.52v and 13.86amp. So, I think I could use 4mm2 wire down to my bus bar next to victron 250/100 blue solar charger. However, when it gets to the bus bars, it will be total 157.52v still but 55.44amp, (this is what will go into my blue solar 250/100 charger), which is too much for the 4mm2 cable. However, the line coming to the bus bar is only carrying 13.86amp from each series isn't it? The combined 55.44 amp will not run back up the lines and change the amp from 13.86 to 55amp correct? Not sure if this makes sense. Please see attached spread sheet. Thanks all. I have spent hours researching this. But cant find the answer.
Thanks so very much.

 

[sunshine_eggo]

You need need a 20A MC4 fuse in the (+) of each string of panels.

 

[micko]

You need need a 20A MC4 fuse in the (+) of each string of panels.

Hi, thanks for that. I forgot to mention, each string has its own 20A pos and neg fuse as these come down through a combiner box first.

 

[400bird]

However, when it gets to the bus bars, it will be total 157.52v still but 55.44amp,

You say bus bar but the pic shows 4 way MC4 splitters. I doubt there are MC4 splitters that will survive at 55 amps.

(this is what will go into my blue solar 250/100 charger), which is too much for the 4mm2 cable. However, the line coming to the bus bar is only carrying 13.86amp from each series isn't it? The combined 55.44 amp will not run back up the lines and change the amp from 13.86 to 55amp correct? Not sure if this makes sense.

It does make sense and no, you should not see 55 amps on the smaller wires running back to each string. However, in a fault condition you can, hence the importance of having fuses on each string to protect the wire.

I didn't look up the spec of 4mm² wire, I assume it's good for 15 amps.

 

[micko]

You say bus bar but the pic shows 4 way MC4 splitters. I doubt there are MC4 splitters that will survive at 55 amps.

It does make sense and no, you should not see 55 amps on the smaller wires running back to each string. However, in a fault condition you can, hence the importance of having fuses on each string to protect the wire.

I didn't look up the spec of 4mm² wire, I assume it's good for 15 amps.

Hi, Thanks for your reply. Yes, Its a picture I found online. I wrote bus bars where the mc4 connectors are. I dont have mc4 connectors there, only pos and neg bus bars.
I do have the bus bars AFTER the 20a fuses. So, in your opionion, even though there will be 55A at the bus bars, it will not run back up through the wiring of the 13.86A lines coming in (unless a fault of course?

 

[sunshine_eggo]

Hi, thanks for that. I forgot to mention, each string has its own 20A pos and neg fuse as these come down through a combiner box first.

Please update your sketch to more accurately convey your configuration.

 

[micko]

Please update your sketch to more accurately convey your configuration.

This is my setup. But, where I have crossed out the mc4 connectors, I have written bus bars. there are no mc4 connectors, just bus bas where they have all them mc4 connectors.

 

[sunshine_eggo]

This is my setup. But, where I have crossed out the mc4 connectors, I have written bus bars. there are no mc4 connectors, just bus bas where they have all them mc4 connectors.

Then what's this?

Hi, thanks for that. I forgot to mention, each string has its own 20A pos and neg fuse as these come down through a combiner box first.

 

[micko]

Then what's this?

I didnt add that as that was irrelevant to my OP. I just want to know, if you have 13a coming in off each string, to a central bus bar, (4 strings in this case), the total amps is multiplied by 4 so around 55a at bus bar. Will this flow back up the 13a lines coming into the central bus bar from each string. That is what I am wanting to find out.

 

[sunshine_eggo]

I didnt add that as that was irrelevant to my OP. I just want to know, if you have 13a coming in off each string, to a central bus bar, (4 strings in this case), the total amps is multiplied by 4 so around 55a at bus bar. Will this flow back up the 13a lines coming into the central bus bar from each string. That is what I am wanting to find out.

The answer is yes. It's possible. That's why fusing or breakers is critical, so the combiner box with over-current protection for each string is critical to your question.

To get good info, you need to provide good info.

 

[micko]

The answer is yes. It's possible. That's why fusing or breakers is critical, so the combiner box with over-current protection for each string is critical to your question.

To get good info, you need to provide good info.

Ok thanks. So it is possible but only if there is some type of issue. In normal situations, the 55a will not flow back up the 13a lines. So, because I have the 20a fusees on each string, I should be covered right?

 

[sunshine_eggo]

Ok thanks. So it is possible but only if there is some type of issue.

Correct.

In normal situations, the 55a will not flow back up the 13a lines.

If you were to blackout shade one string, the other strings would discharge through it.

So, because I have the 20a fusees on each string, I should be covered right?

The 20A would protect the blacked out string.

 

[micko]

Correct.



If you were to blackout shade one string, the other strings would discharge through it.



The 20A would protect the blacked out string.

Ok thanks so much. So, I have those 4 series coming through 20a fuses, both pos and neg, then going to the bus bar. So, in total I have four lines, (8 lines counting pos and neg) coming down from the roof going through individual 20a breaker fuses, then to bus bar. It seems that even though I have 13a coming in on each string to central bus bar, (which is now 55a), it will not "backup" the low amp lines coming in unless something bad happens, and if it does, my 20 amp breakers will activate. Thanks so very much for your help. This has been very hard finding this information. Thanks one again for you assistance ;-)

 

[sunshine_eggo]

Ok thanks so much. So, I have those 4 series coming through 20a fuses, both pos and neg, then going to the bus bar. So, in total I have four lines, (8 lines counting pos and neg) coming down from the roof going through individual 20a breaker fuses, then to bus bar. It seems that even though I have 13a coming in on each string to central bus bar, (which is now 55a), it will not "backup" the low amp lines coming in unless something bad happens, and if it does, my 20 amp breakers will activate. Thanks so very much for your help. This has been very hard finding this information. Thanks one again for you assistance ;-)

You technically only need one fuse per string, but if you have dual pole breakers, that's that.

The (+) wire(s) to the MPPT(s) from the busbar needs a fuse/breaker.

Rule is simply:

If you have more than 2 strings in parallel, you need a fuse on each string.

 

[micko]

Hi, Ok thanks so much for that. So I am covered with the fuses on each string, yes, I do have the neg fused as well. Maybe an over kill anyway, better to be safe then sorry. As for the fused pos line from the bus bar to the victron mppt, I did not think of that, what size fuse or also what type of fuse do I need for that? thanks for pointing that out ;-)
I will be running two 4mm2 lines from the bus bar, down through the dc isolation switch, then to mppt charger. So I will have 8mm2 wire basically. I did not think to fuse that line.

You technically only need one fuse per string, but if you have dual pole breakers, that's that.

The (+) wire(s) to the MPPT(s) from the busbar needs a fuse/breaker.

Rule is simply:

If you have more than 2 strings in parallel, you need a fuse on each string.

 

[micko]

I will also add, that from the mppt victron 250/100 charger, I have my 35mm2 pos cable running through a pull down/out fuse of 160a, 500VAC. Would this be correct? this is in case I need to quickly cut power from batteries to charger if malfunction or something.

Hi, Ok thanks so much for that. So I am covered with the fuses on each string, yes, I do have the neg fused as well. Maybe an over kill anyway, better to be safe then sorry. As for the fused pos line from the bus bar to the victron mppt, I did not think of that, what size fuse or also what type of fuse do I need for that? thanks for pointing that out ;-)
I will be running two 4mm2 lines from the bus bar, down through the dc isolation switch, then to mppt charger. So I will have 8mm2 wire basically. I did not think to fuse that line.

 

[micko]

Off topic, I just read when trying to find what fuse I need for my 48v lithium battery bank, I found this on the victron website
"PV array grounding

The positive and negative of the PV array should not be grounded.

Ground the frame of the PV panels to reduce the impact of lightning.

Do not connect the solar charger to a grounded PV array. Only one ground connection is allowed, and this should be near the battery."
My house only has one ground for the whole house. If I under stand, I need another ground just for this victron charger?

 

[sunshine_eggo]

Hi, Ok thanks so much for that. So I am covered with the fuses on each string, yes, I do have the neg fused as well. Maybe an over kill anyway, better to be safe then sorry.

You're good.

As for the fused pos line from the bus bar to the victron mppt, I did not think of that, what size fuse or also what type of fuse do I need for that? thanks for pointing that out ;-)
I will be running two 4mm2 lines from the bus bar, down through the dc isolation switch, then to mppt charger. So I will have 8mm2 wire basically. I did not think to fuse that line.

Each 4mm^2 line will need its own fuse/breaker. Fuses should be 1.25X wire rating. IMHO, 2X 4mm^2 is pretty marginal at least with regards to U.S. wire sizes. I would personally use a single 10mm^2 wire and fuse accordingly.

EVERY single circuit in your system needs a suitable fuse/breaker based on wire size.

 

[micko]

You're good.



Each 4mm^2 line will need its own fuse/breaker. Fuses should be 1.25X wire rating. IMHO, 2X 4mm^2 is pretty marginal at least with regards to U.S. wire sizes. I would personally use a single 10mm^2 wire and fuse accordingly.

EVERY single circuit in your system needs a suitable fuse/breaker based on wire size.

Yes I agree with the 10mm2, however, here in my area, none of the electrical places carry 10mm2, its either 4 or 6mm2. thats why, I thought I would double the 4mm2, not ideal, but, I cant get the 10mm2 ;-(

 

[sunshine_eggo]

My house only has one ground for the whole house. If I under stand, I need another ground just for this victron charger?

No.

The PV array FRAMES should be grounded to house ground.

The PV (-) should never be grounded to anything.

In the U.S. it's optional to ground your 48V (-) to system ground.