Fusing solar panel cables

[nicknorman]

i am not picking a fight but graham did say use fuses in the early posts and looking at the picture he is right

Graham did say use fuses but didn't say when, where or why, that left us confused. It was chewbakka who explained the issue clearly and that fusing is only appropriate with several strings in parallel where the max current from all the panels exceeds the wiring capacity. Just saying "use fuses" is likely to lead to someone putting a fuse between the combined panel output and the controller, and that would be totally pointless.

[matty40s]

He also recommended the other things which are even less effective.

[nicknorman]

Just thinking about it the max fault current is n-1 x Isc (n being the number of parallel strings). So with 2 strings no point. With 3 strings you would have to start contemplating the max fault current vs each panel's wiring capability. Typically people tend to choose wiring that is "over-rated" in thermal terms, in order to keep the voltage drop down, so in reality it might only be appropriate to fuse with 4 or more parallel strings.

[peterboat]

it worries me because on my first boat i had 5 large panels in parallel and no fuses. its only because i ordered my latest controller from midnite that i had the a pretty picture that showed the fuse box which i then used, [right or wrong its staying]. in the future i am going to change the layout so it will need those fuses to protect the panels.

on another point maybe the regs for panels on boats do need to be written to protect us from an insurance industry that will use any reason not to pay

[nicknorman]

it worries me because on my first boat i had 5 large panels in parallel and no fuses:sick:. its only because i ordered my latest controller from midnite that i had the a pretty picture that showed the fuse box which i then used, [right or wrong its staying]. in the future i am going to change the layout so it will need those fuses to protect the panels.

on another point maybe the regs for panels on boats do need to be written to protect us from an insurance industry that will use any reason not to pay

Of course for there to be a problem you have to have a dead short in the wiring from combiner box to panel, AND lots of lovely sunshine, AND have 3 or more strings. Of course it could happen but one doesn't seem to hear of it. With the advent of MPPT folk tend to wire their panels in series and so there probably aren't a lot with say 4 strings in parallel these days (except you!). Anyway I've learnt something so if we ever get solar I'll know what to do, thanks to chewbacka. Edited December 17, 2015 by nicknorman

[peterboat]

Of course for there to be a problem you have to have a dead short in the wiring from combiner box to panel, AND lots of lovely sunshine, AND have 3 or more strings. Of course it could happen but one doesn't seem to hear of it. With the advent of MPPT folk tend to wire their panels in series and so there probably aren't a lot with say 4 strings in parallel these days (except you!). Anyway I've learnt something so if we ever get solar I'll know what to do, thanks to chewbacka.

i dont have them in parallel at the moment, but did on my old boat which was 10 years ago. up the road is rotherham solar so panels were cheap but large so i did them in parallel because mppt wasnt around then.

now i have 200 volts of solar going into my controller at 65 amps which is dangerous to say the least so i plan to redo the wiring and panels, which will then mean i will have better power in shade also the voltage and amps per string will reduce the very high voltage i currently have coming down the lines

[Chewbacka]

i dont have them in parallel at the moment, but did on my old boat which was 10 years ago. up the road is rotherham solar so panels were cheap but large so i did them in parallel because mppt wasnt around then.

now i have 200 volts of solar going into my controller at 65 amps which is dangerous to say the least so i plan to redo the wiring and panels, which will then mean i will have better power in shade also the voltage and amps per string will reduce the very high voltage i currently have coming down the lines

200v at 65A going into your controller? That is 13Kw. or did you mean 200v going in and 12v 65A coming out?

12v at 65A is 780W.

 

If this is a domestic install the voltage going into a grid tie inverter is often much higher than 200V

Doing a quick google, some grid tie inveters don't even start below an actual input voltage of 150v so you may struggle to reduce the voltage going in.

[peterboat]

my panels are rated at 873 watts but are flexies from america, i get 12 v and 65 amps coming out, but 200 volt going in as they are in series the midnite does its magic and voila i have charged batteries in summer or sunny days. the reason i am going to parallel them is shading whilst i will lose one way i think i will gain more through panels not losing as much due to shading. my panels are rated at 873 watts but are flexies from america hope that makes sense

[FadeToScarlet]

my panels are rated at 873 watts but are flexies from america, i get 12 v and 65 amps coming out, but 200 volt going in as they are in series the midnite does its magic and voila i have charged batteries in summer or sunny days. the reason i am going to parallel them is shading whilst i will lose one way i think i will gain more through panels not losing as much due to shading. my panels are rated at 873 watts but are flexies from america hope that makes sense

My flexible panels have built in bypass diodes for each section, I'd've thought yours would too

[WotEver]

Pete, you may find that with the lower voltage when they're paralleled the MPPT controller can't get quite as much out of them. It'll be an interesting exercise.

[Loafer]

A few pages back I mentioned the results of my trial with the series / parallel thing, changeable by a switch. With an Outback FM60 MPPT controller, it was better when parallel than in series, although with only 2X130W panels.

 

It's worth trying both.

Edited December 17, 2015 by Loafer

[nicknorman]

My guess, and it is only a guess, is that it would be marginally better in strong light to have them in parallel, since the controller will suffer from losses which to some extent will be proportional to the voltage difference between input and output. However in low light conditions series would be marginally better due to actually having enough voltage to do something with.

[Loafer]

My guess, and it is only a guess, is that it would be marginally better in strong light to have them in parallel, since the controller will suffer from losses which to some extent will be proportional to the voltage difference between input and output. However in low light conditions series would be marginally better due to actually having enough voltage to do something with.

 

Well that certainly makes sense. I can't say I tested every light situation, but the two or three occasions I tried, it was always better in parallel so I just left it like that for the next 4 years.

 

I'm now trying to get my head round attempting another trial now, using the same bits of cable that I already have!

[WotEver]

 

I'm now trying to get my head round attempting another trial now, using the same bits of cable that I already have!

Make sure you include plenty of fuses to keep the resident troll fed ?

[peterboat]

they do have bypass diodes in them but was on a solar site yesterday and the losses when in shade are still considerable so as an experiment in futility i am going to have a play also it does remove 200 volts at whatever amps which could be serious if i ever grabbed the cables in the dark electricity cupboard lol

[Graham.m]

My guess, and it is only a guess, is that it would be marginally better in strong light to have them in parallel, since the controller will suffer from losses which to some extent will be proportional to the voltage difference between input and output. However in low light conditions series would be marginally better due to actually having enough voltage to do something with.

 

from memory the panel cables are only 4mm^2, so could another loss be, when in parallel, lower voltage, higher current, higher cable losses. Whereas in series, higher voltage, lower current, lower cable losses

Edited December 17, 2015 by Graham.m

[nicknorman]

from memory the panel cables are only 4mm^2, so could another loss be, when in parallel, lower voltage, higher current, higher cable losses. Whereas in series, higher voltage, lower current, lower cable losses

It depends on where the paralleling is done. If done at the panels then I'd agree with you. However if both sets of cables are taken back to the controller and paralleled there then you effectively have twice the area of copper and so there would be no increase in cable losses.

[Graham.m]

It depends on where the paralleling is done. If done at the panels then I'd agree with you. However if both sets of cables are taken back to the controller and paralleled there then you effectively have twice the area of copper and so there would be no increase in cable losses.

 

logic is saying panels now in series, so the long run is in place. I seem to remember there are 5 panels could be natural is to parallel at the panels, easier etc.

[peterboat]

my cables are 6mm from memory, for neatness it is easier to parallel at panels but not decided yet so both right. my old boat was 5 panels this one has nine the closest 3 are easy to parallel up and feed in the rest will just be twos in parallel, or might try 3s all the way through for ease and put bigger cables to the last 3 as i say depends how i feel and what is neatest

[Graham.m]

my cables are 6mm from memory, for neatness it is easier to parallel at panels but not decided yet so both right. my old boat was 5 panels this one has nine the closest 3 are easy to parallel up and feed in the rest will just be twos in parallel, or might try 3s all the way through for ease and put bigger cables to the last 3 as i say depends how i feel and what is neatest

 

 

Maybe the answer is to split into threes, 3 in series and the parallel the 3 series banks. That would drop the voltage down to about 67V and the current to a manageable figure (4/5A) the cables could handle. Guessing on 12M of cable 4mm would take you into the area of 1% or less loss assuming 67V and 5A. The run for the final parallel point would be 67V & 15A and that would need assuming 12M 16mm^2, if the final run was only 4M , 4mm² should do. A run is there and back and the total length of cable

 

As always my opinion

Edited December 17, 2015 by Graham.m

[peterboat]

 

 

Maybe the answer is to split into threes, 3 in series and the parallel the 3 series banks. That would drop the voltage down to about 67V and the current to a manageable figure (4/5A) the cables could handle. Guessing on 12M of cable 4mm would take you into the area of 1% or less loss assuming 67V and 5A. The run for the final parallel point would be 67V & 15A and that would need assuming 12M 16mm^2, if the final run was only 4M , 4mm² should do. A run is there and back and the total length of cable

 

As always my opinion

and that is exactly what i am doing it is easier because of the mc4 connectors to do this also it is safer. i have had the notebook out this afternoon drawn it up and will do it next week weather permitting. i was just going to post when i saw your post and quoted it hope you dont mind

[Graham.m]

and that is exactly what i am doing it is easier because of the mc4 connectors to do this also it is safer. i have had the notebook out this afternoon drawn it up and will do it next week weather permitting. i was just going to post when i saw your post and quoted it hope you dont mind

 

No problem glad to be of assistance

[Graham.m]

Copy of a email received from BSS today, the recommendation is to follow BS 7671 and I suspect as this is best practice it is what insurance companies would expect.

 

From: *******@boatsafetyscheme.org
To: **********.com
Subject: RE: Solar panels fusing and Isolation
Date: *******************************

Dear *******

Thank you for your emails dated 12 and 26 December.

The BSS requirements at 3.6.2 do refer to solar panels and to comply, panels or solar panel controllers connected directly to the battery would need to be in-line fused.

The BSS currently has no requirements for solar panels to be provided with isolators.

The subject of solar panel fusing and isolation is being considered by the BSS Technical Committee and further published safety advice or even new BSS technical requirements may follow. It seems that the published relevant British, European and International ‘small craft’ electrical installation standards are generally silent on this issue.

In the meantime I recommend you consider applying domestic installation best practice (BS 7671) and take advice from your panel manufacturer, about installing an appropriate, DC-rated (double pole) isolator and in-line fusing; and ensure adequate cable rating, should multiple panels be wired in parallel.

I trust this helps answer your query and sorry for the delay in answering it.

Best regards,

G*******

BSS Manager