Two strings or not too strings, ...

[Mrjmegson]

Hey guys, I think I know the answer to this, but I'm not entirely sure.

I'm going to have 4 rows of 4 panels on my roof, and my inverter only has two inputs, so I've always thought I'd simply make two strings of 8 panels, but I'm now wondering if I can do 4 strings of 4.

Will I benefit from bringing 4 strings of 4 off of the roof, and combining them in a junction box to two strings, so simply putting the two pos together and two negs together?

Will this mean if I have an issue with 1 of the strings, the other 3 will run fine, as they are effectively being combined in parallel?

Also, no matter how many strings I have, I will be bringing solar cable into the loft, and into a junction box, and then running armoured from the loft, through the house to the electrics room, so will need a junction box to change the cable in. Can anyone recommend a junction box to use?

Cheers guys.

 

[johnb2713]

More panels in series = less losses in the cabling and earlier charging as the sun rises.

In terms of failure mode, it would depend on the nature of the failure, if it went open circuit it would function as you say, and other failure mode you may end up back feeding it from the other parallel string.

 

[binky]

If you parallel the strings you double the ampage, so check the MPPT can take the higher ampage. Personally I wouldn't bother with more than 2 strings.

 

[Mrjmegson]

Hey guys, thanks for the replies, I'll leave it at two strings as planned.

Thanks again,

Jay

 

[stuboy]

The only sane reason for having 4 strings and combiners would be if shading was a problem. If the panels have no shading then as Binky says you will be loosing more through IxR losses than you would ever gain, you might also loose in them not switching on until later in the morning due to inverter threshold voltage not being met, & 2 strings of 8 is a happy number. I always use 6mm on solar jobs regardless of the install simply as in the long run it will more than pay for itself as less resistance equals more current and more current equals more (free) watts
I would also try and run the solar cable direct to the DC switch you will be installing next to the inverter, if you should at some later date change your config on the roof you might exceed the voltage rating of house wiring and you have another set of joints which are always nice to avoid if possible.

Cheers
Stu

 

[Mrjmegson]

Hey Stu,

Thanks for the info, unfortunately I've already bought all the cables in 4mm, including the armoured that will run through the house. I'm peed off that I didn't know about the resistance of the 4mm cable.

Can anyone answer the second half of my question, what junction box I should use to switch my cable from the solar to the SWA?

Cheers.

 

[binky]

Hey Stu,

Thanks for the info, unfortunately I've already bought all the cables in 4mm, including the armoured that will run through the house. I'm peed off that I didn't know about the resistance of the 4mm cable.

Can anyone answer the second half of my question, what junction box I should use to switch my cable from the solar to the SWA?

Cheers.

Wouldn't worry too much about the cable, the panels are all 4mm leads and unless cable run is extremely long it will make little difference.

Wiska boxes are good.

 

[Mrjmegson]

Wiska, I'll have a look for those.

Thanks man, I really appreciate it.

 

[stuboy]

Its amazing that Wiska are now a generic term, like hoover was years ago Binky is right, they make some excellent products only ever to be challenged by Wago I suspect?!
I did some calcs once on the payback verses length vs relative resistance of 4 & 6mm I guess it would be worth doing those again with current electricity prices. I will get our top man onto it and report back ;-)

 

[Mrjmegson]

Its amazing that Wiska are now a generic term, like hoover was years ago Binky is right, they make some excellent products only ever to be challenged by Wago I suspect?!
I did some calcs once on the payback verses length vs relative resistance of 4 & 6mm I guess it would be worth doing those again with current electricity prices. I will get our top man onto it and report back ;-)

Lol, cheers.

 

[binky]

Its amazing that Wiska are now a generic term, like hoover was years ago Binky is right, they make some excellent products only ever to be challenged by Wago I suspect?!
I did some calcs once on the payback verses length vs relative resistance of 4 & 6mm I guess it would be worth doing those again with current electricity prices. I will get our top man onto it and report back ;-)

Wiska have threaded holes for armoured glands, just makes life easier.

If you're getting keen on cable calcs, I would be interested in what you calculate for the AC feed, I like to chuck in 4 or 6mm depending on the wattage.

 

[Mrjmegson]

I bought two 40A boxes this aft. They are very tidy, I like the threaded gland holes and the inbuilt rubber membrane for sealing.

Great recommendation, cheers.

Shame that the panel cable will be 4mm, as they will constitute 16m of the run, so about 50%. I almost want to chip them and replace them with 6mm.

Every watt counts.

 

[stuboy]

OK the calcs are in.

In this example I have used 2 lengths of 5m long wires to hook up the array. I chose 8 panels in series producing 1000W (3.3A) and used 4250KWh as the total yearly yield (average for a 3.68Kw system in the uk).

The results are in money saved in 20 years ( @50pence a unit) ... from 1mm squared cable up:-

1.5mm squared cable saves £27.77 on 1mm
2.5mm squared cable saves £21.71 on 1.5mm
4.0mm squared cable saves £12.96 on 2.5mm
6.0mm squared cable saves £7.08 on 4mm
10mm squared cable saves £5.79 on 6mm
16mm squared cable saves £3.15 on 10mm
25mm squared cable saves £1.96 on 16mm

Now taking into account the extra cost of our 10m long cable the 6mm cable (currently) £3.20 more than 4mm. 10mm (if you can get it) £6.54 more than 6mm.

So by my calculations on this install the extra cable thickness has paid for itself after... drum roll......

Using 6mm instead of 4mm cable pays for itself after 9 years and a month
Using 10mm instead of 6mm cable pays for itself after 22 years 6 months (good luck getting 10mm in a standard MC4 though!)

Cheers
Stuart

 

[Mrjmegson]

Ha ha ha, that's a cracking but of work, well done.

So the end result is, don't bother upgrading the 4mm to 6mm.

 

[stuboy]

Ha ha ha, that's a cracking but of work, well done.

So the end result is, don't bother upgrading the 4mm to 6mm.

If its already installed no don't bother, if its a new install its a no brainer ;-)

 

[binky]

Excellent work, but at 4250 kWh at 50p for 20 years that's £42,500. So £7.08 is 0.017%. I would not be losing any sleep over the cable size

 

[Mrjmegson]

Excellent work, but at 4250 kWh at 50p for 20 years that's £42,500. So £7.08 is 0.017%. I would not be losing any sleep over the cable size

I'm northern, 0.017% is a massive saving. Lol