solar cable different thickness & length

[Ssscrudddy]

When wiring 2 solar panels in parallel, do the cables from PV to CC need to be the same thickness & length or can they be different ?

Basically I have 4 panels & they need to be mounted lengthways along my roof (too long to fit sideways, each panel would stick out either side of the roof) & I want to avoid wiring them in series in case of partial shading. But 1 of the cable runs would be quite long around 8m, & then the next panel will be closer to the stern, & so on & so forth. However this will work out quite pricey for the cables.

By my reckoning (allowing 2% V drop) the 8m cable (x2 pos & neg) will need to be at least 6mm2 = approx £30 for 1 panel . So I'm hoping the next panel, 2-3m nearer to the CC & wired in parallel with the 1st, can use a thinner shorter (cheaper) wire?

I know wiring in series will let me use less wire & thinner wire, but if 1 panel gets partial shading this will effectively wipe out both my panels, meaning I lose half my solar power (4 panels to 2 mppt, 2 in parallel per mppt)

panel specs are

PMax: 165Wp
Imax: 4.72A
Vmax: 35V
ISC: 5.18A
VOC: 43.8V

[bottle]

Wire it all in the same size and 'loop' from one to the next.

 

So total run is the same as from the furthest to the controller (plus a bit for connections)

Edited November 9, 2014 by bottle

[MtB]

I know wiring in series will let me use less wire & thinner wire, but if 1 panel gets partial shading this will effectively wipe out both my panels, meaning I lose half my solar power (4 panels to 2 mppt, 2 in parallel per mppt)

 

 

Is that really true of all panels?

 

I have acquired the impression that some panels nowadays have individual cells that cleverly short-circuit themselves when in shadow to prevent this effect. Or something like that. Is this right, anyone?

 

And if so, how can one identify this type of panel?

 

 

MtB

[Ssscrudddy]

Wire it all in the same size and 'loop' from one to the next.

 

So total run is the same as from the furthest to the controller (plus a bit for connections)

Sorry but I just dont understand what you mean by this. Perhaps you could link a wiring diagram that would explain this please.

 

Edit. Is this what you mean (but instead of a battery its a solar panel) ?

 

 

 

Is that really true of all panels?

 

I have acquired the impression that some panels nowadays have individual cells that cleverly short-circuit themselves when in shadow to prevent this effect. Or something like that. Is this right, anyone?

 

And if so, how can one identify this type of panel?

 

 

MtB

I've no idea. Everything I've read so far suggests that partial shading is bad, effectively shutting down the panel that is partially shaded, & wiring them in series effectively makes it 1 big panel with the same effect.

Edited November 9, 2014 by Ssscrudddy

[Quinafloat]

Solar panels, and their use, has so many variables that trying to work it all out is a waste of time and effort. Voltage drops only become serious at the upper end of the scale when you are getting so much energy that loosing a little bit is pretty unimportant. Your batteries will just end up fully charged a few minutes earlier if you spend twice as much on wiring.

 

My installation has a total return length of close on 30 metres and uses 6mm wiring. I reckon I loose more current from the dust that gets on the panels daily than due to the wiring. Clouds also remain a major problem that I have made no headway in solving. As regards shadows causing a problem, park in the open is the easiest solution, it's not really a technical problem.

[Detling]

Sorry but I just dont understand what you mean by this. Perhaps you could link a wiring diagram that would explain this please.

 

Edit. Is this what you mean (but instead of a battery its a solar panel) ?

 

 

Yes not sure what you mean by 2 volts , 600 amp/hours though

 

If you really want you can make the wires to the left hand panel thinner than the ones from the right as they only carry the current for the left hand panel, the ones from the right hand panel to the batteries will be carrying three times as much current and to the controller and then the batteries so need to be bigger.

 

your panel data

PMax: 165Wp

Imax: 4.72A

Vmax: 35V

ISC: 5.18A

VOC: 43.8V

 

The voltage between the + and -ve of the panels will be 35 when connected to the controller and can be 43.8 when not connected. The current from each panel in full sun perpendicular to the panel will be 4.72 Amps so from the right hand panel to the controller you need to allow for 14.6 Amps max (call it 15 ) and fuse the line at say 20 Amps for safety. The controller should convert that to about 30 Amps at 14 Volts to send on to the batteries.

[smileypete]

As said, most (almost all?) panels have 'bypass diodes' to help deal with partial shading. Shading isn't so big a problem unless on a permanent mooring that gets some every day.

 

With 4 panels into 2 MPPTs, I'd just put a pair in series into each MPPT, using 4mm² solar cable. I take it the MPPTs have a max input voltage of 100V or more though, the panels may output near 50V in coldest weather.

 

Even 2.5mm² 'tri-rated' cable might be OK, but won't fit so well into the standard MC4 connector. There's probably sellers on Ebay that do custom lengths of 4mm² solar cable with MC4 connectors ready fitted on the ends, or I s'pose they could be soldered with a bit of care.

 

cheers, Pete.

~smpt~

Edited November 9, 2014 by smileypete

[Ssscrudddy]

Thanks for the replies. The MPPT's are each rated 150V 30A.

As for the 2 Volts 600 amps that was just a picture I found on the internet which was about something else I was just trying to understand what bottle meant when he said 'loop' from one to the next.

I was just trying to work out how much all the wire was going to cost me (& trying to understand it all). It seems I dont need to spend as much as my initial calculations said thankfully.

[Dinz]

Remember that if you undersize you will end up buying/spending twice + if you need to increase. Buy the best you can afford rather than the cheapest you can find! Usually [not always] ends being the most economical in long term.

Tri-rated Cable is so called because it meets the requirements of three different international standards not that it can take 3 X current I think. [Prepare to be proved wrong!]