What gauge wiring to Solar Panels?

FullCircle

FullCircle

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Just installing solar panels in the following configuration to an MPPT controller.

I would like to make the wiring optimal size to maximise efficiency.

50w flexi on Port Quarter, total wiring length to controller 8m (Deck plug through)

50w flexi on Starboard Quarter, total wiring length to controller 3m (Deck Plug through)

100w flexi on Bimini top centre, total wiring length to controller 7m
50w flexi on Bimini top centre, total wiring length to controller 7m
30W on Bimini top port, total wiring length to controller 8m
30W on Bimini top starboard, total wiring length to controller 6m

Ideally, I would like to parallel all the Bimini panels with a junction box before it disappears through the deck level plug.

Any ideas on where to go for advice on wiring gauge?
 
What is the largest cable your regulator can accept?

I would suggest biggest possible to avoid voltage drops, and also make the wiring easier.

I wired all panels to a home made junction box (space issues), nearest the panels, that used a stainless nut/bolt and ring terminals on the wires. I used glands purchased off ebay.

Wires from panels to junction box, then 6mm2 from box to regulator. At your longer (6m+) cable run lengths it might be worth doubling up the box->regulator wires which is what I have seen done on a biggish wind gen.

Your regulator might have more information on suggested sizes.
 
It makes sense to parallel the bimini panels, which gives you a theoretical max output of about 16a. Chances of ever seeing that in the UK is about zero. Allowing for an 8m cable run 6mm sq would be more than enough.

Useful info here : http://www.12voltplanet.co.uk/cable-sizing-selection.html
More here : http://www.autoelectricsupplies.co.uk/product/884/category/159

Not sure what kit you have already, but i'd suggest it would be useful to parallel all the panels and connect them to a dual output controller. Your engine battery will be charged most of the time so you might as well have all the power you can get into the domestic bank. If you already have a split charge setup that won't matter so much.
 
FullCircle said:
Just installing solar panels in the following configuration to an MPPT controller.

I would like to make the wiring optimal size to maximise efficiency.


Any ideas on where to go for advice on wiring gauge?
Click to expand...

This is a handy calculator for determining the wire gauge

http://www.freesunpower.com/wire_calc.php

You can calculate the wire gauge for each panel and for the controller to battery connection but then it make some sense to follow gregcope's advice and economise in the number of different sizes used.

dont forget that some of your acceptable total volts drop will occur between the panels and the controller and some between the controller and the battery.
 
I take a different view applying the following principles and facts:

For a given amount of power, the greater the voltage, the lower the amps
Voltage drop is in inverse proportion to the voltage or in proportion to the amps.
The cross section of cable required is in proportion to the current.
Therefore for a given amount of power, transferring it as higher voltage (and therefore lower amps) means less voltage drop and a smaller size of cable than otherwise is required.
MPPT not only can take a higher voltage than, say 18v (approx open voltage of a solar panel) but much higher - the smallest VICTRON unit is 75/* where 75 is the maximum voltage.
As panels can suffer shade at times or lower light conditions, their voltage can drop and, it may drop below the point at which an MPPT works well or at all. Panels in series (nominally about 28v) will always deliver sufficient voltage for a MPPT controller to provide power to a 12v battery; they convert the voltage down to a v suitable for charging 12v. Therefore there is a further advantage in wiring in series and thereby raising the voltage; their continued utility in marginal conditions.

It all points to maximising the V going into the MPPT rather than the A. On the basis suggested in posts above, you have loads of 12v inputs to be joined as one before the MPPT - some in parallel - in order to reduce the number of wires coming back to the charge controller.

YOu need to work in either nominally 12v, 24v or 36v. For the reasons above, I suggest abandon 12v feed. If you thought your plan through then I think you will find you could have three x two-panels-in- series.
If these three (nominally 24v) feeds are joined up at the MPPT then you have a higher voltage, lower amp set up in order to save loss of power. The MPPT works better in that it will 'always' work and you save on cable size and can use the MC4 stuff which is easy and can be bought readily off the bay. Also you reduce by 100% the number of wires through glands and back to the controller.

You will know best from the layout of your panels how it can be achieved but, if you made three x 24v supplies back to a busbar and into the controller then you would have a very good system indeed.
I think you know but, just in case others don't, the output from a Victron MPPT is determined by the battery voltage when first connected. The controller reduces down the incoming voltage to the correct charging voltage for the battery system. Therefore nominal 24v (or 36 or 48v etc) input is no problem when you want a nominal 12v output. It reduces the voltage and increases the current accordingly.
 
So is the view that it is better to go for MPPT on the house battery, and ignore the engine battery? (I can't find any MPPT controllers that will do both batteries separately)
 
Poecheng said:
I take a different view applying the following principles and facts:

For a given amount of power, the greater the voltage, the lower the amps
Voltage drop is in inverse proportion to the voltage or in proportion to the amps.
The cross section of cable required is in proportion to the current.
Therefore for a given amount of power, transferring it as higher voltage (and therefore lower amps) means less voltage drop and a smaller size of cable than otherwise is required.
MPPT not only can take a higher voltage than, say 18v (approx open voltage of a solar panel) but much higher - the smallest VICTRON unit is 75/* where 75 is the maximum voltage.
As panels can suffer shade at times or lower light conditions, their voltage can drop and, it may drop below the point at which an MPPT works well or at all. Panels in series (nominally about 28v) will always deliver sufficient voltage for a MPPT controller to provide power to a 12v battery; they convert the voltage down to a v suitable for charging 12v. Therefore there is a further advantage in wiring in series and thereby raising the voltage; their continued utility in marginal conditions.

It all points to maximising the V going into the MPPT rather than the A. On the basis suggested in posts above, you have loads of 12v inputs to be joined as one before the MPPT - some in parallel - in order to reduce the number of wires coming back to the charge controller.

YOu need to work in either nominally 12v, 24v or 36v. For the reasons above, I suggest abandon 12v feed. If you thought your plan through then I think you will find you could have three x two-panels-in- series.
If these three (nominally 24v) feeds are joined up at the MPPT then you have a higher voltage, lower amp set up in order to save loss of power. The MPPT works better in that it will 'always' work and you save on cable size and can use the MC4 stuff which is easy and can be bought readily off the bay. Also you reduce by 100% the number of wires through glands and back to the controller.

You will know best from the layout of your panels how it can be achieved but, if you made three x 24v supplies back to a busbar and into the controller then you would have a very good system indeed.
I think you know but, just in case others don't, the output from a Victron MPPT is determined by the battery voltage when first connected. The controller reduces down the incoming voltage to the correct charging voltage for the battery system. Therefore nominal 24v (or 36 or 48v etc) input is no problem when you want a nominal 12v output. It reduces the voltage and increases the current accordingly.
Click to expand...

You need to check that the charger's efficiency is as high with the higher input voltage though.
Also more 12V feeds may be more shade tolerant than a smaller number of feeds at a higher voltage. This may influence the number of hours per day you get as much charge as the batteries will take, which may be a better goal than simple efficiency.
 
Polly's Kettle said:
So is the view that it is better to go for MPPT on the house battery, and ignore the engine battery? (I can't find any MPPT controllers that will do both batteries separately)
Click to expand...

I just bought the Votronic - it does have dual charging outputs, one is a trickle
http://www.votronic.de/index.php/en...ng-controllers/boat-version-sr/sr-530-duo-dig
SR_530_1625.jpg


and also the remote output meter

http://www.votronic.de/index.php/en/products/solar-technology/lcd-solar-computer

LCD-Solar-Computer_S_1250.jpg
 
Yes just charge the house bank.

Engine batteries are usually way over spec for just starting if your use case is engine bat to start then switch to both to charge.

I sometimes switch to both when using solar if there are lots of amps coming in.

I have a Victron 75/15 which I am very happy with.
 
lw395 said:
You need to check that the charger's efficiency is as high with the higher input voltage though.
Click to expand...
True but there is no particular reason to think it would be less efficient at 24v nominal than 12v nominal.

lw395 said:
Also more 12V feeds may be more shade tolerant than a smaller number of feeds at a higher voltage. This may influence the number of hours per day you get as much charge as the batteries will take, which may be a better goal than simple efficiency.
Click to expand...

I am not sure how this can be correct but apologise in advance if I am wrong.

A 12v panel in sunlight gives an open circuit voltage of about 18v (for the sake of argument). If it is in shade or low light and its voltage is reduced to 12v or even 9v then it will be doing nothing at all - 12v is too low for an MPPT regulator to work in MPPT mode (I think they need about 16-17v) so it will achieve nothing or practically nothing. THe fact a number are in parallel still means you have 12v or 9v and the charger still cannot achieve anything.

However, link them in series and your two panels in shade (each giving 12v or 9v) are together giving 24v or 18v both of which are enough to kick the MPPT going and to put charge in.

So the advantage in good conditions is less voltage drop, less cable cross section for the same power transfer and less cable glands. In marginal conditions, they are still supplying a charge voltage in circumstances where paralleled panels would have long ceased.
 
PaulRainbow said:
It makes sense to parallel the bimini panels, which gives you a theoretical max output of about 16a. Chances of ever seeing that in the UK is about zero. Allowing for an 8m cable run 6mm sq would be more than enough.
Click to expand...

Maybe not 100% of rated current in the UK, but I was surprised to get 80% of rated current from a panel laid on the side deck in late March in Ireland. A panel optimally aligned on the south coast of England in mid summer should definitely beat that, so maybe safest to use rated current for any calculations.
 
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