Poly mono and cable size ( yes solar again!)

[pcatterall]

Considering my solar requirements again! Spain 10m ketch.
Currently we have about 100 watts on stern fittings plus 2 small ( 20watts each ) on the dog house roof.
I have started leaving the boat long term with electrics off and just the small units topping up the batts.
Cruising we have auto pilot and small waeco chest fridge plus lights and instruments. ( and all the dam mobile phones that the crew find indispensable
With care we can be self sufficient under sail ( but sometimes hand steer to reduce the load)
We are considering getting 2x 100 watt semiflexible panels which we can use on or off a backing board and mount on rails or other suitable spots when we are moored (or even cruising 'serenely').
We may fit a power socket somewhere centrally on deck so that the panels can be connected by a trailing lead ( yes trip hazard!) ( there is a live outlet near the bow which could do but I need to check the cable size)
The panels could be stored flat on a berth under the mattress.

so...… I will appreciate general comments and suggestions but specifically would like advice on cable size for example a 10m run with max 200 watts load. I can use the sizing chart but what would be a sensible compromise in % efficiency?
Also what is the practical difference between mono and poly units.
The usual thanks!

 

[gregcope]

Mono or poly not relevant for wiring, but not sure your asking that.

You want the thickest cable to avoid a voltage drop vs the cost/fitting effort.

Outside I have some twin core 4mm2 (expensive) from panel to deck.

Inside I settled on using 6mm2. Brought a real of red and black. Partly cost, partly this is the biggest my victron 75/15 can take (check yours).

I do save cost/effort by placing the regulator near the electrical bank and wiring it back to the batteries via thick cables present there. For example instead of wiring 6mm2 back to the batteries, I have wired negative to the main negative bus bar behind the electrical panel (that has 25m2) and positive to the back of the 1-2-1 switch (has 25mm2 too) via a fuse. Saves lots of work and benefits from bigger cables.

 

[peteK]

I have just done similar and wired the socket for the solar panel across one of the existing feeds for the regulator,so it cant overchage the battery.

 

[pcatterall]

Thanks guys. The charts suggest that a mix of 4mm and 6 should do the job ok.
The poly/mono question was separate from the loading issue..

 

[William_H]

Actually the size of the cable and voltage drop is nopt nearly as much concern as say for mast head incandescent lamp. A 100 watt solar panel will exhibit an apparent internal resistance of around 4 ohms. That looks like a resistor in series that you can not get rid of. It explains why when testing you can simply put an amp meter across the output and measure max current with no apparent voltage and no concerns. If you have a tiny supply cable from the panel such that 1 volt is dropped at 5 amps that power will be lost 5 watts. However in the simple case of solar panel connected to battery (as in a pulse width regulator) you get approximately 6 volts dropped in the panel resistance because your battery at 14 volts draws current to drag down the voltage at the panel terminals. To drop 1v at 5 amps is a large loss from a tiny long wire and loss will in practice be much less.
I am advocating OP use what wiring is already fitted to the sockets. However if fitting more wire, larger wire will be more robust and resistant to corrosion etc and that should be the criteria. In practice forget about voltage drop from a solar panel. ol'will

 

[Pete7]

12v Planet have a voltage drop calculator on their website, Victron also have a spread sheet you download and play around with.

Cable sizing and selection | 12 volt planet

 

[pcatterall]

Thanks all. I have all the charts but from time to time have read some specific tips re solar saying that you can accept a bit more voltage drop and similar things.William H ( above ) hints at this ( I think) but I'm afraid I don't understand it all!!

 

[gregcope]

My advice is for optimal. It should work fine with thinner cables as @William_H points out.

It depends if you want to eek out all the efficiency you can. I run a fridge, at anchor on a small install (100W) and I am trying to minimise the amount of engine charging required to top up.

One minor thing to check. Your regulator may need a min voltage to start (17V) and this could be a factor with long runs and thin cable if you have one panel or are going parallel. Serial is less of an issue. YMMV. Probably will not be a problem.

 

[noelex]

One minor thing to check. Your regulator may need a min voltage to start (17V) and this could be a factor with long runs and thin cable if you have one panel or are going parallel. Serial is less of an issue. YMMV. Probably will not be a problem.

The start up voltage of popular Victron MPPT controllers is set at +5v over battery voltage. This creates the potential concern that start up can occur late with “12v” panels connected in parallel. In practice this does not seem to be a problem, but even if it is, the voltage drop in battery cables is not a factor. Voltage drop is dependent on current. At start up the current is effectively zero.

The controller is essentially measuring Voc.

So larger or smaller cables will not influence start up voltage.

 

[gregcope]

I stand corrected! Thank you.

 

[noelex]

For wire size.

Look up the panel’s or the arrays Imp. This is the maximum current the solar panel will produce when connected to the controller.

This maximum current will be seen only rarely so rather than calculating the wire size for a 3% loss, somewhere around 5% is generally used. There is nothing wrong with going to a larger wire than this and you will see a small real world gain if do so. The only thing to watch is some solar controllers will only accept a small wire gauge, but there are ways around this problem. You can also go smaller if installation of thicker wire is difficult. This will reduce the output, but you will not do any damage or cause any problems unless you go below the size where the wire cannot handle the current, which is very unlikely, but do check this as the consequences are serious (overheating of the wire and fire).

From the controller to the battery the current will change if you are using an MPPT controller. This new current needs to be calculated to determine the correct wire size from the controller to the battery. The distance will obviously be different as well. Here the voltage drop needs to be kept as low as possible because this will effect the battery set points. Use 3% as the maximum voltage drop calculating on the maximum current. There is nothing wrong with going larger and, as above, always check that the maximum current rating of the wire is below the maximum possible current. If this is a problem there are devices available for some controllers that will communicate the correct battery voltage.

Note that these percentage voltage drops are cumulative. So if you have a 5% loss between the solar panels and the battery and a 3% loss between the controller and the battery, once you allow for some further voltage drop over fuses connections etc it is likely that about 10% of power has been lost in the wiring alone. This full loss only applies at maximum output, but it is significant.