Correct Wire Size and Type for 100w Solar Panel

PaulRainbow said:
An MPPT controller is not the best choice with a single panel. Not all controllers require a 5v differential. 2.5mm cable, up to about 5m to the controller is correct for a 100w panel.

Of course he won't. I advised 6mm from controller to the batteries, if he fits enough panels so that 6mm is under spec the controller will be overloaded. If he fits another panel in parallel that panel will be wired to the controller directly, it will not require the existing wiring to be altered in any way. If, on the other hand, he fits a second panel and wires it in series with the existing panel the 2.5mm cable will still be as adequate as it is now.

In short, 2.5mm cable from the panel to the controller, max length 5m. 6mm cable from controller to batteries. Job done.
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Do you know the controller make, for the OP didn't say? A slightly heavier gauge cable has got to be a heck of a lot cheaper than a new controller! Re your parallel point; there is most certainly no need to run a separate cable to the controller. Either way OP can now make his mind up so please excuse me if this little matter is getting needlessly heated.
 
dom said:
True re initial startup, but once the controller activates it will have another algorithm requiring panel voltage to exceed VBatt by 1 or 2 volts. Excessive losses in the cabling can risk the MPPT measuring an unexpected drop in voltage upon start-up to below its limit thereby triggering a shutdown for a while. That's why with a single 12V panel I would personally try and minimise voltage drops. At even 6A over say 6m with a couple of connections one wld be looking at a c.0.75V drop with 2.5mm^2 which one could halve by going to 4mm. Guess I'm a bit lost as to the preference for smaller size,

As for the bog comment; not sure I get that?
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Let's assume you are correct, just for a second. Let's also assume the OP is using a Victron controller (although he isn't), which requires a 5v differential to start.

Firstly, he'd be foolish to be using such a controller with a single panel. Typical output for that panel would be circa 18v, very inefficient to use a controller with a 5v startup differential. That 18v doesn't take shading into account and being coachroof mounted shading is going to have a considerable impact on panel performance.

But on to you point of a .75 volt drop triggering a shut down. If the controller requires a 5v differential to start, then how does a .75v drop make the controller shut down when it only needs a 1v differential to continue charging ? It doesn't.

The question was "Correct Wire Size and Type for 100w Solar Panel" i gave that answer.
 
dom said:
Do you know the controller make, for the OP didn't say? A slightly heavier gauge cable has got to be a heck of a lot cheaper than a new controller! Re your parallel point; there is most certainly no need to run a separate cable to the controller. Either way OP can now make his mind up so please excuse me if this little matter is getting needlessly heated.
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No, i don't know the make, model or type of controller. Nor do i need to. He asked about cable sizes, you brought controllers up, which are totally irrelevant to cable sizes.

There is no point fitting heavy cables where heavy cables have no benefit. Go buy a solar kit and see what size cables they supply.

I currently have 4 solar panels, a Victron MPPT controller and Victron monitoring via Bluetooth. I can assure you beyond any doubt that 2.5mm cable, up to 5m, is correct for a single 100w panel.

BTW i don't see anything getting heated, i'm just correcting you misconceptions and misunderstandings. Sorry if you see it otherwise.
 
The recommendation "fit the controller close to the battery" does not mean it has to be mounted in the battery compartment. Victron (75/15), for instance, state :
●Mount close to the battery, but never directly above the battery (in order to prevent
damage due to gassing of the battery).
● Use cables with 6mm² cross section. Do not exceed 5m cable length.
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VicS said:
I am surprised that the OP has room on the coach roof for the 100watt panel he is intending to buy!
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I could just squeeze one on the companionway hatch garage on my Discus, just. Coachroof mounted panels are horribly inefficient though, mine seem to be shaded to some degree almost all of the time. Either the sprayhood, the boom or the mast, sometimes mast and boom. A 50w panel on a pushpit mounted pole would yield more than a 100w panel on the coachroof, i think.
 
Dougal Tolan said:
Best size of cable is that which fits the plugs properly.
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This might be one of the better answers to the cable size question. The other imperative is cable size that is robust in resisting damage and corrosion. Actual wire resistance ie length and diameter is not nearly so significant as people might imagine. Yes wire size and so volt drop is significant in say an incandescent mast head lamp where volt drop means current drop which reduces lamp power.
However a solar panel of 100w has effectively an internal resistance of 4 ohms. 20 volts divided by 5 amps when shorted.
That means that for a 10% loss of solar power in cables you would need .4 ohm resistance which is for most runs quite a small cable size. Or put another way 1 volt drop at 5 amps will give a 5% loss of power. We hope you will have much less than 1 volt drop with reasonable sized wires even on the small side.
Likewise the wiring from controller to battery a volt drop will mean less current into the battery which means less volt drop so reducing the effect of the volt drop. As opposed to the previously mentioned mast head light where volt drop means current drop means large power drop.
So to the OP use whatever wire seems to suit best and don't worry so much about volt drop. It won't matter. olewill
 
VicS said:
But the panel the Op is planning to buy will not deliver 8 amps! Read the tech spec!

The current at MPP is 5.5 amps and the short circuit current is 5.9 amps ... and those will have been quoted for standard conditions.

The regulated volage will be 14.4 volts or thereabouts so even with a MPPt controller the current will be no more than 7amps in the controller to battery wiring
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8 smps or 7 amps, the difference is inconsequential. If the controller is 20' from the battery even with a 6 amp current the voltage drop will still be close 0.4V. using 12 AWG wire and that is too much to lose in a charging circuit.

What difference do you think it will make if the battery is charged at 14.0V instead of 14.4V?

That's why I said it is important to determine the distance from controller to battery to choose wire size.
 
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My opinion, most of the recommendations for cable size are at the bare minimum size acceptable. Charging the battery will be much more efficient if you maintain maximum voltage at the battery terminals. To do this is facilitated by mounting the controller as close to the batteries as practical and feasible and going for more than the minimum size cable. Why invest a few hundred in panels and controllers and scrimp on wire size to save a few dollars.

Don't forget that you have to figure the round trip from controller to batteries so a 10' run uses 20' of wire. 6 meters from controller to batteries using 12 AWG (3mm) wire you will drop well over 0.3V. If you upgrade to 10 AWG (5mm) the drop is slightly over 0.2V.

If you don't think a few tenths of a volt matter why do you think these expensive, smart controllers regulate the charging voltages so exactly?

The difference in cost between 10 and 12 AWG in the US is less than $0.20/ft. Another $0.25/ft you can get 8 AWG. If you're 20' from panel to battery, 40' of wire you'll spend another $8.00 to go from 12 to 10 or less than $20 to go from 12 to 8. How much did you say you spent on panels and controllers? Don't you want the most you can get from your investment?

Using small wire is penny wise and pound foolish.
 
PaulRainbow said:
BTW i don't see anything getting heated, i'm just correcting you misconceptions and misunderstandings.
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Oh my; well let's clear a few up then:

PaulRainbow said:
100w panels are fitted with 2.5mm cable and MC4 connectors.
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Some are, some aren't. OP's Ebay panels look suspiciously like the ones fitted with 4mm tails:
http://www.friendlygreengiant.com/index.php?route=product/product&path=25&product_id=49
Here is another example of 4mm. For sure many use 2.5mm, but some may ask why an incredibly price sensitive manufacturer would ever needlessly overspec.
http://www.sunstore.co.uk/files/pdf/Sunstore 100-12 spec sheet.pdf


PaulRainbow said:
An MPPT controller is not the best choice with a single panel. Not all controllers require a 5v differential. 2.5mm cable, up to about 5m to the controller is correct for a 100w panel.
...Dull days yield less power, so i can't see your point here.

If he fits another panel in parallel that panel will be wired to the controller directly, it will not require the existing wiring to be altered in any way.
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Okay first your comment that PWM and not MPPT is best for single panel installations. The MPPT is a little more expensive for sure, but we're talking about which is the better here. A correctly specced MPPT (ie not oversized with too much internal consumption) should theoretically be better in pretty much all conditions, and especially in the colder UK climate. As the solar panel operating temperature comes down, the Vmp increases. Solar panels operating at their peak power point in Standard Testing Conditions (25C) deliver about 17.5V, while the charging battery voltage is around 13.3-14V. The MPPT controller is specifically designed to capture this excess voltage. As a consequence an MPPT controller in cool conditions (panel can reach 25C on a surprisingly cold day) can produce up to 20-30% more charging than a PWM controller. The effect is magnified when it's cloudy and dull.

Here's a fun little test which demonstrated a c.30% MPPT efficiency gain over the PWM for a little 80W panel and that the gain is indeed most pronounced on dull days. I would be genuinely interested to see any 'scientific' studies which contradict this.
https://www.sunstore.co.uk/MPPT-vs-PWM-test/

And finally turning to point re parallel wiring. I typed "solar panels in parallel" into Google and here is the first of 1.3 million hits that came up: http://hespv.ca/blog/wire-solar-panels-parallel-vs-series/

What you say just doesn't make sense, sorry.

Edit: oops forgot that still leaves the question as to whether 2.5mm^2 would work and it would with a 100W panel. It would at a push if total length is kept below 5m. Now let us assume OP puts his panel on the coachroof in front of the mast to avoid boom shade: that's 1m in the panel tails, 1m to side of boat; 2.5m back, 1.5m down to batt level, another 1.5m back to batts in centreline ...that's 7.5m already!
 
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If you are even considering MPPT vs PWM then scraping the bottom of the barrel for every dreg of electricity seems to be important (whats the cost differential between MPPT and 30% more panels?) In that case any losses should be addressed, and fixing cable loss is by far the cheaper option.
Given that, I find the Victron documentation about VBatt + 5V to start and VBatt + 1V to run, somewhat alarming. I would expect a device designed to extract all possible energy from solar PV to run in any situation where the PV could power the controller. I can only guess that Victron only use DC down converters and not any of the vast array of simply implemented DC/DC buck converter that can convert up and down in the same unit as required.
 
dom said:
Some are, some aren't. OP's Ebay panels look suspiciously like the ones fitted with 4mm tails:
http://www.friendlygreengiant.com/index.php?route=product/product&path=25&product_id=49
Here is another example of 4mm. For sure many use 2.5mm, but some may ask why an incredibly price sensitive manufacturer would ever needlessly overspec.
http://www.sunstore.co.uk/files/pdf/Sunstore 100-12 spec sheet.pdf
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Funny you should quote Sunstore, that's one of the companies that i quoted as supplying 2.5mm cable with their 100w kits :)

Okay first your comment that PWM and not MPPT is best for single panel installations. The MPPT is a little more expensive for sure, but we're talking about which is the better here. A correctly specced MPPT (ie not oversized with too much internal consumption) should theoretically be better in pretty much all conditions, and especially in the colder UK climate. As the solar panel operating temperature comes down, the Vmp increases. Solar panels operating at their peak power point in Standard Testing Conditions (25C) deliver about 17.5V, while the charging battery voltage is around 13.3-14V. The MPPT controller is specifically designed to capture this excess voltage. As a consequence an MPPT controller in cool conditions (panel can reach 25C on a surprisingly cold day) can produce up to 20-30% more charging than a PWM controller. The effect is magnified when it's cloudy and dull.

Here's a fun little test which demonstrated a c.30% MPPT efficiency gain over the PWM for a little 80W panel and that the gain is indeed most pronounced on dull days. I would be genuinely interested to see any 'scientific' studies which contradict this.
https://www.sunstore.co.uk/MPPT-vs-PWM-test/
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That's one very simple test, that only takes one factor into account. It does not take into account the voltage differential required for a MPPT controller to start. You previously mentioned the need for a 5v differential, using your figure of 17.5v above, if the battery was down to 12.5v the controller would not start until the panel reached max voltage, if the battery was 12.6v the controller would never start, it would be up to full output and still do nothing until the battery voltage dropped to 12.5v. With at least 2 panels, wired in series, the controller would start much earlier and the benefits of MPPT would be realised. Victron state "Recommended number of cells for highest controller efficiency: 72 (2x 12V panel in series or 1x 24V panel)." Other manufacturers recommendations may differ.[/quote]

And finally turning to point re parallel wiring. I typed "solar panels in parallel" into Google and here is the first of 1.3 million hits that came up: http://hespv.ca/blog/wire-solar-panels-parallel-vs-series/

What you say just doesn't make sense, sorry.
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No idea what your point of posting random Google hits for domestic solar panels is. No idea what point you are trying to make. Whether to connect multiple panels parallel or series is dependent upon a number of factors on a boat. Mostly the location of the panels, regarding shading and the type of controller being used.

I have four panels, wired as two pairs in series, connected together in parallel. The layout minimises the effects of shading on the overall system, whilst maximising the benefits of the MPPT controller.

That said, that's just about all irrelevant to the OP with his single 100w panel.

Edit: oops forgot that still leaves the question as to whether 2.5mm^2 would work and it would with a 100W panel. It would at a push if total length is kept below 5m. Now let us assume OP puts his panel on the coachroof in front of the mast to avoid boom shade: that's 1m in the panel tails, 1m to side of boat; 2.5m back, 1.5m down to batt level, another 1.5m back to batts in centreline ...that's 7.5m already!
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Firstly, i have consistently said a max of 5m of cable from the panel to the controller, thereafter 6mm cable to the battery.

Secondly, with a panel ahead of the mast your cable run is nonsense. The 1m tails would get the the side of the boat, but why would any sane person run the cables on such a route ? Cable gland just ahead of the mast, then run the cable under deck to the controller. I have a Westerly, a little bigger than the OPs and 4m of cable would get from just ahead of my mast to beneath my port side sofa, if i fitted the controller there it would be 1.5m to the batteries.

As it is, he won't be fitting it ahead of the mast, because there is no room for it there. He will be fitting it ahead of the sprayhood, athwartship. I doubt he'll need more than 3m of cable.
 
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The Op's ebay panels look like they've got fairly fat cables coming out.
That's either because there's fairly fat copper in the cables, or more likely, the cable is double insulated, because these things are often wired in series to rack up more than 50V, so different regs kick in.
Here's a handy voltage drop calculator:
http://www.epanorama.net/index.php?index=calc_cable

I leave it as an exercise to the reader to work out the effects of voltage drops at various points in the circuit.

The A ratings of various cables may be in free air, if they are well thermally insulated or bundled with lots of other hot cables, a de-rating factor may be called for.
But the standard current rating for equipment wire is around 10A per sqmm.

BTW with solar, brand new panels often put out upto 120% of spec'd power, it's all about achieving something like 80% of spec'd power when the panel is at end of life.
 
PaulRainbow said:
Funny you should quote Sunstore, that's one of the companies that i quoted as supplying 2.5mm cable with their 100w kits :)
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Then read that data sheet !!!

The rest of your argument is equally full of holes, sorry, and if you have some scientific evidence supporting PWM over MPPT than please post it. Science after all trumps opinions, I think.
 
dom said:
Then read that data sheet !!!

The rest of your argument is equally full of holes, sorry, and if you have some scientific evidence supporting PWM over MPPT than please post it. Science after all trumps opinions, I think.
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The panel has 4mm tails, the kit is supplied with 5m of 2.5mm cable, ffs.

If you want evidence of MPPT v PWM, use you friend Google.

The OPs question was regarding cable size for his panel on his boat. It had nothing to do with PWM v MPPT or the size of panel tails. Those are just red herrings thrown in to confuse things.

I'm not supporting PWM or MPPT, each has it's place. That place will depend on various circumstances, not least panel voltages and the controller algorithm. From the figures you have quoted (5v PV to battery differential and 17.5v max PV voltage) a single 12v panel with an MPPT controller makes no sense.

My panels output around 38v (wired in series) and the MPPT algorithm starts early in the day. Even with the dull days we are currently experiencing my controller is charging at 0700, that does make sense.
 
RichardS said:
See post #15. :)

Richard
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I saw that Richard. I didn't mount mine in the battery compartment. Mine is 3m away, wired with 6mm cable, well within the Victron recommendation. This also keep the PV to controller cable runs short, my coachroof panel cables are just 1m long. The reason the controller needs to be reasonably close to the batteries is to minimise voltage drop. It's the voltage drop from controller to batteries that would have a serious effect on performance. A fraction of an amp between PV and controller is insignificant, as Will mentioned in an earlier post.
 
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