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Solar - mc4 cable / mppt and panel ratings


Johny London

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Currently confusing myself over what cable thickness is needed for what application.

For example, looking at a table from an ebay seller ratings for mc4 cable as follows...

2.5mm = 39a

4mm = 52a

6mm = 67a

10mm = 93a

 

Whereas the Tracer instruction manual states...

10a controller = Battery wire 4mm, load wire 4mm

20a controller = Battery wire 6mm, load wire 6mm

30a controller = Battery wire 10mm, load wire 10mm

40a controller = Battery wire 16mm, load wire 16mm

 

I would have thought that due to higher voltage/lower current on panel side the wire ratings would be higher on the battery side and lower on the panel side, not sure what they mean by battery wire, and presumably load is irrelevant to most of us though probably equivalent to cable going to batteries?

 

For example, if I have 2x260w panels in series, each rated at a s/c current of 9a (at 35v) what size cable do I need? Surely not that thick because it will be carrying only 9 amps at 70v or if in parallel 18amps at 35v?

 

On top of that I was under the impression that mppt controllers would deliver up to their maximum current rating but can handle much more powerful panels, basically just not delivering the available surplus current. Epever manual states that...

 

20amp controller: 260w delivered with a maximum of 390w of panels (12v)

30amp controller: 390w delivered with a maximum of 580w of panels (12v)

40amp controller: 520w delivered with a maximum of 780w of panels (12v)

 

So the mppt presumably can only handle dumping/ignoring a certain amount of power? I'm more interested in just getting something on a poor day rather than big maximum numbers on good days.

 

 

Edited by Johny London
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As with all extra low voltage installs the cable size is driven by voltage drop, not the current capacity of the cable. 

You might expect the MPPT to sort it out, but if it is seeing a significantly different voltage to that at the panel it will be offset from the MPP, and mistracking. Or, if there is voltage loss between the controller and the battery the battery will get less charge than it could/should.  Or both in some set ups.

 

So to choose a cable you need to know how long it is as well as the expected curent.  There are then several on line volt drop calculators, or someone here will advise.

 

N

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In the Tracer manual, both battery wires and load wires are 12V nominal. The controller can be used to supply a load, for example a fridge, directly, disconnecting it when the battery voltage drops too low. That is why the cable sizes are the same. You'd size them for acceptable voltage drop over the distance to the load and batteries. No mention of the solar panel connection wire size there. For them, as with all wire sizing for boats you are more interested in voltage drop, than maximum current carrying capacity. As the voltages for a set of series connected panels are high, then that is not so much of a concern and you can use relatively thin wire. Personally I'd size them for easy use with the MC-4 connectors and robustness on the roof. Probably 4mm2. I'd not take a table of cable sizes in a random ebay vendors listing as gospel! Random Ebay sellers tend not to be experts... This is more likely to be maximum currents over short distances in an automotive application than anything else.

 

Jen

Edited by Jen-in-Wellies
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So does 6mm cable sound ok for 2x260w panels on a 15 metre run? (either series or parallel?)

And I already have 4mm on 300w of panels 15m,  and 4mm on 300w panels 10m

 

My two sets of panels (total 600w) don't seem to have done any harm to my 30a controller. Is it that the controller has to work harder dumping current if its also supplying higher current?

 

Presumably the tracer figures specify large cables because they want their device to work within spec, at its very best?

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Voltage drop = resistance of the wires there and back x current flowing.

Resistance of the wires = resistivity of copper 1.72x10-8 ohm.m x length (m) there and back / cross sectional area (m2).

 

Assuming when you say 6mm wire you actually mean 6mm2, then:

 

Voltage drop at 9A, = 9 x 1.72x10-8 x 30 / 6x10-6 = 0.796V, or 1.1% at 70V. Series connection. 7.1W of wasted power.

Voltage drop at 18A = 1.592V, or 4.4% at 35V. Parallel connection. 28.6W of wasted power.

 

Are these acceptable to you?

 

Jen

Edited by Jen-in-Wellies
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If you want to get the highest efficiency of converting sunlight into stored energy in your battery then you don’t want to be dissipating energy in the connecting cables as heat.  So thick cables are advised, more so between the controller and batteries as the controller controls the voltage at it’s output terminals and so any volts lost in the cable to the batteries will result in lower voltage across the batteries and hence an increase in time to charge.

 

The mppt controllers were often designed for solar powered street lights, so mine has the option to turn on the load (street light) as the panels voltage drops (because it gets dark).  Most boaters do not use the load connections.

 

controllers don’t dump power as the batteries charge, they just stop transferring power into the batteries allowing the panel voltage to rise.  As to what will happen if more panels are connected than the spec, it depends on the quality of the design, but if you have a lot of extra panels, the controller could well be running at it’s max output for a long time, so it may get hot and may have a reduced life.  Again depends on the design, I would guess the cheapest controllers have saved cost at the expense of thermal management 

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Thanks for some great info - especially Jen's calculations. So, series it is! Though that'll be at 70volts dc so I'll be careful.

I was looking at the instructions I have for each of my controllers.

The instructions that came with the 30amp tracer (AN series, blue buttons) states much lower excess wattage is acceptable: 390w max usaeble and 580w maximum panels.

The instructions that came with the older one I have (A series, identical other than yellow buttons and apparently common positive architecture) states (for a 30amp controller) 390w max useable and 1170w maximum panels! 

Certainly something to consider if building an overly large array for those grey days.

 

Yes sorry I meant 6mm squared but don't know where to find a little "2"

 

Next thing - I'm looking for suitable dc breakers instead of the inline fuses I currently use.

 

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21 minutes ago, Johny London said:

I meant 6mm squared but don't know where to find a little "2"

{sup}2{/sup} but use square brackets - [ ] - instead of squiggly ones. 

Or type 6mm^2 if you prefer. 

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The instructions for my 20A Tracer (XTRA2206N) state that the maximum power output is 260W (i.e 20A at 13V), but that you can connect up to 780W of panels (for a 12V system). That way you can still get up to 20A into your batteries in lower light conditions, but you won't get more than 20A even in very bright sun.

You still have to observe the maximum open circuit voltage limits, which in practice means the extra panels may have to be in parallel, not series.

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1 hour ago, Mike the Boilerman said:

One point not mentioned so far, the cable sizing charts are generally for copper. The cable sold for connecting solar panels to the controller is usually steel, so all bets are off regarding sizing.

 

 

Pretty sure mine are tinned copper and not steel.  I didn’t know steel was commonly used.

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7 hours ago, Mike the Boilerman said:

One point not mentioned so far, the cable sizing charts are generally for copper. The cable sold for connecting solar panels to the controller is usually steel, so all bets are off regarding sizing.

 

 

Are you sure? All the ones I've used have been tinned copper. The manufacturers data sheets I've looked at just now for solar specific cable say copper, not steel. The resistivity of steels are up to an order of magnitude worse than annealed copper, which would increase voltage drop by up.to ten times. Can't see any advantages in using steel over copper that can compensate for that.

 

Jen

Edited by Jen-in-Wellies
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1 hour ago, Jen-in-Wellies said:

Are you sure?

 

I’m fairly sure (but not certain) that all the solar panel wiring cable, bought from various sources, i have On my boats is steel but I’ll check. I still have some un-used in stock at home but I’m away for a few days at alvecote. Will check when i get back. 

 

 

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2 hours ago, Jen-in-Wellies said:

Are you sure? All the ones I've used have been tinned copper. The manufacturers data sheets I've looked at just now for solar specific cable say copper, not steel. The resistivity of steels are up to an order of magnitude worse than annealed copper, which would increase voltage drop by up.to ten times. Can't see any advantages in using steel over copper that can compensate for that.

 

Jen

 

56 minutes ago, Mike the Boilerman said:

 

I’m fairly sure (but not certain) that all the solar panel wiring cable, bought from various sources, i have On my boats is steel but I’ll check. I still have some un-used in stock at home but I’m away for a few days at alvecote. Will check when i get back. 

 

 

My last reel end. Looks like copper. Feels like steel

IMG_20190822_092059.jpg

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17 minutes ago, rusty69 said:

 

My last reel end. Looks like copper. Feels like steel

IMG_20190822_092059.jpg

 

 

Have to say I only assumed the conductors are steel due to how difficult the strands are to cut and to bend.

 

They behave like steel, and look like tinned steel to my eye. I guess they could be hardened copper. I think I might be able to find a magnet, and test the cables on this boat later.

 

 

 

 

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6 minutes ago, Mike the Boilerman said:

 

 

Have to say I only assumed the conductors are steel due to how difficult the strands are to cut and to bend.

 

They behave like steel, and look like tinned steel to my eye. I guess they could be hardened copper. I think I might be able to find a magnet, and test the cables on this boat later.

 

 

 

 

Oooh, a scientific test. 

 

Just put my reel next to the ships compass. No deflection, variation or deviation. 

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3 minutes ago, Captain Fizz said:

Shit and derision, that's your reputation out the window then!!?

 

I think that happened long ago!

 

Still v curious about what the conductor is, given it is SO stiff to bend and difficult to cut. 

 

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