What cable size for wind charger and solar panels?

[whipper_snapper]

Dear all,

I am sizing cables for a nominal 400W wind charger and around 240W of solar panels. That means that they could *theoretically* deliver 33A and 20A respectively, and Capt Slartey’s excellent spreadsheet tells me that for a 3% drop on a 20’ 2-way run I should use 6AWG and 8AWG wire. That is damn thick stuff! It seems to me that I do not need such heavy wire as I can live with a bigger Voltage drop when the current is high, it is when I am trying to squeeze the last bit of juice into the battery and the current is tiny that I really need to worry about the drop. So if I allow for a 10% drop at full current, I can get away with 12 and 14AWG, that will give me less than a 1% drop at 1 Amp. I am tempted to just use 14AWG for both.

Comments ? Suggestions?

Thanks

 

[sarabande]

I am just installing a couple of 40 watt Kyocera panels from Aire and Sun (wwwaireandsun.co.uk). Using 4mm sq cabling, as I am a risk-averse person.

Their instructions state that the modules can be interconnected with 14 to 10 AWG, (2mmsq to 5.5 mm sq). A&S caution that solar panels in the wild can produce more current and/or voltage than under test conditions, and advice applying a safety factor of 1.25 to the system/wiring. Apparently the National Electrical Code pp690-698 agrees.

http://www.aireandsun.co.uk/pdf/basic_solar_system_guide.pdf

I have just checked with the manufacturer of the charge controller (Steca, Germany) They suggest 6 mm sq cable for lengths of 2 m between the module and controller and controller and battery. Any longer distance will need more mmsq to avoid loss.

I guess that Capn Slarty is right. Go for the thick stuff.

 

[oldsaltoz]

Too heavy will be a problem, too light will be. You need to check if the insulation is a hazard if heated in a confined space, also its UV rating and best if tinned.

I would run the heavy cable on both, at least you will an interchangeable cable if needed at sea.

Avagoodweekend......

 

[hightech]

lighter cables = higher resistance = heat

Heating up cables is not good for the cables and definitely not good for the boat if the overheat and catch fire.

Think hard about down sizing.

 

[oldsaltoz]

[ QUOTE ]
Too heavy will be a problem, too light will be. You need to check if the insulation is a hazard if heated in a confined space, also its UV rating and best if tinned.

I would run the heavy cable on both, at least you will an interchangeable cable if needed at sea.

[/ QUOTE ]

Oops, That should read "Too heavy will <span style="color:red"> not </span> be a problem, too light will be.

Avagoodweekend......

 

[alan]

I think that your calculation for your solar panels may not be correct: 240W panels at around 18v [max load votlage] (not 12v !!!!) = 13 A current (max load).
From a table that I have :
2.5mm^2 cable can carry 20Amps (IEC 332)
4.0 mm^2 cable can carry 27 Amps

From my calculation:
13A on 2.5mm^2 cable can be run for 12 metres (2 X 6m) for a 1.02v or 6% drop (referenced to 18v)

13A on 4.0mm^2 cable can be run for 12 metres (2 X 6m) for a 0.64v or 3.6% drop (referenced to 18v)

I doubt you will ever get 13A from your panels and so probably 2.5mm^2 cable (12AWG) will be enough in practice but if it was me I would use 4mm^2 (10 AWG) for safety and have less drop (in practice, on a perfect day with panels orthogonal to the sun, probably about 0.4v or 2% ref to 18v).

Alan.

 

[whipper_snapper]

Thanks,

I am well aware of that! I will not go anywhere near the point where the maximum possible current will actually threaten the wire. And of course there will be a fuse sized to protect the wire. It's just a question of identifying the expected 'real world' conditions.

 

[whipper_snapper]

Thanks, yes I had assumed 12V. When it's tied to a battery you will presumably never see 18V, but I suppose 14V is possible, but at that stage, preumably the current will be tiny?? I was assuming that the 'worst case' was flat battery and full panel output.

10AWG sound like a good compromise!!

 

[alan]

I was just measuring my new panels today (Kyocera 65Watts) and there was around 18v going into the regulator and 14.4v going from regulator to battery. I was assuming that with that much power that you would be fitting a regulator??
Hence my calcs using 18v for cable sizes from panel to regulator; of course from reg to battery you should consider 12v for when the battery is flat ...... but typically the run from reg to battery is short (mine is < 1m) hence volt drops are minimal and smaller section cable can be used (within the bounds of safety).

Alan.

 

[whipper_snapper]

Thanks, excellent point. Yes there is a common regulator for wind and solar and if that is just dumping power when the batteries are fully charged, then the full current may often be seen, but a drop across the cable would not be a problem under those circumstances. The regulator is inches from the batteries, so the drop there will be tiny with 10AWG or even thinner.