12v cable thickness for solar panel installation

[welshgeekboy]

Hi there, I'm trying to install a 12V solar panel in my caravan. I've worked out that I'll need a 100W panel, so presuming it's working at 100% efficiency that gives me a current of 100W / 12V = 8.33A. I reality the current will be more like 6A (I think).

I'll be running the cable 7-8m to the charge controller, then ~1.5m to the battery. I've looked on ebay for some cable to go from the panel to the charge controller and found many options. I then looked at some websites to learn about the importance of thick cable, and found a few cable thickness guides. Apparently for a 8.8m run of cable at 12V and 8A I would need an 8 mm2 (2.8mm diameter) cable. However I see on ebay things like this:

2 Core Twin 11A 16.5A 25A 12v THINWALL Automotive Cable Auto Wiring Loom Marine | eBay
That states that a 0.5 mm2 cable is enough for 11A! Could someone please help me to understand what diameter cable I need? I'm figuring the calculators on websites will be more reliable than an ebay listing. And I don't want anything to start melting...

Thanks a lot, forumgoers!

 

[telectrix]

1.5mm2 cable is rated to 16A. you could use 2.5mm2 up to 26A. won't hurt to over spec the cable size.

 

[Richard Burns]

The cable size for a solar system tends to be based on limiting the drop of voltage that occurs in a thin cable this means cables tend to be thicker than is necessary for carrying the current load.
So the current the cable can take is always way above the actual current loading.
Even though a 1mm2 cable can carry the planned load there would be a significant drop in voltage over the run to the battery.
8mm² cable is not generally available, 4mm², 6mm², 10mm² are available.
If you were running a commercial solar system the expected limit on volt drop is 1% (i.e 0.12 V) but this may be a bit extreme for your purpose.
The cable you use should be a flexible cable so it is not stressed during caravan movement and you should aim to have the thickest cable you think you can run easily and that you can afford.
Personally I would be aiming for at least 4mm², but this is just my personal preference with very little science behind it.

The solar panel will almost guaranteed never produce 100 W of power and the majority of the time will be a lot lower unless it is angled at 90° to the incident sunshine and there is a perfectly clear sky.

 

[UKMeterman]

4mm2 fine stranded sounds good, the important thing to remember never connect directly to the battery always go via a fuse.

 

[Gavin A]

4mm2 fine stranded sounds good, the important thing to remember never connect directly to the battery always go via a fuse.

and blocking diode to stop the battery discharging via the panel at night.

 

[Kamikaze]

Cast your eye over this thread. It might be helpful. You will probably encounter the same problem.

http://www.electriciansforums.co.uk...urrent-drain-leisure-battery.html#post1084552

 

[solarfred]

you need a decent solar charge controller. It will control and protect the battery charging regime. 4mm2 solar cable is plenty for 100w panel over that distance. The charging lead should be heavy (limited by the terminal size) and fused.