Here's a Very Nice Cable Calculator

[WotEver]

Unpinned, due to out-of-date link which no longer works.

One here: http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

 

And here: http://www.energymatters.com.au/climate-data/cable-sizing-calculator.php

 

And here, but results in AWG so you'll need a conversion chart: http://www.calculator.net/voltage-drop-calculator.html

[alan_fincher]

One here: http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

 

And here: http://www.energymatters.com.au/climate-data/cable-sizing-calculator.php

 

And here, but results in AWG so you'll need a conversion chart: http://www.calculator.net/voltage-drop-calculator.html

 

Stating the obvious, perhaps, but when I was looking around amongst the offerings available.most seemed to assume the length you entered was the out and back distance, (so for a 20 metre run of twin core cable you should enter 40 metres), whereas a few were actually geared for you just entering the cable length, (so you put in 20 metres, even though the current has to travel 40 metres).

 

The problem was that not all of them made it clear what they were expecting you to enter for an out and back run, and if you assume wrongly then you will get the wrong answer, ending up with cable either over-specced or under-specced for your application.