Which size cable?
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I assume you have two items drawing 6 amps each, if you mean two items drawing three amps each, just change 12 to 6 in the calculations below. This is not as straightforward as you might think because the length of cable run, out and back, comes into it as well. Almost any cable supplier’s catalogue will give you the current rating for a cable, but that is only the maximum current that it can carry without danger of overheating and fire. For instance, vehicleproducts.co.uk ( Vehicle Wiring Products) say that for 12 amps 32/ 0.20, 1sq mm conductor cross sectional area in thin wall or 20/ 0.30 2sq mm conductor cross sectional area in standard PVC will be
AIs there a chart of what I can safely use for two items on a cable carrying 6 amps? safe. If the total run of both the positive and negative cable is more than a metre or two, you are likely to get volt-drop and that gives symptoms of a flat battery. To calculate volt-drop use this formula: 0.0164 X current (amps) X total cable run length
The CSA of the cable Try to get below 0.3-volt for starting and charging circuits while others can be up to 0.5-volt, but the lower the better.
Let’s say your items are 8 metres from the fusebox, that gives a 16-metre run using 2 sq mm cable so: 0.0164 X (2 x 6) X16
2 sq mm This gives a volt-drop of about 1.6 volts and that is too high. 4sq mm cable gives 0.8-volt while 6sq mm cable gives 0.52 volts of volt-drop so, for most circuits, this will do.
There are tables on the Internet if you Google ‘12v cable size chart’.
Canal Boat Magazine, Archant Specialist, Evolution House, 2-6 Easthampstead Road, Wokingham, RG40 2EG