Current rating of wires.

If you’re designing a circuit that has to pass any significant current you will be faced by the prospect of choosing wire to carry that current, and will probably notice that the current rating of insulated wire doesn’t seem to make sense from type to type. For instance, looking at vehicle wiring products thinwall cable, a 16/0.2 wire (0.5 mm2 area) is rated at 11 amps, but a 16/0.2 cable from maplin is rated at 3A, one from Rapid at 6A … what’s going on?

Well one answer could be that they apply the rating arbitrarily but that’s unlikely. To find the real answer we need to look at what happens to cable when you pass current through it. The simple answer is that it heats up – and the more current you pass then then the hotter it gets. How hot, well that depends on the “nominal area” (0.5mm2 in our example) and how easily the heat can escape – so if you get really excited and read manufacturers data sheets you’ll see that many will have different ratings depending on if the cable is in free air, or bundled with others, or buried in walls etc. The other factor is the nature of the insulation – thicker insulation will mean less heat escape and a lower current rating.

Of course, the maximum current rating of a cable is the current at which the manufacturer feels the integrity of the insulation is compromised – over which it will melt or burn or harden and crack… That doesn’t mean its a good idea to do so, because there’s the “elephant in the room” to consider – wires are not perfect, they have resistance!

For example, the data sheet for the 16/0.2 wire says that the wire is 39ohms/km. That doesn’t sound a lot, but lets say you have 5m of wire and you use VWP’s thinwall cable because its carrying 10A – total resistance = 39 x 5/1000 = 0.195 ohms, which at 10 amps will drop a voltage of 1.95v – which will probably be an issue as well as the fact that the cable will get noticeably warm. Move up to 32/0.2 wire which has a resistance of 19.2 ohms/Km and that voltage drop goes down to 0.96 volts (still significant!).

It’s probably more important, in fact, to calculate the voltage drop and make sure thats acceptable. if you reduce the voltage drop then your going to get less heating in the cable and therefore strain the insulation less.

The other observation is also to make sure that you keep runs of cable carrying high currents as short as practical. In a project I worked on, we used a HUGE cable to take 12v to the back of a car, and used relays near to the lamps to switch them; the 4-5m switching cables to the relays carried little current and could be small wires, with all the current being carried by a length of starter cable which had negligible voltage drop at the 30 or so amps it was carrying.

So, in summary, the manufacturers maximum current rating should be obeyed because above it the insulation may melt or burn. But you should use cable that is over-rated, especially for long runs, because of the voltage drop. If you’ve got short runs (e.g. inside a case) then you can run nearer the maximum, but bear in mind all that heat generated!

Version 2 updated 5 Aug 2012, 9:42 a.m.