How many times have you asked an engineer a question, and had that for an answer? Well, there’s usually a bit more to this statement, and it’s usually “on where you’re starting from”. And this is the case when we try to decide upon a conductor size for a given current.
We are looking for the Goldilocks condition, not too big, not too small, but just right. If the conductor we choose is too big, we will increase the cost of the materials, the weight and the size of the finished unit. If the conductor we choose is too small, we will reduce the life of the unit, and will incur a serious risk of fire. (I know, I did it myself as a very young, very green, design engineer, but only once!) So, we need to look at the factors that choose the conductor size for us.
When we draw power through a conductor, its inherent resistance will cause a rise in temperature, irrespective of the material. This is why, unless other factors decide otherwise, annealed copper is the first choice as a conductor. It has a low conductivity, 94% at 0°C, at £3.60/kg, in comparison to 100% for silver, at £382.30/kg. (The latter figure is why we use copper!) However, that loss of energy will be converted into heat, and as we’ve discussed before, heat is the enemy of cable. The major factor will be the insulation material, PVC for instance, will rapidly fail at temperatures above 105°C, so any current that causes a PVC insulated cable to go above this temperature is too high. Bear in mind, also, that 105°C is pretty hot, especially if you touch it, so you will need to consider what temperature rise the system can tolerate.
But, “that depends on where you’re starting from.” If the ambient temperature of the environment in which the cable is being used is, say 20°C, then you can deploy a higher temperature through the conductor, than if the ambient temperature is say 50°C.
Added to this, we have bundling. Penguins on an ice floe huddle together to keep warm, and take turns at being on the inside, where it is more difficult for the heat to escape. So it is with cable. On the inside of a bundle, the heat generated is shared rather than escapes, in effect, raising the ambient temperature. So, we have to apply a de-rating factor to the applicable current, dependent upon the size of the bundle.
So, the size of a conductor required for a given current depends upon the ambient temperature of the environment in which the device will be used, the temperature increase that can be tolerated in normal usage, the ability for the generated heat to escape, and the insulation material that has been chosen.
If you’re not sure after all of that, (I refer you to my fire), call us at Aerco Ltd, and we shall be only glad to help you choose the right products for your application. And sell you them.
(by David Cairns)
To learn more cable and wire services and products at Aerco, please message us or call us on 01403 260206.