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Using electricity Copper cables
Copper and energy efficiency

Copper is a very good electrical conductor. This means that the resistance of a length of copper cable is relatively low. An aluminium cable would have nearly twice the resistance of a copper cable with the same dimensions. Therefore the energy losses in the aluminium cable will be higher than in the copper cable. The copper cable is more energy efficient.

Photo of cables
Picture 1.3 Cables made from aluminium and copper. The aluminium cable has nearly twice the area to give it the same resistance as the copper cable. You can see the strands of conductor and the armour.

To make an aluminium cable with the same energy losses as a copper cable, we have to make it fatter. The larger cross sectional area reduces its resistance and brings the energy losses down to the same as a narrower copper cable.

The two cables in the photograph have similar current-carrying capacity. They are each designed to be able to carry up to 500  A without the conductor going above 90  °C; they will have to carry this current when demand is high but should not be used for larger current.

The copper cable (on the right) is thinner than the aluminium one, because copper is a better conductor. Its cross sectional area is 300  mm2 as opposed to 500  mm2 for the aluminium.

There are four groups of wires within the cable and the groups are insulated with cross-linked high-density polyethene (XLPE). The whole cable is reinforced with steel wire armour.

What are the advantages of copper?

The copper cable has several advantages over the aluminium one. Because it is thinner, it can fit into smaller duct spaces. It can be bent around tighter corners. Also, copper can be easily jointed.

Copper is three times as dense as aluminium. An aluminium cable is half the weight of a copper one with the same rating, so aluminium cables (reinforced with steel) are often preferred for hanging between overhead pylons.

For a complete comparison of energy-efficiency, we should also take into account the energy used in mining, refining and transporting these metals.

What is the safe operating current?
The current rating of a cable is the current it can carry safely without overheating. The main issue is that the temperature of the conductor should not rise above 90 °C because this will cause the insulation to degrade and eventually break down. A cable should always be protected by a protective device – a fuse or circuit breaker – that corresponds to its current rating. The transmission cables in picture 1.3 will be in series with a 500  A protective device which will break the circuit before the current is high enough to cause the cables to overheat.

The operating current of a cable is determined by how hot the cable gets. This is affected by a number of variables:

  • the resistance of the cable - a higher resistance cable will get hotter at a given current
  • the insulation on the cable - this will tend to keep it warm
  • the environment of the cable - if it is in a duct (especially with no airflow) it will tend to get hotter.

Copper plays a part in improving the current rating of cables because it is such a good conductor.

Question 2
These questions relate to the copper cable shown in the photograph. The voltage drop across 1 km of cable is 30  V at 500  A.

a) You should recall the equation which defines electrical resistance. Use it to calculate the resistance of 1  km of the copper cable.

Equation reminder
voltage = current × resistance
V = I R
This equation defines the ohm - the unit of resistance.

b) You should also recall the equation for the rate of resistive heating (power dissipation) when a current flows through a wire. Use it to calculate the power dissipated when the maximum current flows through 1  km of the copper cable.

Equation reminder
power = current2 × resistance
P = I2 R

c) Explain why a cable like this has a maximum current rating.

d) Are there circumstances in which the current should be kept even lower than the current rating?

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