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Electric current Energy efficient cables
Choosing a cable
Photo of coper cables.
Picture 2.10 Copper cables.

Industrial premises such as factories, office blocks and shopping centres are supplied with electricity through thick cables. These cables may get hot as currents of hundreds of amps are made to flow through them.

How is the best cable chosen? We need to consider:

  • installation costs
  • running costs.

In summary, the thick cable costs more to install but is less expensive to run.

Installation
Most of cost of installing a cable goes on accessories, digging trenches, installing cable trays, and labour. The cost of cable may be 10 to 15% of the installation cost. So even doubling the cost of the cable has only a small effect on total cost. Thicker cables also allow for increased demand in the future.

Running costs
Thicker cables have a lower resistance. This means that they are less wasteful.

Imagine a factory that needs to draw 200 amps from the National Grid to run its machines. This current will heat up the cables that supply the factory. The amount of power wasted in these cables by joule heating is given by the expression:

P = I squared R

To keep this power as small as possible, the resistance of the cable should be as small as possible. So thicker is better. This also allows for an increase in demand in the future.

Also, a thinner cable gets hotter. So fans or air-conditioning may be needed to remove the heat. These add to the running costs.

Wires for question 7
Picture 2.11 See question 7.
Question 7
Look at the wires in picture 2.11. Then answer the questions below.
Wire A Wire B Wire C
a)  Which wire has the lowest resistance?
b)  Which heats up most for a given current?
c)  Which cable would be most expensive to buy?
d)  Which cable would be most expensive in the long run?
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