Choosing the cable cross-section is a cornerstone of safety and efficiency in an electrical installation. A cable with too small a cross-section overheats, causes voltage drop and creates a fire risk. A cable that is too thick means unnecessary cost. To find the right balance you need to know the basic calculation methods.
The first step in choosing a cable cross-section is to determine the total power the line will carry. Add up the watt values of all the devices that will be fed from that line. For example, a lighting line may total five hundred watts, while a kitchen line with an oven, a dishwasher and a microwave can have five thousand watts or more.
After finding the total power, calculate the current. In single-phase circuits, current is found by dividing power by voltage. On a two-hundred-and-twenty-volt supply, one thousand watts draws about four and a half amperes. According to this current value you pick the right thickness from the cable cross-section table.
The cross-sections commonly used in homes are as follows: For lighting lines, one and a half square millimetres is enough and is protected by a ten-ampere breaker. For socket lines, two and a half square millimetres is the standard choice and works with a sixteen-ampere breaker. For high-power devices such as air conditioners, ovens and water heaters, four or six square millimetre cable is needed and these lines must be protected by their own breaker.
Cable length also affects the choice of cross-section. Over long distances the voltage drop increases and to make up for it a larger cross-section should be chosen. According to the regulations, voltage drop must not exceed three percent.
The cable type is important too. In homes, NYM-type multicore cables are standard. NYM is used for concealed installation, while NH-type cables are used for surface-mounted installation or in cable trunking. Copper-conductor cables conduct better than aluminium and are safer.
Never choose cables by eye. A professional electrical engineer or technician can analyse your home's power needs and determine the correct cross-sections. This small investment brings a big safety gain in the long run.
