Calculation of voltage drop and cable size

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Motor / Electrical load

HOW TO CALCULATE VOLTAGE DROP

What is it and how do I calculate voltage drop?

You may have heard of the term. Voltage drop is the known phenomenon where the voltage at the end of a run of cable is lower than at the start. Any length or size of cable will have a resistance, and running a current through this resistance will cause the voltage to drop. As the length of the cable increases, its resistance increases in proportion; so voltage drop is particularly a problem with long cables runs, for example in larger buildings or on larger properties such as farms.

Australian Standards require the total voltage drop from the point of supply (i.e. where the power enters the site from the grid) to anywhere in the installation is kept below 5% of the full line voltage. Voltage drops higher than 5% are liable to cause issues such as dim or flickering lights, electric motors running hot and potentially burning out and heating elements heating poorly.

Remember that the 5% maximum is from the point of supply – so if you’re adding a new cable from an existing switchboard, part of this 5% will likely have already be taken up in the cables feeding this switchboard.

• Calculate Voltage drop of Cable.
• Calculate  Size of Cable.
• Calculate Current Capacity of Cable.

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