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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.

What can I do about it?

To lower the voltage drop in a circuit, you need to increase the size (cross section) of your conductors – this lowers the overall resistance of the cable. Of course, larger cable sizes increase cost, so it’s important to calculate voltage drop and find the optimum conductor size that will reduce voltage drop to safe levels while remaining cost-effective.

How do I calculate voltage drop?

To accurately calculate the voltage drop for a given cable size, length, and current, you need to accurately know the resistance of the type of cable you’re using. However, AS3000 outlines a simplified method that can be used.

The table below is taken from AS3000 – it specifies ‘Am per %Vd‘ (amp metres per % voltage drop) for each cable size.  To calculate the voltage drop for a circuit as a percentage, multiply the current (amps) by the cable length (metres); then divide this number by the value in the table.

For example, a 30m run of 6mm2 cable carrying 3 phase 32A will result in 1.5% drop: 32A x 30m = 960Am / 615 = 1.5%.

AS3000 Table C7

Cable conductor size Single Phase (230V) Am per %Vd Three phase (400V)Am per %Vd
1 mm2 45 90
1.5 mm2 70 140
2.5 mm2 128 256
4 mm2 205 412
6 mm2 306 615
10 mm2 515 1034
16 mm2 818 1643
25 mm2 1289 2588
35 mm2 1773 3560
50 mm2 2377 4772
70 mm2 3342 6712
95 mm2 4445 8927

 

Example

A cable from a main switchboard to a feed mill is 160m long. The feed mill requires a 415VAC, three phase, 80A supply. As there is already a voltage drop before the main switchboard, we need to limit the voltage drop of this new cable to 3%. What size cable is required?

80A x 160m = 12800Am / 3 (%) = 4267 Am per %Vd.

Looking up this value in the table, 50mm2 is the smallest suitable size.