Voltage Drop Explained

Voltage drop (VD) is a common issue in electrical systems where the voltage (V) at the end of a circuit is lower than at the beginning due to resistance in the wiring. This can lead to inefficient equipment operation or even failure. Solving voltage drop involves ensuring proper wire sizing, minimizing long-distance wiring runs, and using materials with lower resistance. Calculating the voltage drop for specific circuits and adjusting the installation accordingly helps maintain optimal performance and prevent power loss.

Any length or size of wires will have some resistance, and running a current through this dc resistance will cause the voltage to drop. As the length of the cable increases, so does its resistance and reactance increase in proportion. Hence, voltage drop is particularly a problem with long cables runs, for example in larger buildings or on larger properties such as farms. This technique is often used when properly sizing conductors in any single phase, line to line electrical circuit. This can be measured with a voltage drop calculator.

Electrical cables carrying capacity of current always present inherent resistance, or impedance, to the flow of current. VD is measured as the amount of loss which occurs through all or part of a circuit due to what is called cable "impedance" in volts.

Too much voltage drop, otherwise known as " excessive voltage drop ", in a cable cross sectional area can cause lights to flicker or burn dimly, heaters to heat poorly, and motors to run hotter than normal and burn out. This condition causes the load to work harder with less energy pushing the current.

How is this solved?

To decrease the voltage drop in a circuit, you need to increase the size (cross section) of your conductors – this is done to lower the overall resistance of the cable length. Certainly, larger copper or aluminum cable sizes increase cost, so it’s important to calculate voltage drop and find the optimum wire size that will reduce VD to safe levels while remaining cost-effective.

How do you calculate voltage drop?

Voltage drop is the loss of electricity caused by the flow of current flow through a resistance. The greater the resistance the greater the VD To check the VD, use a voltmeter connected between the poinL where the VD is to be measured. In DC circuits and AC resistive circuits the total of all the voltage drops across series-connected loads should add up to the voltage applied to the circuit (Figure 1).

Each load device must receive its rated power to operate properly. If not enough is available, the device will not operate as it should. You should always be certain the voltage you are going to measure does not exceed the range of the voltmeter. This may be difficult if the V is unknown. If such is the case, you should always start with the highest range. Attempting to measure voltage higher than the voltmeter can handle may cause damage to the voltmeter. At times you may be required to measure a V from a specific point in the circuit to ground or a common reference point (Figure 8-15). To do this, first connect the black common test probe of the voltmeter to the circuit ground or common. Then connect the red test probe to whatever point in the circuit you want to measure.

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 electrical code – it specifies ‘Am per %Vd‘ (amp metres per % voltage drop) for each cable size. To calculate the VD for a circuit as a percentage, multiply the current (amps) by the cable length (metres); then divide this Ohm 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%.