# What Causes Voltage Drop?

**Post By: ** Ryan King
**On:** 01-08-2024

If you force anything through a length of pipe or cable, the amount of power at the supply end will inevitably diminish by the time it gets to the exit. Electrical cables are no different. When electric current travels through cables, its flow is always impeded by the cable's inherent DC resistance. This impedance means the conductor can't sustain a consistent voltage level throughout its length. Loss of voltage through all or part of the circuit is measured as voltage drop.

## What Causes Voltage Drop?

Voltage drop occurs normally when a current passes through an electrical cable. It results in the amount of voltage being reduced over a run of cable from input to output. This means that any electrical equipment will receive an input voltage below the value of the input supply. The level of voltage drop is also affected by passive elements in the circuits, such as other cables, connectors and contacts.

The resistance and reactance increase proportionally as you increase the length of the cable. Voltage drop is, therefore, a recognised problem with long cable runs, like those you might need in larger buildings or extended properties. Voltage drop can also have a significant impact on your electrical equipment. Your motors may run too slowly, overheat and burn out. Your heaters wont heat as well as they should, and your lights might start to flicker or dim.

This loss of voltage can easily be measured using a voltage drop calculator. You need to use this calculation for single-phase, line-to-line electrical circuits so you may properly size the conductors.

## How To Calculate Voltage Drop

So, what causes voltage drop is impedance in the flow of current through a length of cable. The voltage drop will also be greater in proportion to the resistance. You can use a voltmeter to check the level of voltage reduction, which you connect between the points where you want to measure the voltage drop. If you connect loads in series, the total of all the voltage drops in AC resistive circuits and DC circuits should add up to the total amount of voltage applied to the whole circuit.

When using a voltmeter to measure the voltage drop, you should always be sure that the proposed measurement isn't beyond the range of the voltmeter. If you dont know how much voltage is in question, this can make it difficult to keep within the voltmeters range. If you do set out from this position, you should always go from the top and start with the highest range. Trying to measure a higher voltage than it can handle could cause damage to the voltmeter.

You might occasionally have to measure a voltage drop between a specific point in your electrical circuit and some common reference point or ground. If this is the case, you should connect first to the circuits ground or common point, using the voltmeters black common test probe. The red test probe should only then be connected to the point in the circuit that you need to measure.

## Voltage Drop Formula

When you want to calculate the percentage of voltage drop precisely for any cable length, size and current, you must know the exact resistance of your cable type. To simplify these calculations, you can use this formula to work it out:

In this formula...

- X represents the voltage drop
- mV/A/m stands for milliVolts per metre per Amp
- Ib represents the design current
- I represents the total length of the circuit cable conductor

Therefore, an electrical conductor loses a amount of voltage for every metre it travels, measured in the number of millivolts (mV) for every amp of electrical power (design current) that flows through it. The whole calculation is then divided by 1,000 to get volts instead of millivolts.

The design current represents the maximum current that you expect the circuit to use (watts/voltage). If you dont know this amount, you can go with the rating of the protective device, as the design current will never exceed this amount. In order to arrive at a percentage voltage drop for your circuit, you can multiply the amount of design current (amps) by the length of your cable (metres). Take the resulting number and divide it by the value shown in the table to get the percentage.

The maximum amount of voltage drop permitted in the UK under BS7671 is 3% for lighting circuits (6.9V) and 5% for all other purposes (11.5V).

## How To Fix Voltage Drop

As with any kind of pressure loss over distance, the amount of voltage drop you experience relates directly to the length of cable and circuit you are using. To compensate for this loss, you can try using larger diameter cables, which will offer less impedance to the flow of current. You could also opt for copper conductor cables, as these are less prone to voltage drop. Your costs will inevitably rise as you try to minimise the problem, because copper is more expensive, as are larger-diameter cables. Its therefore of great importance to calculate the voltage drop over various cable options to find the optimum size and type.

## Counteracting Voltage Drop

Voltage drop in general causes the electrical load to work harder, as there is less voltage than there should be pushing the current. Each load device on your circuit can only operate properly when it receives its correctly rated voltage. Without sufficient voltage, an electrical device will present problems such as overheating motors, flickering lights or ineffective heaters. This means that they present a safety hazard and aren't working cost-effectively.

Any length or size of cable you use has some innate resistance, and the voltage drop is unavoidable when you run a current through it. However, using a larger diameter cable will mean the voltage drop is less than using a small size. To keep within British Standard prescribed limits, and for optimal performance of your electrical equipment, this is often the simplest solution. It will greatly improve the functionality, reliability, and safety of your electrical installations.

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