The Voltage Drop Calculator

Voltage drop is the reduction in voltage that occurs as electricity flows through a cable, caused by the cable’s resistance. The longer and thinner the cable, and the higher the current, the greater the voltage drop meaning less usable voltage reaches your appliance or equipment.

In this article we explain the math behind our handy voltage drop calculator as well as how to use it, we hope you find it useful!

Disclaimer:

This calculator is for general information and educational purposes only and does not constitute professional electrical advice. Results are estimates only and may not be suitable for all systems or installations. Electrical systems can be hazardous so always follow manufacturer instructions, relevant UK regulations, and consult a qualified professional before installation or modification.

Any value for the voltage drop above 5% should be considered unsafe. To ensure compliance with BS 7671 value must be less than 3%.

How To Use Our Voltage Drop Calculator

To use our voltage drop calculator correctly you first need to know and understand the terms shown in the calculator which are:

• Cable Resistivity: It’s a measure of how much the cable resists electrical flow (see next section)
• Cable Area: The cross sectional area of the cable
• Total Cable Length: The length of the ground and live cable
• Load power: It’s the rated power of the appliance(s) you want to run
• Supply Voltage: In DC systems this is usually the battery voltage.

Which Cable Is The Voltage Drop Calculated For?

This depends on what you enter for Load Power, as the Supply Voltage is normally fixed (battery voltage).

For example:

• If you are powering a DC device (e.g. phone charger) then use the battery voltage as the supply voltage, and the rated power of the device as the load power.
• If you are planning to power an AC appliance using an inverter connected to a battery, use the battery voltage as the supply voltage and the inverter rated power as the load power.
• If you plan to have solar panels then the load power will be the maximum power the charge controller can harness from the panels and pass to the battery.

The Math Behind Our Voltage Drop Calculator

The aim of this section is to show you how our tool calculates the voltage drop in a 12V DC circuit with a load of 50W connected using a 2m AWG2 silver cables.

To calculate the voltage drop in a dc circuit we need to know the following:

• The cable’s characteristics: Material, area, length and resistivity
• The resistance of the cable
• Our circuit voltage (battery voltage)
• The loads we are running (their power in W)
• The current flowing in the circuit

According to the IET wiring regulations found on the BS 7671 the voltage drop for DC systems must be ≤ 3% to ensure safe system operation (it is a serious fire risk if voltage drops are high)

Also keep in mind that in our calculations we have to take into account the live and ground cable and anything that is connected to the circuit which includes the fuses, lugs, switches etc though we will assume for the sake of this example that they have negligible effect, though in practice you have to take them into account.

The Cable’s Characteristics

The cable’s characteristics can be found on the technical specification issued by the manufacturer. Those include the cable’s material, the cross sectional area of the cable and the length of the cable, using this information we can then calculate the resistance of the cable.

To calculate the resistance of the cable we use the formula below:

Resistance = Resistivity x Length / Area

We recommend using the manufacturer specification (or contact them) to obtain accurate values for the resistivity as most cables are made of alloys and thus not 100% pure.

Also keep in mind that length refers to the total length of the cables used in the circuit i.e both the live and ground cables.

In our hypothetical example we assumed we are using 100% pure silver which we can obtain its resistivity from the table below.

Material	Resistivity (10^-8 ohm.m)
Silver	1.6
Copper	1.7
Gold	2.3
Aluminium	2.7
Zinc	6
Brass	6.3
Nickel	7
Iron	9.9
Tin	11.5

Please note: For wires made of alloys (tinned copper) always consult the manufacturer specification, it is not usually as high as one may first perceive as long as it comes from a trusted manufacturer. For example a google search showed that tinned copper has a resistivity very similar to that of pure copper as the tin coating doesn’t constitute much of the material.

Finding The Circuit Voltage

This is the most important step as we use the circuit voltage to assess whether our voltage drop is acceptable or not for example in 48V systems the voltage drops can be up to 4 times larger than that in 12V systems and still be compliant with safety regulations.

The circuit voltage is determined by the power source, in a DC circuit it will be the battery and or the PV system.

We assumed we are using a 12V battery for this example. Keep in mind that the battery voltage varies depending on the state of charge of the battery. For example a fully charged lithium battery may have a voltage of approximately 13V.

The Circuit Current

To calculate the maximum current in our circuit we have to know the maximum loads we are running at any given time for this example its a 50W appliance. The current is calculated as:

Power = Current x Voltage

This result in a maximum current of 4.2A

The voltage drop

The voltage drop is calculated using this formula:

Voltage drop = Current x Cable Resistance

We assume our cable is made of silver with resistivity 1.6 x 10^-8 and sized at AWG2 (34 mm^2) and is 2 m long (live and ground)

This gives a cable resistance of 0.0009 ohms and thus a voltage drop of 0.004V which is within the acceptable 3% range (~0.033% of the total voltage)

Please note the more components you add to the circuit the higher the voltage drop so please take into account voltage drops caused by switches, fuses and anything else connected to the circuit. This is especially important for complex high power circuits.

Thank you so much for reading our article! Please feel free to share your feedback below!