Voltage Drop Calculator

Where:

• Vd​ is the voltage drop in Volts (V).
• $mV/A/m$ is the millivolts per amp per metre for the specific cable you’re using. You can find this value in Appendix 4 of BS 7671.
• Ib​ is the design current of the circuit in Amps (A).
• $L$ is the length of the circuit in metres (m).

We divide by 1000 to convert the result from millivolts to Volts.

How to Calculate Voltage Drop

There are two primary ways to calculate voltage drop. The first is a detailed method based on the fundamental physics of the cable, which is how our calculator works. The second is a simplified, ‘worst-case’ method using values from BS 7671, which is the standard for proving compliance in the UK.

Let’s look at both, using the same example for each: a 20A radial circuit using 2.5mm² copper cable over a distance of 22 metres.

Method 1: The ‘First Principles’ Calculation

This method calculates the cable’s actual resistance from scratch and then uses Ohm’s Law to find the voltage drop. It is very precise for the conditions you specify.

Step 1: Calculate the cable’s total resistance (R). The formula for resistance is: R=Aρ×L​

• $\rho$ (Rho) is the resistivity of the material. For copper, we know it to be about 1.72×10−8Ω⋅m at 20°C.
• $L$ is the total length of the wire in metres. For a circuit, the current travels out and back, so we must double the circuit length ($22m\times 2=44m$).
• $A$ is the cross-sectional area of the wire in square metres (2.5mm2=2.5×10−6m2).

R=2.5×10−6(1.72×10−8)×44​=0.302Ω

Step 2: Calculate the voltage drop (Vd​). Now we use Ohm’s Law, $V=I\times R$:

• $I$ is the current (20A).
• $R$ is the resistance we just calculated (0.302 Ω).

Vd​=20×0.302=6.04V

This result is very close to what a detailed calculator would show for a 20°C ambient temperature.

Method 2: The BS 7671 Compliance Calculation

This is the standard method used by UK electricians to ensure an installation is safe and compliant. It’s simpler because it uses a pre-calculated value from the regulations that already accounts for a worst-case scenario.

Step 1: Find the (mV/A/m) value for your cable. Look up the value in Appendix 4 of BS 7671. For 2.5mm² copper twin and earth cable, this value is 18 mV/A/m.

Important: This value is based on the cable’s total impedance (resistance + reactance) at its maximum operating temperature (e.g., 70°C), making it a safe, all-in-one figure.

Step 2: Use the BS 7671 voltage drop formula. The formula is: Vd​=1000(mV/A/m)×Ib​×L​

• $(mV/A/m)$ is 18.

• Ib​ is the design current (20A).

• $L$ is the circuit length (22m).

Vd​=100018×20×22​=7.92V

What is Voltage Drop?

Voltage drop is the reduction in electrical potential (voltage) along the path of a current-carrying wire. In simple terms, as electricity travels down a cable from the consumer unit to an appliance, it loses a bit of energy, resulting in a slightly lower voltage at the far end.

This matters for a few key reasons:

• Compliance: BS 7671 (The Wiring Regulations) sets specific limits on acceptable voltage drop to ensure safety and proper functioning of installations.

• Performance: Too much voltage drop can cause equipment to run inefficiently, flicker lights, and lead to poor performance from motors or electronics.

• Safety: In extreme cases, excessive voltage drop can indicate an undersized cable for the load, which can lead to overheating and pose a fire risk.