Voltage Drop Calculator - BS 7671 Compliant

Calculate voltage drop and check BS 7671 compliance for electrical installations

Calculate Voltage Drop

Check if your cable installation meets BS 7671 voltage drop requirements

Amps

The current flowing through the cable

meters

One-way cable run distance

Cross-sectional area of the conductor

How to Use the Voltage Drop Calculator

Use this calculator to verify that your cable installation meets BS 7671 voltage drop requirements before energizing the circuit.

  1. Enter the load current - The actual current (in Amps) flowing through the cable under normal operation.
  2. Specify the cable length - The one-way distance (in meters) from the distribution board to the load point.
  3. Select the cable size - The cross-sectional area of the conductor in mm² (e.g., 2.5mm², 4.0mm², etc.).
  4. Choose the voltage - 230V for single-phase or 400V for three-phase installations.
  5. Select conductor material - Copper is standard in the UK. Aluminium has higher resistance.

Understanding Voltage Drop

Voltage drop is the reduction in voltage that occurs as electrical current flows through a conductor due to the conductor's resistance. Excessive voltage drop can cause:

  • Poor performance of electrical equipment
  • Dimming of lights
  • Overheating of motors
  • Reduced equipment lifespan

BS 7671 Limits

  • Lighting circuits: Maximum 3% voltage drop
  • Other circuits: Maximum 5% voltage drop

BS 7671 Regulation 525 - Voltage Drop

This calculator checks compliance with BS 7671:2018+A2:2022 Regulation 525 - Voltage drop in consumers' installations.

Regulation 525.1

Under normal service conditions, the voltage drop between the origin of the installation (usually the supply terminals of the consumer unit) and any socket outlet or other point of utilization shall not exceed the values given in Appendix 12:

  • 3% for lighting circuits - To prevent visible dimming
  • 5% for other uses - Power circuits, heating, etc.

How Voltage Drop is Calculated

The calculator uses the following formula:

Single-phase: Vdrop = 2 × I × L × R
Three-phase: Vdrop = √3 × I × L × R

Where: I = Current (A), L = Length (m), R = Resistance (Ω/m)

Note: Voltage drop is only one consideration. Cable selection must also account for current-carrying capacity, overcurrent protection, and fault protection requirements.