⚡ Voltage Drop Calculator UK (BS 7671)

Calculate cable voltage drop and check compliance with UK wiring regulations

How it works: Enter your circuit details. Use the sliders to see how cable length, thickness, and current affect the voltage drop in real-time. The traffic light shows if your cable meets BS 7671 standards.
Circuit Parameters
Interactive Cable Settings
1m25 m200m
1mm²2.5 mm²50mm²
0.1A10.0 A100A

Live Cable Visualizer & BS 7671 Status

Source Load PASS
Voltage Drop (V)
0.00 V
Total loss
Voltage Drop (%)
0.00%
Of supply voltage
End Voltage
0.00 V
At the load
BS 7671 Status
PASS
Limit: 5%

Calculations based on DC resistance approximation at 20°C. For precise AC calculations, impedance (Z) should be used. Always consult a qualified electrician for final installations.

Understanding Voltage Drop in UK Electrical Installations

Voltage drop is a critical factor in electrical design. When current flows through a cable, resistance causes a loss of voltage. If the drop is too high, appliances won't work efficiently, lights will dim, and cables can overheat. Our free Voltage Drop Calculator helps electricians, engineers, and DIYers ensure their circuits comply with the UK wiring regulations (BS 7671).

Step-by-Step Instructions

  1. Supply Voltage: Select 230V for standard UK single-phase circuits or 400V for three-phase industrial supplies.
  2. Conductor Material: Choose Copper (standard for most UK wiring) or Aluminum (often used for large service tails).
  3. Application Type: Lighting circuits have a stricter limit (max 3% drop) compared to power/socket circuits (max 5% drop).
  4. Use the Sliders: Adjust cable length, cross-sectional area (mm²), and load current (Amps) to see the real-time effect on the cable visualizer.
  5. Check the Status: The traffic light turns Green (PASS) if within BS 7671 limits, or Red (FAIL) if the cable is too long or too thin.

Understanding Your Results

  • Voltage Drop (V): The actual volts lost between the consumer unit and the appliance.
  • Voltage Drop (%): The percentage loss relative to your supply voltage. This is what BS 7671 regulates.
  • End Voltage: The actual voltage arriving at your appliance. A 230V supply with a 5% drop leaves only 218.5V at the socket.
  • BS 7671 Status: A quick compliance check based on Appendix 4 of the IET Wiring Regulations.

Frequently Asked Questions

What is the maximum allowed voltage drop in the UK?

According to BS 7671 (IET Wiring Regulations), the maximum voltage drop is 3% for lighting circuits (6.9V on a 230V supply) and 5% for other uses like power sockets (11.5V on a 230V supply).

Why does cable thickness (mm²) matter?

A thicker cable (higher mm²) has lower electrical resistance. This means less voltage is lost as heat over the same distance, allowing you to run longer cables or higher currents safely.

Copper vs Aluminum: Which should I use?

Copper has lower resistivity (0.01724 Ω·mm²/m) compared to Aluminum (0.0282 Ω·mm²/m). Aluminum cables need to be roughly 1.6 times larger in cross-section to carry the same current with the same voltage drop.

Does temperature affect voltage drop?

Yes. This calculator uses resistance at 20°C. In real-world scenarios, cables operating at their maximum temperature (e.g., 70°C for PVC) will have higher resistance and therefore a slightly higher voltage drop. Professional software accounts for this.

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