Cable Voltage Drop Calculator

Table of contents

• What is the Cable Voltage Drop Calculator?
• How millivolts per amp-metre (mV/A/m), circuit length, phase factors, and conductor materials are calculated
• Example Calculation: 32A shower circuit over 25m using 4.0mm² copper cable on a 230V single-phase supply
• BS 7671 Voltage Drop Limits: Understanding the 3% lighting and 5% power rules
• Conductor cross-sectional area (CSA) upsizing: Fixing excessive drop across long runs
• The Essential UK Cable Sizing & BS 7671 Electrical Compliance Checklist
• How to use the Cable Voltage Drop Calculator
• Frequently Asked Questions (FAQs)

Use this Cable Voltage Drop Calculator to compute total electrical voltage drop, determine end-of-line terminal voltage, evaluate percentage drop against supply voltage, and verify strict BS 7671 IET Wiring Regulations compliance for single-phase and three-phase AC circuits. Calculating voltage drop accurately is a critical electrical design requirement for electricians, electrical engineers, and DIY installers across the UK. Excessive voltage drop causes equipment overheating, flickering lighting, electric motor failure, and non-compliance with UK building regulations.

What is the Cable Voltage Drop Calculator?

The Cable Voltage Drop Calculator is an advanced electrical engineering verification tool designed to evaluate voltage drop across low-voltage AC electrical installations according to British Standard BS 7671.

By selecting your electrical supply system (230V Single Phase AC or 400V Three Phase AC), specifying circuit function limits (Power / Sockets / Motors max 5% vs Lighting Circuit max 3%), entering design load current (Ib in Amps), setting cable route run distance in metres, choosing conductor cross-sectional area (1.5mm² through 35mm²), and selecting conductor material (Copper or Aluminium), this calculator provides immediate voltage loss figures, percentage drops, end-of-line load voltages, and pass or fail compliance statuses.

How millivolts per amp-metre (mV/A/m), circuit length, phase factors, and conductor materials are calculated

The calculator evaluates electrical voltage drop across low-voltage installation cables by multiplying design current, route distance, and specific conductor voltage drop reference factors derived from BS 7671 Table 4D2B and Table 4D5.

To maintain complete transparency, the tool follows these logical electrical engineering calculation steps:

• Millivolts per Amp-Metre (mV/A/m) Reference Lookup:
  • Every conductor size and type possesses a specific internal resistance value expressed in millivolts per amp per metre (mV/A/m). For standard single-phase 230V copper cables:
    • 1.5mm² Conductor: 29 mV/A/m
    • 2.5mm² Conductor: 18 mV/A/m
    • 4.0mm² Conductor: 11 mV/A/m
    • 6.0mm² Conductor: 7.3 mV/A/m
    • 10.0mm² Conductor: 4.4 mV/A/m
    • 16.0mm² Conductor: 2.8 mV/A/m
• Conductor Material Adjustment Factor:
  • Aluminium conductors have higher resistivity than copper conductors (~1.64x multiplier). The calculator automatically scales the base mV/A/m factor up by 1.64x if aluminium is selected.
• Single-Phase vs Three-Phase System Factors:
  • • For 230V Single-Phase AC, current flows out along the live phase conductor and returns along the neutral conductor (two-wire voltage drop).
    • For 400V Three-Phase AC, balanced line currents cancel out in the neutral conductor. The calculator applies a balanced line factor multiplier of 0.866 (√3 / 2) to compute line-to-line voltage drop across 400V three-phase systems.

  Voltage Drop (Volts) = (mV/A/m Factor x Design Current Ib x Route Length L) / 1000

Example Calculation: 32A shower circuit over 25m using 4.0mm² copper cable on a 230V single-phase supply

To observe how cable cross-sectional area and circuit length affect total voltage loss and regulatory compliance, consider this standard domestic installation calculation.

Example: An electrician installs a 32 Amp electrical shower circuit fed by a 230V single-phase consumer unit over a one-way cable run distance of 25 metres using 4.0mm² twin and earth copper cable.

• Circuit Parameters:
  • Supply Voltage: 230 V Single-Phase AC
  • Design Current (Ib): 32 Amps
  • Cable Length (L): 25 Metres
  • Conductor Size: 4.0mm² Copper (mV/A/m factor = 11 mV/A/m)
  • Circuit Function: Power Circuit (BS 7671 limit = 5.0% / 11.50 Volts)
• Step-by-Step Mathematical Evaluation:
  • Calculated Voltage Drop: (11 mV/A/m x 32 Amps x 25 Metres) / 1000 = 8.80 Volts
  • Percentage Drop of Supply Voltage: (8.80 V / 230 V) x 100 = 3.83% Drop
  • End-of-Line Terminal Voltage: 230 V – 8.80 V = 221.20 Volts
  • BS 7671 Compliance Status: PASS (3.83% is well below the maximum allowed 5.0% threshold of 11.50 Volts).

The calculation proves that running 4.0mm² copper cable over 25 metres at 32 Amps delivers 221.20 Volts to the appliance terminals, satisfying BS 7671 regulatory standards.

BS 7671 Voltage Drop Limits: Understanding the 3% lighting and 5% power rules

Compliance with BS 7671 (IET Wiring Regulations) requires that the voltage drop between the origin of an installation (typically the consumer unit or main switchboard) and the terminal load outlet does not exceed specific percentage thresholds:

• Lighting Circuits (3.0% Maximum Allowed Drop):
  • 230V Single-Phase Limit: 6.90 Volts
  • 400V Three-Phase Limit: 12.00 Volts
  • Lighting circuits carry a strict 3% limit because voltage fluctuations cause severe lamp flickering, LED driver instability, reduced lumen output, and premature driver failure.
• General Power, Sockets, Heating & Motor Circuits (5.0% Maximum Allowed Drop):
  • 230V Single-Phase Limit: 11.50 Volts
  • 400V Three-Phase Limit: 20.00 Volts
  • Power circuits are permitted up to a 5% drop. Excessive voltage drop on power circuits causes electric motors to draw higher current to maintain torque, leading to motor winding overheating, nuisance breaker tripping, and inefficient operation.

Conductor cross-sectional area (CSA) upsizing: Fixing excessive drop across long runs

When a calculated voltage drop exceeds BS 7671 legal thresholds on long cable runs, increasing the conductor cross-sectional area (CSA) is the primary method to reduce internal resistance:

• Halving Conductor Resistance: Increasing cable CSA from 2.5mm² (18 mV/A/m) to 4.0mm² (11 mV/A/m) reduces internal resistance and voltage drop by roughly 39%. Moving up to 6.0mm² (7.3 mV/A/m) reduces voltage drop by over 59% compared to 2.5mm².
• Outbuilding & Sub-Main Feeders: Supplying detached garages, garden offices, or EV chargers over 30 to 50 metres often requires upsizing steel wire armoured (SWA) cables to 10mm² or 16mm² purely to satisfy voltage drop limits, even if a smaller cable could safely carry the continuous current rating.

The Essential UK Cable Sizing & BS 7671 Electrical Compliance Checklist

Adhering to a structured verification process ensures safe electrical designs that pass initial inspection and testing:

✅ Design Load & Route Distance Verification

• Verify Design Current (Ib) and Protection Rating (In): Ensure design current calculations reflect actual maximum simultaneous demand rather than overestimating circuit load.
• Measure Accurate One-Way Route Length: Measure the true physical cable routing path including vertical wall drops and bend radii, rather than simple straight-line distances.

✅ Installation Conditions & Correction Factors

• Apply Thermal Derating Factors (Cg, Ca, Ci): Ensure cable current-carrying capacity is derated for grouping, high ambient temperatures, or thermal insulation contact before verifying voltage drop.
• Account for Cable Temperature Rise: Operating conductors at maximum load increases internal copper resistance. Ensure calculations account for conductor operating temperatures under full load.

✅ Inspection & Terminal Testing Verification

• Test End-of-Line Terminal Voltage: Measure actual operating terminal voltage under load during EIC or EICR commissioning to verify physical results match calculated design figures.
• Check Consumer Unit Supply Voltage: Verify baseline supply voltage at the main intake switch; UK nominal voltage is 230V (+10% / -6%), giving an acceptable supply range between 216.2V and 253.0V.

How to use the Cable Voltage Drop Calculator

1. Electrical Supply System: Select your supply phase system (“230V Single Phase AC” or “400V Three Phase AC”) to set the base voltage calculation.
2. Circuit Function (BS 7671 Limit): Choose your circuit application type (“Power / Sockets / Motors Max 5%” or “Lighting Circuit Max 3%”) to apply regulatory threshold limits.
3. Design Current – Ib (Amps): Input your circuit design load current in Amps (or the circuit protective device rating, such as 16A, 32A, or 40A).
4. Cable One-Way Length (Metres): Enter the one-way physical cable route length from your consumer unit or distribution board to the terminal load point.
5. Conductor Cross-Sectional Area (CSA mm²): Select your conductor cable size (from 1.5mm² up to 35mm²).
6. Conductor Material: Choose your conductor material (“Copper” for standard Twin and Earth / SWA or “Aluminium”).
7. Review Voltage Drop Analysis: Examine the results panel to view total voltage loss in Volts, percentage drop, calculated end-of-line load voltage, and BS 7671 pass or fail status.

Frequently Asked Questions (FAQs)

What is the maximum allowed voltage drop under UK BS 7671 Wiring Regulations?
Under BS 7671 (IET Wiring Regulations), the maximum permitted voltage drop between the installation origin and terminal outlets is 3% (6.90 Volts on a 230V supply) for lighting circuits and 5% (11.50 Volts on a 230V supply) for general power, sockets, motors, and heating loads.

Why do lighting circuits have a stricter voltage drop limit than power circuits?
Lighting circuits carry a stricter 3% limit because voltage fluctuations cause visible lamp flickering, LED driver instability, reduced light output, and premature electronic ballast failure. Power circuits allow up to 5% because heavy motors and resistive heaters operate safely within slightly broader voltage tolerances.

How do I fix a cable circuit that fails the BS 7671 voltage drop check?
If a circuit fails voltage drop compliance, the primary solution is to increase the cable conductor cross-sectional area (CSA)—for example, upgrading from 2.5mm² to 4.0mm² or 6.0mm². Alternative remedies include shortening the physical cable route or redistributing heavy single-phase loads across a 400V three-phase supply.

What happens if electrical equipment is operated with excessive voltage drop?
Operating appliances under excessive voltage drop causes electric motors to draw higher current to compensate, leading to internal winding overheating, nuisance breaker tripping, reduced motor torque, slow heating element performance, and equipment failure.

Sources

• IET Wiring Regulations (BS 7671) – Official UK electrical installation standards, Table 4D2B and Table 4D5 millivolt drop reference data
• NICEIC Technical Publications – Guidance notes on cable sizing, voltage drop calculations, and electrical inspection testing
• NAPIT Code of Practice – Practical electrical installation guidelines for single and three-phase low-voltage circuits

This calculator provides estimated voltage drop values and regulatory compliance statuses based on standard BS 7671 mV/A/m reference tables. Final cable design verification must always consider installation method derating factors (thermal insulation, grouping, ambient temperature) and be certified by a qualified electrician.