Voltage Drop & Cable Size Calculator

Calculate voltage drop and select optimal cable size for electrical installations. Supports single and three-phase systems. Our tool helps ensure safety and efficiency in your electrical projects.

100% Accurate • Industry Standard

Why Voltage Drop Matters in Electrical Systems

What is Voltage Drop?

Voltage drop is the reduction in voltage that occurs as electricity travels through a conductor. Excessive voltage drop can cause equipment malfunction, reduced efficiency, and potential safety hazards.

Importance of Proper Cable Sizing

Selecting the correct cable size is crucial for:

Preventing overheating and fire hazards
Ensuring equipment operates at peak efficiency
Complying with electrical codes and standards
Reducing energy waste and operational costs

Key Electrical Standards

Our calculator follows industry standards including:

NEC (National Electrical Code)
IEC (International Electrotechnical Commission)
BS 7671 (UK Wiring Regulations)

VD Calculator

Enter your electrical system parameters to calculate voltage drop and optimal cable size

System Parameters

System type

Supply voltage (V)

Load input mode

Power factor (for kW → A)

Load (A)

Cable one-way length (m)

Cable Specifications

Conductor material

Conductor size (mm²)

Cable core type

Derating factor

Note: Multi-core cables typically require derating (80-85%). Single-core cables are generally used for higher currents.

Environmental Factors

Temperature (°C) — conductor ambient

Allowed voltage drop (%)

Conductor Data Reference

Default: copper @20°C

The table below uses typical DC resistances at 20°C (Ω/km) converted to Ω/m.

mm²	Ω/km	Ω/m

Default Ampacity Guide

These are rough typical continuous current values for copper insulated cables (single-core in free air / typical conditions). Use only as guide.

mm²	Ampacity (A)

Core Type Information

Single-core cables:
• Used for high current applications
• Typically installed in conduits or trays
• Require derating when bundled together

3-core cables:
• Used for balanced three-phase systems
• Includes three phase conductors
• Common in industrial applications

4-core cables:
• Used for three-phase + neutral systems
• Includes three phases and a neutral
• Used where neutral current is significant

Understanding Voltage Drop Calculations

Voltage Drop Formulas

Single-phase (L-N):
VD = 2 × I × L × R

Three-phase:
VD = √3 × I × L × R

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

Temperature Correction

Conductor resistance increases with temperature:

R_T = R₂₀ × [1 + α × (T - 20)]

Where:
α = Temperature coefficient
(0.00393/°C for copper)
T = Conductor temperature (°C)

Industry Standards

Recommended voltage drop limits:

Lighting circuits: 3% max
Power circuits: 5% max
Critical systems: 1-2% max

Always consult local electrical codes for specific requirements.