Cable Size Calculator

Cable Size Calculator: How to Select the Right Wire

Selecting the correct cable size is a fundamental requirement for any electrical installation, ensuring both safety and optimal performance. An accurate cable size calculator helps electricians, engineers, and DIY enthusiasts determine the appropriate wire gauge based on key factors such as current carrying capacity, voltage drop, installation method, and ambient temperature. In the UK, compliance with BS 7671 (18th Edition Wiring Regulations) is mandatory, making precise calculations crucial to avoid hazards like overheating, excessive voltage drop, or non-compliance with legal standards. Whether for residential wiring, commercial installations, or industrial projects, using a reliable cable size calculator ensures that conductors can handle the expected load while maintaining efficiency and longevity. Understanding how to calculate cable ampacity, derating factors, and voltage drop percentages prevents undersized wiring (a fire risk) and oversized cables (an unnecessary cost), making this an indispensable tool for anyone working with electrical systems. Proper cable sizing also impacts energy efficiency, circuit protection coordination, and long-term reliability, ensuring that installations meet both IET (Institution of Engineering and Technology) guidelines and practical performance expectations.

This guide explains how to calculate cable size for any UK electrical installation.

Key Factors in Cable Sizing

1. Current Carrying Capacity

Depends on:

• Load current (amps)
• Installation method (clipped direct, conduit, etc.)
• Ambient temperature
• Grouping factors
• 2. Voltage Drop Calculation

Maximum allowable drop:

• Lighting: 3%
• Other circuits: 5%

Formula:

Voltage Drop = (mV/A/m) × Current × Length ÷ 1000

(mV/A/m values from BS 7671 Tables)

Short Circuit Protection

Cable must withstand fault currents until protective device operates

UK Cable Size Calculator Steps

Determine design current (Ib)

• For motors: Full load current
• For circuits: Sum of connected loads

Select protective device rating (In)

• MCB/fuse rating ≥ Ib

Apply correction factors

• Ambient temperature (Ca)
• Grouping (Cg)
• Thermal insulation (Ci)

Calculate minimum current capacity (It)

It ≥ In ÷ (Ca × Cg × Ci)

Select cable from tables

• BS 7671 Appendix 4
• IET On-Site Guide tables

Verify voltage drop

• Ensure within permitted limits

Example Calculation

Scenario:

• 32A ring final circuit
• 20m run in conduit with 3 other circuits
• Ambient temperature: 35°C
• PVC-insulated cable

Calculation:

1. Ib = 32A
2. In = 32A MCB
3. Correction factors:

• Ca (35°C) = 0.94
• Cg (4 circuits) = 0.65

1. It = 32 ÷ (0.94 × 0.65) = 52.4A
2. From tables: 6mm² (53A capacity)
3. Voltage drop check:

• 6mm² = 7.3mV/A/m
• Drop = (7.3 × 32 × 20) ÷ 1000 = 4.67V (2% of 230V) – OK

Common UK Cable Types

Cable Selection Mistakes to Avoid

1. Ignoring voltage drop on long runs
2. Overlooking correction factors for installation method
3. Underestimating future loads
4. Mixing conductor materials (copper/aluminium)
5. Using undersized CPC (circuit protective conductor)