Cable Cross-Section Sizing Calculator

Cable Sizing in the IEC/Metric World Outside North America, cable size is specified in mm² (square millimetres) rather than AWG. Standard sizes in IEC 60364 are: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300 mm². A higher number means a larger, thicker conductor with more capacity.

Two Selection Criteria As with AWG selection, IEC cable sizing must satisfy two requirements simultaneously:

1. Ampacity, the cable must carry the load current without overheating based on installation method and ambient temperature.
2. Voltage drop, the voltage at the load must not fall more than 4% below the supply voltage (IEC guideline; some standards allow 3%).

The selected size must pass BOTH tests.

Ampacity by Installation Method How a cable is installed dramatically affects how quickly heat dissipates:

• In conduit/raceway: worst cooling: use base ampacity values.
• Clipped direct to surface: 20% more ampacity than conduit (better cooling).
• In free air (suspended): best cooling: approximately 30% more than conduit.
• Underground: approximately 10% less than conduit (soil thermal resistance). These factors are multiplied against the base ampacity for the selected cross-section.

Material Factor Copper conductors carry approximately 25% more current than aluminum conductors of the same cross-section. Or equivalently, aluminum requires a cross-section approximately 1.6× larger for the same ampacity. Copper is standard for fixed wiring; aluminum is used for large service cables and overhead lines.

Voltage Drop Formula ΔV = 2 × ρ × L × I / A Where: ρ = resistivity of copper = 1.72 × 10⁻⁸ Ω·m (aluminum = 2.82 × 10⁻⁸ Ω·m) L = one-way cable length in metres I = load current in amps A = conductor cross-sectional area in m² (so convert mm² by dividing by 1,000,000)

The factor of 2 accounts for the outgoing and return conductors. Voltage drop percentage = (ΔV / V_supply) × 100. IEC 60364-5-52 recommends keeping the drop below 4% for final circuits (3% for some applications).

Reading the Results The calculator finds the smallest standard cross-section that satisfies both ampacity and voltage drop. If voltage drop forces a larger size than ampacity requires, the table will show which criterion is binding. For very long runs, voltage drop can require a cable two or three sizes larger than ampacity alone demands.

Practical Tips Always use the NEXT size up from the calculated minimum — cable sizes are a fixed series. For three-phase systems, voltage drop is reduced by a factor of √3 compared to single-phase for the same power. In hot climates (ambient above 30°C), derate ampacity by approximately 3–5% per 5°C above 30°C. In cable trays with many cables touching, further derating applies — consult IEC 60364-5-52 correction tables.