Cable Ampacity Calculator Formula

Understand the math behind the cable ampacity calculator. Each variable explained with a worked example.

Use the Cable Ampacity Calculator

Formulas Used

Derated Ampacity

ampacity = cross_section_mm2 * current_density * derating_factor

Base Ampacity

base_ampacity = cross_section_mm2 * current_density

Max Power at This Ampacity

max_power = cross_section_mm2 * current_density * derating_factor * voltage

Max Power

max_power_kw = cross_section_mm2 * current_density * derating_factor * voltage / 1000

Variables

VariableDescriptionDefault
cross_section_mm2Conductor Cross Section(mm²)6
current_densityAllowable Current Density(A/mm²)5
derating_factorDerating Factor0.8
voltageSystem Voltage(V)230

How It Works

Cable Ampacity Estimation

Ampacity is the maximum continuous current a conductor can carry without exceeding its temperature rating.

Formula

Ampacity = Cross Section x Current Density x Derating Factor

Current Density Guidelines

  • Open air: 5-8 A/mm^2
  • In conduit: 3-5 A/mm^2
  • Underground: 2-4 A/mm^2

    • Derating Factors

  • Ambient temperature above 30 C: 0.7-0.9
  • Multiple cables in conduit: 0.5-0.8
  • Direct burial: 0.7-0.85

    • Always verify with NEC/IEC tables for your specific installation.

    Worked Example

    6 mm^2 copper cable, 5 A/mm^2, derating 0.8, at 230 V.

    cross_section_mm2 = 6current_density = 5derating_factor = 0.8voltage = 230
    1. 01Base ampacity: 6 x 5 = 30 A
    2. 02Derated: 30 x 0.8 = 24 A
    3. 03Max power: 24 x 230 = 5,520 W = 5.52 kW

    Frequently Asked Questions

    Is this a substitute for NEC/IEC tables?

    No. This provides estimates. Always use the applicable electrical code tables for final cable sizing in real installations.

    What happens if I exceed ampacity?

    The cable overheats, insulation degrades, and eventually a fire hazard develops. Overcurrent protection (breakers/fuses) should prevent this.

    How does cable length affect sizing?

    Longer cables have more resistance, causing voltage drop. You may need to upsize the cable beyond ampacity requirements to limit voltage drop.

    Ready to run the numbers?

    Open Cable Ampacity Calculator