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

    Ready to run the numbers?

    Open Cable Ampacity Calculator