Cable Ampacity Derating Factor

Adjusts a cable's rated current-carrying capacity for installation conditions such as grouping and ambient temperature.

Written by the Vorzep Team

Electrical InstallationsCable Sizing

Cable Ampacity Derating Calculator I_derated = I_rated · k₁ · k₂

I_derated = I_rated × k₁ × k₂

I_derated = derated ampacity (A) · I_rated = rated ampacity (A) · k₁ = temperature derating · k₂ = grouping derating

⟹ Solve ⟷ I_derated, I_rated, k₁, k₂

Rated Current (I_rated)

k₁ (Temp. derating)

—

k₂ (Grouping derating)

—

Derated Current (I_derated)

Please fix the errors above.

Solve for: I_derated I_rated k₁ k₂

Presets:

Derated Current —

—

I_rated: — k₁: — k₂: — I_derated: —

✓ Copied!

Derated Current Gauge —

Low (< 50 A) Moderate (50–150 A) High (> 150 A)

I_derated = I_rated × k₁ × k₂ · Derating factors account for temperature and cable grouping.

Variables

Symbol	Quantity	Unit
I_derated	Derated ampacity	A
I_rated	Rated ampacity (base)	A
k₁	Temperature correction factor	dimensionless
k₂	Grouping correction factor	dimensionless

What it means

A cable’s current‑carrying capacity (ampacity) must be derated when conditions differ from standard (e.g., higher ambient temperature, grouping of cables, or burial). The derated current is I_derated = I_rated × k₁ × k₂, where k₁ and k₂ are correction factors. For example, if the temperature is above 30°C, a factor less than 1 is applied. If cables are bundled, a grouping factor reduces the ampacity. Example: A cable rated at 100A at 30°C, with temperature factor 0.9 and grouping factor 0.8, would have a derated ampacity of 100 × 0.9 × 0.8 = 72A. This ensures the cable does not overheat under the actual installation conditions.

Worked example

Cable Ampacity Derating – Practical Example

Real‑World

Scenario: A cable rated 100 A in free air is installed in a cable tray with 4 other cables (grouping factor 0.8) and ambient temperature 45 °C (temperature factor 0.94). Calculate the derated ampacity.

Parameter	Value
I_rated	100 A
k_temp	0.94
k_group	0.8
Formula	`I_derated = I_rated × k_temp × k_group`

1I_derated = 100 × 0.94 × 0.8 = 75.2 A

Final Design 75.2 A ✓ Derated

Why: Derating factors account for ambient temperature and grouping – actual safe current is reduced accordingly.

Common mistakes

Watch for unit mismatches (W vs kW, single- vs three-phase) and remember to include power factor or efficiency where the formula requires it.

Applications

Cable ampacity derating: I_derated = I_rated × k₁ × k₂ applies correction factors for temperature and grouping to avoid overheating. Engineers use it to size cables in non‑ideal conditions, ensuring safe operation and compliance with standards. This is essential for practical cable installation.

Cable sizing for derating factors (temperature, grouping)
Installation design in cable trays and ducts
Compliance with ampacity tables and standards
Prevention of overheating and insulation damage
Educational understanding of derating principles