Cable Derating Calculator

Electrical cables are rated for current under ideal lab conditions, but real-world installation factors — heat, bundling, burial depth, and more — reduce how much current they can safely carry. Enter your Initial Cable Current Rating, then select your ambient temperature, grouping, thermal insulation, burial depth, soil resistivity, harmonics, and cable type correction factors into the Cable Derating Calculator to find the Derated Cable Current. Secondary outputs include the Total Derating Factor, Derating Percentage, and Current Reduction in amperes.

A

Base ampacity of cable under ideal conditions

Results

Derated Cable Current

--

Total Derating Factor

--

Derating Percentage

--

Current Reduction

--

Results Table

More Electrical & Electronics Tools

Frequently Asked Questions

How do you calculate cable derating?

Cable derating is calculated by multiplying the initial cable current rating by all applicable derating factors. The formula is: Derated Current = Initial Current × K1 × K2 × K3 × K4 × K5 × K6 × K7, where each K factor represents a specific environmental or installation condition.

How is the derating factor calculated?

The total derating factor is the product of all individual correction factors including ambient temperature (K1), grouping (K2), thermal insulation (K3), burial depth (K4), soil resistivity (K5), harmonics (K6), and cable type (K7). Each factor accounts for specific conditions that affect heat dissipation.

What is the derating ampacity formula?

The derating ampacity formula is: Final Ampacity = Base Ampacity × (K1 × K2 × K3 × K4 × K5 × K6 × K7). This gives you the maximum safe current the cable can carry under the specific installation and environmental conditions.

What does 50% derating mean?

50% derating means the cable can only safely carry half of its rated current capacity. This occurs when the total derating factor equals 0.5, typically due to high ambient temperatures, multiple cables grouped together, or poor heat dissipation conditions.

What is the 125% rule for conductors?

The 125% rule requires that continuous loads be calculated at 125% of their actual value when sizing conductors and overcurrent protection devices. This provides a safety margin and accounts for the fact that continuous loads operate for 3 hours or more at their maximum rating.

Why is cable grouping important for derating?

When cables are grouped together, they generate collective heat that reduces each cable's ability to dissipate heat effectively. The grouping factor accounts for this thermal interaction, with more cables in a group requiring greater derating to prevent overheating.

How does ambient temperature affect cable capacity?

Higher ambient temperatures reduce a cable's current-carrying capacity because the cable has less temperature difference available for heat dissipation. Standard cable ratings assume 30°C ambient temperature, so higher temperatures require derating while lower temperatures may allow increased capacity.

When should harmonics derating be applied?

Harmonics derating should be applied when the electrical system has significant non-linear loads like variable frequency drives, LED lighting, or computer equipment. These create harmonic currents that increase heating in the neutral conductor and require additional derating for safety.