Cable Derating Calculator
Calculate cable current capacity based on environmental and installation factors
Cable Derating Calculator
Compliant with IEC 60364 Standards
Results:
Total Derating Factor: 0.000
Cable Derated Current (A): 0.00
How to Use the Calculator
Step 1: Initial Current
- Enter the initial current rating of the cable (e.g., from manufacturer data or standard tables).
- This is the current the cable can carry under ideal conditions.
Step 2: Select K-Factors
- k1 (Ambient Temperature): Choose the factor based on the surrounding air temperature.
- k2 (Grouping): Select the factor based on the number of cables or circuits grouped together.
- k3 (Thermal Insulation): Account for the thickness of any thermal insulation around the cable.
- k4 (Depth of Burial): For buried cables, select the factor based on the burial depth.
- k5 (Soil Resistivity): For buried cables, choose the factor based on the thermal resistivity of the soil.
- k6 (Harmonics): Select the factor based on the Total Harmonic Distortion (THD) in the system.
- k7 (Cable Type): Use the factor specific to the cable insulation type (e.g., PVC, XLPE).
Step 3: Calculate
- Click the "Calculate" button to determine the total derating factor and the derated current.
- The derated current is the maximum safe current the cable can carry under the specified conditions.
How to Calculate Cable Derating
Practical steps to move from nameplate ampacity to a derated, code-aligned value
1. Start from Base Ampacity
Pull the cable’s rated ampacity from the manufacturer or the governing code table at standard conditions (typically 30°C in free air or conduit).
Base ampacity = catalog or code rating at reference temperature
2. Apply Ambient Temperature (k1)
Use the correction factor that matches the site temperature profile. Hotter locations mean smaller k1 values and a lower allowable current.
Temperature-adjusted ampacity = Base ampacity × k1
3. Account for Grouping (k2) and Harmonics (k6)
Bundled circuits and nonlinear loads elevate heat, so apply both grouping and harmonic factors before trusting the final number.
Bundle or THD present → multiply by k2 and k6 before sizing
4. Include Installation Effects (k3, k4, k5)
Thermal insulation, burial depth, and soil resistivity all reduce heat dissipation. Choose realistic factors for each instead of best-case values.
Installation factors compound: insulation × depth × soil
5. Multiply for the Total Derating Factor
Combine every applicable k-factor to get one multiplier. This is the cornerstone that scales the initial current into a safe, corrected ampacity.
Total DF = k1 × k2 × k3 × k4 × k5 × k6 × k7
6. Derive the Final Derated Current
Multiply the initial current rating by the total derating factor, then confirm it still satisfies load and code requirements with margin for continuous duty.
Derated current = Initial current × Total DF
Cable Derating Chart
Reference multipliers for common temperature, grouping, and installation scenarios
| Condition | Typical Factor / Limit | Quick Note |
|---|---|---|
| Ambient 20°C | k1 ≈ 1.05 | Cool environments slightly boost ampacity. |
| Ambient 35°C | k1 ≈ 0.96 | Common indoor setting; modest reduction. |
| Ambient 45°C | k1 ≈ 0.87 | Hot plant rooms require notable derating. |
| 3–4 Cables Bundled | k2 ≈ 0.65–0.70 | Heat stacking quickly trims allowable current. |
| 10 Cables Bundled | k2 ≈ 0.48–0.52 | Use conservative spacing or ventilated trays. |
| 100 mm Insulation Wrap | k3 ≈ 0.85 | Insulation traps heat; avoid if possible. |
| Buried at 1.0 m | k4 ≈ 0.90 | Depth slows heat rejection in many soils. |
| Soil 2.0 K·m/W | k5 ≈ 0.80 | Drier soils run hotter; consider backfill. |
| THD at 20% | k6 ≈ 0.80 | Non-linear loads demand harmonic derating. |
| 50% Derating Scenario | DF ≈ 0.50 | Triggered by severe heat, bundling, or harmonics combined. |
Values are indicative; always verify against the applicable standard (IEC/NEC) and manufacturer data for the exact installation.
Cable Derating FAQs
How do you calculate cable derating?
Identify all relevant k-factors (temperature, grouping, insulation, burial, soil, harmonics, cable type), multiply them to form the total derating factor, and then multiply the cable’s base ampacity by that factor.
I_derated = I_initial × (k1 × k2 × k3 × k4 × k5 × k6 × k7)
How is the derating factor calculated?
Each environmental or installation condition has a published multiplier. Select the value that matches your scenario and multiply them together—this product is the total derating factor applied to the initial current rating.
What is the derating ampacity formula?
The common approach is the base ampacity from code or manufacturer tables multiplied by the total derating factor, which captures all applicable corrections.
Derated ampacity = Base ampacity × Total DF
What does 50% derating mean?
A 0.50 total factor means the cable can only carry half of its nameplate current under the stated conditions. Severe heat, tight bundling, or high harmonics often drive this outcome.
What is the 125% rule for conductors?
For continuous loads, codes typically require sizing conductors so the ampacity is at least 125% of the expected load, ensuring thermal headroom after derating is applied.
Required ampacity ≥ 1.25 × continuous load
What size wire for 300 amps?
Size selection depends on insulation type, installation, temperature rating, and derating factors. Large conductors such as 350–500 kcmil copper are common starting points, but always confirm with the derated ampacity and local code tables.
What is the 1.25 rule in electrical?
It is another way of referring to the 125% requirement used for continuous loads and certain overcurrent protection calculations, providing extra thermal margin.
How many amps is a 4 AWG wire rated for?
Typical base ampacity for 4 AWG copper THHN is around 85–95A depending on temperature rating, but the final allowable current must be adjusted using the total derating factor for the actual installation.
Final rating = Base ampacity × Total DF
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