Part 6 – Cable Derating Factors
Introduction
After calculating the Full Load Current and selecting a preliminary cable size, many engineers believe that the cable selection process is complete. However, this assumption is one of the most common mistakes in electrical design.
The current carrying capacity published in cable manufacturer catalogues is determined under standard reference conditions. These reference conditions are carefully controlled during testing and rarely represent the actual installation environment found in utility-scale solar power plants.
In real projects, cables may be installed:
- Underground in high thermal resistivity soil.
- Inside cable trenches.
- In cable ducts.
- On cable trays carrying multiple circuits.
- In areas with high ambient temperatures.
- At varying burial depths.
- Along routes exposed to direct sunlight.
Each of these conditions affects the cable's ability to dissipate heat. When heat dissipation decreases, the conductor temperature rises for the same load current. To maintain safe operating temperatures, the permissible current carrying capacity must therefore be reduced.
This reduction is known as Derating or Correction of Ampacity.
Professional cable sizing always includes derating calculations before the final cable size is approved.
Whether calculations are performed manually or verified using OneCalcApp.com, derating remains an essential engineering step because it reflects actual site conditions rather than ideal laboratory conditions.
What Is Cable Derating?
Cable derating is the process of adjusting the manufacturer's rated current carrying capacity to account for actual installation conditions.
The corrected current carrying capacity is lower or, in some cases, equal to the catalogue rating.
In simple terms:
Manufacturer Ampacity = Current under Standard Conditions
Corrected Ampacity = Current under Actual Site Conditions
The corrected ampacity is the value that should be compared with the calculated Full Load Current—not the catalogue rating.
Why Is Derating Necessary?
Every conductor generates heat whenever current flows through it.
This heat must continuously escape into the surrounding environment.
If heat cannot escape efficiently:
- Conductor temperature increases.
- Insulation temperature rises.
- Cable life reduces.
- Voltage drop increases.
- Power losses increase.
- Long-term reliability decreases.
The purpose of derating is to ensure that the conductor temperature always remains within the permissible operating limit specified for the insulation type.
Ignoring derating may result in a cable that appears adequate on paper but overheats under actual operating conditions.
Heat Generation Inside a Cable
Electrical current flowing through a conductor produces heat because every conductor has electrical resistance.
The heat generated is proportional to:
- Current
- Conductor resistance
- Operating duration
As current increases:
- Heat generation increases rapidly.
- Conductor temperature rises.
- Insulation ages faster.
The cable reaches a stable operating temperature only when:
Heat Generated = Heat Dissipated
If heat generation exceeds heat dissipation, conductor temperature continues increasing.
This is the fundamental reason why installation conditions influence cable ampacity.
Standard Reference Conditions
Manufacturer ampacity tables are generally based on standard installation conditions specified by applicable standards.
Typical reference conditions may include:
- Standard ambient temperature.
- Standard soil thermal resistivity.
- Single isolated cable.
- Specified installation method.
- Standard burial depth.
- No abnormal external heat sources.
Actual projects almost never satisfy all these assumptions simultaneously.
Therefore correction factors become necessary.
Major Derating Factors
Professional cable sizing generally considers several correction factors.
The most common are:
- Ambient Temperature Correction
- Soil Thermal Resistivity Correction
- Burial Depth Correction
- Cable Grouping Correction
- Installation Method Correction
- Thermal Environment Correction
Each correction factor represents one environmental influence on cable cooling.
Ambient Temperature Correction
One of the most significant derating factors is ambient temperature.
Cable manufacturers usually determine current ratings at a specified reference ambient temperature.
When the surrounding temperature increases:
- Heat dissipation decreases.
- Cable temperature increases.
- Current carrying capacity reduces.
For example:
A cable installed inside an electrical room operating at high ambient temperature cannot carry the same current as the same cable installed in a cooler environment.
Therefore an ambient temperature correction factor must be applied.
Engineering Principle
Higher Ambient Temperature
↓
Reduced Cooling
↓
Higher Conductor Temperature
↓
Lower Allowable Current
Soil Thermal Resistivity Correction
For underground cables, soil becomes the primary cooling medium.
Different soils transfer heat differently.
Examples include:
- Wet Clay
- Moist Sand
- Dry Sand
- Rocky Soil
- Backfilled Soil
Dry soil generally has higher thermal resistivity than moist soil.
Higher thermal resistivity means:
- Poor heat transfer.
- Higher conductor temperature.
- Lower cable ampacity.
This is why geotechnical information is often required for utility-scale underground cable design.
Burial Depth Correction
The depth at which the cable is buried also affects cooling.
A shallow cable can release heat more easily.
A deeply buried cable has:
- Longer heat transfer path.
- Reduced heat dissipation.
- Higher operating temperature.
Consequently, greater burial depths may require correction factors depending on the installation standard and manufacturer recommendations.
Cable Grouping Correction
Grouping occurs when several loaded cables are installed close together.
Instead of dissipating heat independently, grouped cables warm each other.
This mutual heating significantly reduces their current carrying capacity.
Typical grouped installations include:
- Cable trays
- Underground trenches
- Cable ducts
- Tunnel installations
The greater the number of loaded cables, the lower the permissible current per cable.
Grouping correction is therefore one of the most important factors in large solar power plants.
Installation Method Correction
Cable cooling depends strongly on how the cable is installed.
Common installation methods include:
- Free Air
- Cable Tray
- Underground Direct Buried
- Cable Duct
- Conduit
- Trench
A cable suspended freely in air usually dissipates heat more effectively than a cable enclosed inside a conduit.
Therefore installation method directly affects current carrying capacity.
Overall Derating Factor
After determining the applicable correction factors, they are combined into one overall derating factor.
The general relationship is:
Where:
- K = Overall Derating Factor
- K1 = Ambient Temperature Factor
- K2 = Soil Thermal Resistivity Factor
- K3 = Burial Depth Factor
- K4 = Grouping Factor
- K5 = Installation Method Factor
Each project may require a different combination depending on the installation conditions.
Corrected Cable Ampacity
Once the overall derating factor has been determined, the corrected current carrying capacity is calculated.
Where:
- Icorrected = Corrected Current Carrying Capacity
- Irated = Manufacturer Rated Current
- K = Overall Derating Factor
This corrected ampacity represents the maximum continuous current the cable can safely carry under actual site conditions.
Acceptance Criterion
The selected cable is acceptable only if:
Icorrected ≥ Iload
If the corrected ampacity is lower than the Full Load Current:
- Reject the cable.
- Select the next larger conductor size.
- Repeat the verification process.
This iterative approach is standard engineering practice.
Common Mistakes During Derating
Many cable sizing errors occur because derating is overlooked.
Typical mistakes include:
- Using catalogue ampacity directly.
- Ignoring ambient temperature.
- Ignoring grouped cable installations.
- Ignoring soil thermal resistivity.
- Applying only one correction factor when multiple factors are applicable.
- Using correction factors from different standards without consistency.
- Forgetting to document assumptions.
Engineering Best Practices
Professional engineers generally follow these recommendations:
- Use correction factors from the applicable project standard or approved manufacturer data.
- Apply all relevant correction factors.
- Verify installation conditions during site design.
- Recalculate corrected ampacity whenever installation conditions change.
- Keep calculation records for future verification.
- Coordinate cable sizing with civil and construction teams to avoid unexpected installation changes.
How Onecalcapp Helps Engineers
Applying multiple correction factors manually for every design iteration can become repetitive, especially when evaluating different installation methods or cable sizes.
The OneCalcApp.com AC Cable Sizing Calculator helps engineers organize the required design inputs, apply the selected derating factors and compare corrected ampacity values efficiently.
This allows engineers to evaluate multiple design options quickly while still understanding the engineering principles explained in this guide.
Summary
By the end of this chapter, we have learned that:
- Manufacturer ampacity is based on reference conditions.
- Actual site conditions require correction.
- Derating ensures safe conductor operating temperature.
- Multiple environmental factors influence cable capacity.
- The corrected ampacity—not the catalogue rating—must be compared with the Full Load Current.
Engineering Takeaway
A cable that passes the manufacturer's basic ampacity rating cannot automatically be considered suitable for the project.
The actual installation environment must be evaluated and the applicable correction factors must be applied before confirming the final cable size.
The next stage is to verify the selected cable under actual operating conditions and continue with the remaining cable sizing checks.
Calculate. Verify. Design with confidence.