Power cable rating examples for calculation tool verification

Introduction

Accurate current rating (ampacity) calculations are fundamental to the safe and economical design of power cable systems. The current rating defines the maximum continuous current a cable can carry without exceeding its permissible operating temperature. While IEC standards such as IEC 60287 provide the mathematical framework for calculating cable ampacity, practical implementation often leads to inconsistent results due to interpretation differences, modelling assumptions, and software variations.

To address this challenge, CIGRE Study Committee B1 established Working Group B1.56 to develop guidance and standardized case studies for verifying current rating calculation techniques and software tools. The outcome is a comprehensive Technical Brochure providing structured verification methodology, harmonized guidance points, and a broad set of practical examples covering AC and HVDC cable systems.

This article presents the background, objectives, methodology, key technical guidance, and case studies developed by WG B1.56, aimed at improving transparency, consistency, and confidence in power cable rating calculations.

Why verification of cable current rating calculations is essential

Unlike voltage withstand capability, cable current rating is not physically tested. It is determined entirely by calculation. Consequently, the reliability of the rating depends directly on the correctness of the calculation method, the interpretation of IEC standards, the assumptions and input data selected, and the implementation within software tools or spreadsheets.

In practice, very similar cable systems have been observed to produce significantly different ampacity results, particularly during tender evaluations. Differences may arise from interpretation ambiguities in IEC formulae, simplifications inherited from historical hand calculations, inconsistent treatment of losses or thermal resistances, and tool-specific modelling choices.

WG B1.56 identified that even small deviations in intermediate parameters can significantly affect final ratings. Therefore, a structured and transparent verification framework is required.

Objectives of CIGRE WG B1.56

The Working Group developed 49 detailed guidance points clarifying or supplementing IEC standards, 11 representative case studies covering a wide range of cable types and installation conditions, and a formal verification methodology for tools and calculation techniques. Intermediate results are provided with high numerical precision to identify deviations.

The goal is not to replace IEC standards, but to remove ambiguities while remaining consistent with their technical foundation.

Key benefits for the Cable Industry

The Technical Brochure delivers major practical advantages.

Fair comparison in tender evaluations

Using standardized assumptions and verified tools enables consistent ampacity calculations for comparable cable designs, reducing ambiguities during contract award processes.

Increased trust between stakeholders

Verified calculation tools minimize discrepancies between consultants, manufacturers, and operators.

Improved confidence in software tools

Both in-house and commercial calculation tools can be benchmarked and improved against the case studies, creating a solid foundation for more complex analyses.

Scope of the case studies

The 11 case studies cover a broad spectrum of cable technologies, including 10 kV to 400 kV AC cables, 320 kV HVDC cables, XLPE insulated cables, SCFF and pipe-type cables, and both land and submarine installations.

Installation configurations include trefoil and flat formations, direct buried systems, backfill installations, ducts and troughs, and free air and seabed installations. Bonding arrangements cover single-point bonding, multiple bonding, and cross-bonding systems.

Together, the cases allow verification of a large portion of the IEC 60287 calculation framework.

Verification methodology

The verification approach applies to commercial software, in-house tools, spreadsheets, and custom-developed calculation platforms.

The recommended process includes selecting relevant case studies within the tool’s domain, performing full calculations, comparing intermediate and final results with Technical Brochure values, investigating deviations, and producing a structured verification report.

All intermediate results are provided with high numerical accuracy to enable precise benchmarking.

Key technical guidance points

Accuracy and rounding

Although calculations may be performed with high numerical precision, practical uncertainties—particularly in soil thermal parameters—limit real-world accuracy. Final ratings should therefore be rounded down according to defined current ranges.

This reinforces an important principle: improving input data quality often has greater impact than refining mathematical precision.

Avoiding historical simplifications

IEC standards include simplifications originally designed for hand calculations. WG B1.56 recommends avoiding these simplifications in modern computational environments by not neglecting eddy current losses in continuous metallic sheaths, always including dielectric losses, and using general thermal resistance formulations instead of simplified expressions for non-touching cables.

These measures ensure robustness for modern cable designs.

Interpretation of input data

The Technical Brochure provides standardized methods for deriving cable dimensions from incomplete data sheets, handling missing material properties, and treating thin layers and tapes individually rather than combining materials. Separating layers avoids unrealistic averaging of thermal properties.

Lay length effects

Lay lengths significantly influence electrical resistance, capacitance, thermal resistances, and sheath and screen parameters. For three-core cables, a lay length factor must be applied to relevant parameters.

For stranded conductors, an average lay length factor of approximately 1.05 is recommended when manually calculating DC resistance. Corrugated sheaths also require a corrugation factor to correctly adjust electrical resistance.

Losses in screens, sheaths and armour

Modern cable designs frequently include multiple metallic layers with complex interactions. WG B1.56 provides flowcharts to determine appropriate loss calculation methods.

For magnetic armour wires, guidance extends IEC limitations beyond standard diameter ranges by recommending linear scaling of AC resistance factors. Specific recommendations are also provided for flat wire armour.

Thermal resistance T2 in three-core cables

The calculation of T2 (thermal resistance between sheath and armour) is not clearly defined in IEC standards. The Technical Brochure redefines this procedure by adding core jacket resistances (divided by three), including filler and bedding thermal resistances, and applying worst-case dry thermal properties.

Careful evaluation of filler void conditions (air-filled versus water-filled) is emphasized.

HVDC cable rating specificities

HVDC cables introduce an additional limiting factor: maximum allowable temperature difference across the insulation.

Unlike HVAC cables, electric field distribution in HVDC insulation depends on resistive properties rather than permittivity. The temperature drop across insulation—typically limited to 10–15 K—may restrict current capacity before conductor temperature limits are reached.

Both constraints must be respected in rating calculations.

Future developments – WG B1.72

Further verification work continues under WG B1.72, addressing more complex situations such as dynamic, cyclic, and emergency ratings; horizontal directional drilling installations; multiple parallel circuits; and external heat sources.

These developments aim to further enhance standardization and reliability in advanced cable rating scenarios.

Conclusion

CIGRE WG B1.56 has delivered a structured and practical framework for verifying power cable current rating calculations. By providing harmonized guidance points, detailed case studies, and a rigorous verification methodology, the Technical Brochure strengthens consistency, transparency, and industry confidence in IEC-based ampacity calculations.

For engineers, manufacturers, consultants, and system operators, this work represents a major step toward fair comparison in tenders, improved software reliability, and more robust cable system design.

As power networks expand and cable systems become increasingly complex—including large-scale HVAC and HVDC projects—the importance of standardized, verifiable current rating methodologies will only continue to grow.