Cable Sizing, Ampacity and Thermal Rating Studies

Overview

APS provides specialist cable sizing, ampacity and thermal rating studies for underground, subsea, industrial and utility-scale cable systems. The service supports the selection and verification of MV, HV and EHV cable circuits by determining the maximum permissible continuous, cyclic, emergency and short-time current that a circuit can carry without exceeding the thermal limits of the conductor, insulation, sheath, screen, armour, joints or the surrounding installation environment.

Cable rating is not governed by conductor cross-section alone. A thermally sound cable design requires systematic consideration of cable construction, installation depth, trench and duct configuration, ambient temperature, soil thermal resistivity, load profile, bonding arrangement, sheath and armour losses, grouping derating, thermal backfill performance, soil dry-out risk, and mutual heating from adjacent circuits, buried services and nearby heat sources. Failure to account for these factors can lead to sustained over-temperature, premature insulation ageing and reduced asset life, or to unnecessary over-sizing and avoidable cost.

The thermal rating assessment is carried out using recognised engineering methods aligned with IEC 60287 for steady-state current rating, IEC 60853 for cyclic and emergency loading, the Neher-McGrath thermal rating approach, CIGRE technical guidance and relevant IEEE engineering practice. These are not competing methods but complementary analytical frameworks applied according to installation complexity, load profile and project requirement. Where the installation geometry or loading conditions warrant detailed simulation, specialist cable rating platforms including CYMCAP and Cableizer are used for detailed thermal modelling, mutual heating analysis and cross-verification. National sizing requirements such as AS/NZS 3008 can also be applied where the project demands it.

For cable bonding and sheath assessment, EMT simulation using EMTP® and PSCAD™/EMTDC™ is applied where transient analysis is required — including sheath overvoltage under switching and fault conditions, bonding conductor performance during earth faults, cross-bonding transient behaviour, and sheath voltage limiter (SVL) assessment under lightning and switching impulse. This allows bonding arrangements and SVL protection to be evaluated against both steady-state circulating current limits and the transient voltages that occur under fault and switching conditions.

APS supports projects from early-stage cable option studies through to detailed design verification, independent review of third-party calculations, connection application support and asset uprating assessment.

Key Services

APS provides study-led cable rating services spanning early-stage option studies through to detailed design verification and independent technical review. The service combines thermal modelling, installation-environment assessment, loss calculation, bonding analysis and practical installability review.

• Cable ampacity and thermal rating — continuous, cyclic, emergency and short-time current rating studies using IEC 60287, IEC 60853 and Neher-McGrath methods, covering steady-state, transient and emergency loading conditions More info
• Underground trench and duct-bank thermal modelling — multi-duct, multi-circuit arrangements including mutual heating effects, grouping derating and unequally loaded circuit assessment More info
• Subsea cable thermal rating and burial-depth assessment — offshore export cables, inter-array circuits and subsea interconnectors, including burial depth optimisation and seabed thermal environment More info
• Soil thermal resistivity, backfill and dry-out risk assessment — soil thermal characterisation, critical isotherm and moisture migration analysis, dry-out risk evaluation and thermal backfill specification More info
• Sheath, screen and armour loss calculation and bonding analysis — single-point, both-end and cross-bonded system assessment; circulating current, sheath standing voltage and induced voltage analysis; EMT simulation using EMTP® and PSCAD™/EMTDC™ for transient sheath overvoltage, bonding conductor performance and SVL evaluation under fault, switching and lightning conditions More info
• Magnetic losses in metallic enclosures and structures — eddy current and hysteresis losses in steel pipes, cable ducts, troughs, trays and nearby metallic infrastructure More info
• Thermal interaction with nearby infrastructure — mutual heating from adjacent cable circuits, buried services, foundations, drainage systems and external heat sources More info
• Cable installation configuration studies — direct-buried, ducted, trough, tunnel, riser, bridge-mounted and air-installed cable rating across all common installation geometries More info
• Cable crossing and route constraint assessment — thermal hotspot identification at crossings, route feasibility assessment and rating impact of route constraints More info
• Cable pulling, mechanical installation and route feasibility review — pulling tension, sidewall pressure, bending radius and installability checks against cable manufacturer limits More info
• Technical due diligence and independent verification — independent review of manufacturer cable data, installation drawings, design assumptions and third-party cable rating calculations More info

Typical Applications

These studies are applicable to any project where cable thermal performance, current-carrying capacity or installation feasibility needs to be verified — particularly where route constraints, soil conditions, load profile or adjacent heat sources increase the complexity of the rating assessment.

• Underground HV and EHV transmission cable circuits
• Distribution-connected MV cable systems
• Offshore wind farm export and inter-array cable systems
• Subsea cables and HVDC land and sea cable links
• Renewable energy generation and storage connection cables
• Data centre and critical facility HV and MV cable infrastructure
• Industrial plant and manufacturing site power cable systems
• Multi-circuit cable routes and shared duct banks
• Cable tunnel, riser, bridge-mounted and air installations
• Cable routes constrained by soil conditions, mutual heating or route geometry
• Cable replacement, uprating and asset life extension projects
• Independent review and technical due diligence on cable designs

Technical Value

The objective of the study is to confirm that the proposed cable system is thermally suitable, practically installable, technically justified and capable of supporting the required operating duty with adequate design margin — providing clear, defensible evidence to support design review, connection approval and asset management decisions.

APS supports early-stage option studies, detailed design verification, connection applications, asset uprating, cable replacement projects and independent review of third-party calculations. By combining IEC- and CIGRE-aligned rating methods with realistic installation-environment modelling — soil resistivity, backfill performance, grouping, bonding arrangement and mutual heating — the assessment captures the factors that determine real-world thermal performance, not just the factors that are straightforward to calculate.

The study identifies whether the cable system will operate within safe thermal limits under steady-state, cyclic and emergency loading conditions, and whether the design margin is appropriate for the required asset life and operating regime. Where a constraint is identified, the assessment evaluates the impact and supports the selection of a technically justified and cost-effective mitigation — whether a revised conductor size, an adjusted installation arrangement, a thermal backfill specification, a modified bonding scheme or a revised operating limit.

By combining recognised thermal rating methods with detailed installation modelling and practical engineering judgement, APS helps clients avoid both under-rating, which creates thermal risk and reduces asset life, and over-rating, which results in unnecessary cost — and provides the technical evidence required for design sign-off, connection approval and long-term asset management.