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Cable sizing for HV schemes: what a cable study checks

By Saqib Shah · · 4 min read

What a cable study checksA cross-section of three cables in a trench under ground level with heat arrows, and three checks: continuous rating, short-circuit withstand, and cyclic and emergency load, with the standards that cover each. What a cable study checksGround cross-section, with the inputs that set the answerGround levelDepthHeat leaves the cables through the soil.Warmer, drier soil means less current.Continuous ratingSteady load, no time limitIEC 60287Short-circuit withstandA fault for a short, set timeIEC 60949Cyclic and emergency loadDaily swings and short overloadsIEC 60853Inputs that move the answerDepth, spacing and grouping, soil thermalresistivity, ground temperature, sheath bonding

TL;DR. A cable study asks whether a chosen cable can carry the load continuously, survive a fault, and handle the way the load changes over a day or in an emergency. Different IEC standards cover each question. The answer depends on inputs that you must source and record for the actual route, and your network operator may specify its own requirements on top. This post gives no ratings, because ratings belong to the project.

What is a cable study?

It is the calculation that shows a specific cable, installed in a specific way, is adequate for the duty it will see. It is not a table lookup. The same cable can have different ratings in different trenches, so the study is tied to the installation, not only to the cable datasheet.

Which standards does the method come from?

Three IEC standards cover the core questions. Your network operator or client may point to others or add their own requirements.

  • Continuous rating: IEC 60287-1-1. The 2023 edition (3.0, published 22 May 2023) applies to steady-state operation of cables at all alternating voltages and direct voltages up to 5 kV. It covers cables buried directly, in ducts, troughs or steel pipes, and cables in air, and gives formulae for current ratings and losses. Steady state means a constant 100 per cent load that brings the conductor to its maximum temperature under constant conditions. The thermal resistance calculations are in the companion part, IEC 60287-2-1.
  • Short-circuit withstand: IEC 60949. It calculates the thermally permissible short-circuit current. The catalogue description says it starts from the adiabatic short-circuit current and applies a factor for heat flowing into neighbouring materials.
  • Cyclic and emergency load: IEC 60853. Part 1 gives the cyclic rating factor for cables up to and including 18/30 (36) kV, where thermal capacitance can be neglected. Part 2 covers cyclic rating factors above that and an emergency rating method for cables of any voltage.

Check the edition and any amendments your client requires. Standards are revised.

Why is the answer different for every project?

Because the standards leave important choices to the user. The IEC 60287-1-1 abstract says several parameters are left for users to select: construction-related values, site conditions that depend on the country of use, and margins agreed between manufacturer and user, such as the maximum conductor temperature.

In practice the site inputs are the things a team will recognise: how deep the cables are, how they are arranged and grouped, the thermal properties of the soil and the ground temperature, and how the metallic screens are bonded. Each one moves the result. None of them should be a default nobody remembers choosing.

What should the team write down?

For every input, record the value, where it came from and who accepted it:

  • Soil and ground temperature: a survey, a client document or a standard, with its date.
  • Installation arrangement: the drawing and revision it came from.
  • Cable construction: the manufacturer’s datasheet and its revision.
  • Load profile: who supplied it and whether it is a continuous or cyclic duty.
  • Fault data: the fault level and clearance time, with the network operator’s reference.
  • Margins: the maximum conductor temperature you used and why.

This is the same discipline as any engineering default: a value with a source, accepted by the engineer. If a value is unknown, say “owner to confirm” rather than filling the gap.

Does the network operator have its own rules?

Often, yes. Network operators publish design requirements for connections, and the Energy Networks Association publishes framework documents for underground connections, for example the EREC G81 series, including a part on design and planning for new industrial and commercial underground connections. Whether a particular document applies depends on the network operator and the type of connection. Ask which standards they want you to design to before the study starts, not after.

What should the report show?

A reader should be able to check the study without redoing it. That means:

  • the three checks, each with its result and the standard and edition it used;
  • every input with its source;
  • the worst case that governs the cable size, and the margin left;
  • any assumption that needs confirming, marked as such.

Whether the margin is acceptable is the responsible engineer’s decision, and your network operator may have a view too. The study should make that decision easy to take and easy to audit.

Sources

Accessed 14 October 2026.

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