IEC 60287 Current Rating Explained: Why Cable Ampacity Is a Site Decision, Not a Catalogue Number

Customers ask "how many amps can this cable carry?" The honest answer is: it depends where you put it. The same cable can carry very different currents buried in dry sand, pulled into a duct, laid in free air in the sun or grouped with other circuits.

The international method for answering that question is IEC 60287, Electric cables — Calculation of the current rating. This article explains the idea behind it and, more importantly, which inputs actually move the answer.

The idea in one paragraph

A cable heats up because current flows through the conductor's resistance and because insulation, screens and armour have losses of their own. That heat has to escape through the cable's layers and into its surroundings. The current rating is the continuous current at which the conductor just reaches its maximum permitted temperature. IEC 60287 models this as a chain of thermal resistances — insulation, bedding, sheath, and then the environment — much like an electrical circuit in which heat plays the role of current and temperature the role of voltage.

Insight 1: the environment usually matters more than the cable

Engineers naturally focus on the cable construction, but for buried cables the largest single thermal resistance is often the ground itself. The factors that move a rating most are frequently outside the cable:

• Soil thermal resistivity. Moist clay and dry sand behave very differently. Ground that dries out around a hot cable can make things worse over time.
• Burial depth and ducts. Ducts protect the cable but add thermal resistance through the air gap and duct wall.
• Neighbouring circuits. Grouped cables heat each other; the hottest cable in a group sets the limit.
• Ambient and ground temperature. A project in the Gulf is not a project in Scandinavia.
• Cables in air. Sunlight, enclosure and arrangement on trays or cleats change the picture completely.

The practical lesson: a rating is only as good as the site data behind it. When a tender gives no installation conditions, the right move is to ask — or to state clearly which assumptions the offer is based on.

Insight 2: bonding can matter as much as conductor size

For single-core cables, how the metallic screens are earthed has a large effect:

• Solid bonding (earthed at both ends) is simple, but lets circulating currents flow in the screens. On large conductors this can cost a substantial share of the rating.
• Single-point bonding and cross-bonding largely eliminate those currents, at the price of extra hardware such as sheath voltage limiters and link boxes.
• Trefoil versus flat formation and phase spacing also change screen losses.

Sometimes the cheapest way to more amps is not a bigger conductor but a different bonding arrangement — a conversation worth having with the customer.

Insight 3: at high voltage, the insulation itself becomes a heat source

At low voltage, insulation losses are negligible. At high and extra-high voltage, dielectric losses grow with voltage and take a real share of the thermal budget, leaving less for the conductor. This is one reason HV and EHV designs need careful, full calculations rather than scaled-up LV rules of thumb.

Insight 4: big conductors are less efficient than they look

AC current crowds towards the surface of large conductors (skin effect) and is disturbed by neighbouring conductors (proximity effect). Doubling the conductor size does not double the rating. For very large HV conductors, segmental designs are used to limit the effect — see IEC 60228 explained for the conductor side of this story.

What IEC 60287 does not cover

IEC 60287 calculates the steady-state rating at a constant, full load. Real networks also need:

• cyclic and emergency ratings, covered by IEC 60853, which can allow higher currents for loads that vary through the day;
• short-circuit capability of the conductor and the metallic screen, checked separately;
• voltage drop, which can decide the conductor size on long LV feeders regardless of temperature.

A complete check usually looks at all of them. And because IEC 60287 is a calculation, type tests remain the reference for proving a design; the calculation is the engineering basis for design, selection and offers.

Why catalogue ratings cause trouble

Catalogue ratings assume standard reference conditions that rarely match a real project. Used directly in a tender, they lead to one of two outcomes:

• Over-sizing — the offer is technically safe but loses on price to a competitor who calculated properly.
• Under-sizing — the offer wins, and the problem appears later, in the customer's own design review or in service.

Calculating the rating with every design, for the customer's own conditions, removes both risks.

Current ratings in CablePLM

In CablePLM, IEC 60287 ratings are calculated with every design, using your laying method, soil, burial depth, ambient temperature and bonding. Change the conductor or the screen and the rating updates together with the construction, bill of materials and datasheet. Short-circuit capability is calculated with the design too, and the Cable Selection & Optimization add-on compares alternatives to find the cheapest design that meets the required current.

Read next: anatomy of a power cable, layer by layer and IEC 60502-2 for MV cables.

Want to see ratings calculated for your own projects? Book a demo.