Cable Ampacity and Derating Factors Explained

What does a cable ampacity rating represent?

A rating is a property of the whole cable system under stated operating and installation conditions. The IEC 60287-1-1 current-rating method addresses steady-state operation: continuous constant current at full load factor under constant surrounding conditions, which approaches the maximum permitted conductor temperature asymptotically. It should not be presented as a short-time emergency or cyclic rating.

IEC 60853-2 provides separate methods for cyclic and emergency current ratings. A project specification should state the duty being calculated, the conductor temperature limit, the load profile, and the duration and recovery assumptions for any emergency case.

The project engineer must select the applicable code, national reference conditions, temperature limits and installation assumptions. A catalogue value cannot confirm the permissible loading of a completed route.

How does heat balance determine the rating?

Electrical losses produce heat in the conductor and, depending on cable construction and system arrangement, in the insulation, metallic screens, sheaths and armour. Heat then crosses the cable layers and passes to the external environment. IEC 60287-1-1 covers current-rating equations and losses, while IEC 60287-2-1 covers thermal resistance for cables in air, in ground, in ducts, troughs and pipes.

The rating can change even when conductor size stays the same. A different sheath-bonding arrangement, duct layout, circuit spacing, soil condition or ambient temperature changes either the heat produced or the path available to remove it.

For single-core AC cables, metallic sheath losses depend on bonding and physical arrangement. IEC 60287-1-2 and IEC 60287-1-3 address specific eddy-current and circulating-current cases. A datasheet rating may therefore be inapplicable when the project changes formation, spacing, parallel conductors or sheath bonding.

What inputs does a defensible calculation need?

A calculation can only be reviewed if every input is written down. A minimum input register covers the following groups.

• Cable construction: conductor, insulation, screen, metallic sheath, armour, oversheath and relevant dimensions.
• Electrical system: frequency, phase arrangement, bonding method, parallel conductors and harmonic or unbalanced loading.
• Installation: air, tray, burial, duct, pipe or tunnel; spacing, depth, geometry, nearby circuits and heat sources.
• Environment: ambient temperature, soil or backfill thermal resistivity, moisture assumptions and enclosure effects.
• Duty: continuous current, load factor, cyclic profile, emergency duration and future circuits.
• Method: standard edition, national reference conditions, software and calculation options.

How do soil conditions affect a buried route?

CIGRE notes that the rating depends on both the cable parameters and the thermal environment, and that soil thermal resistivity can vary strongly with density and moisture along a route. A calculation with many decimal places is not reliable if the route input is an unverified generic value.

For critical circuits, state how soil or engineered-backfill properties were obtained, which seasonal or dry condition governs, where route hot spots may occur, and what installation quality control preserves the assumed geometry. IEC 60287-3-1 defines site reference conditions but notes that general values can be superseded by national requirements.

How do ambient temperature and grouping derate a cable?

Derating starts from a reference condition. The reference ambient is 30 °C for cables in air and 20 °C for buried cables (IEC 60364-5-52). At 40 °C in air, the correction factor is 0.87 for PVC insulation and 0.91 for XLPE insulation (IEC 60364-5-52, Table B.52.14).

Correction factors multiply: corrected capacity equals the tabulated capacity times the ambient factor, the grouping factor and any other applicable factors. The insulation also sets the conductor temperature limit, which is 70 °C for PVC and 90 °C for XLPE in normal operation (commonly cited from IEC 60502-1). The table below shows what each factor depends on and where its value comes from.

The 0.87 and 0.91 values apply only to the 40 °C ambient case in the named standard edition. The grouping factor is not quoted here because it depends on the installation method; confirm it from the tables or the manufacturer's datasheet.

For sizing steps that combine ampacity with voltage drop and short-circuit checks, see the cable sizing guide.

When are published tables enough, and when is a project calculation needed?

Published tables can be appropriate when cable construction and installation match the reference case and all correction factors come from the same code or standard system. Do not multiply unrelated correction factors.

A project calculation is more appropriate for non-standard spacing, multiple circuits, mixed duct banks, variable soil, external heat, single-core sheath losses, tunnels, cyclic duty or emergency operation. If the geometry falls outside an analytical method, document the alternative model and its validation.

A useful report includes an input register and route drawing, the standard and software version, applicable electrical losses, internal and external thermal resistances, the governing ambient or soil condition, the conductor temperature, sensitivity to uncertain inputs, and a final rating clearly labelled as continuous, cyclic or emergency. CIGRE also warns that interpretation, assumptions, rounding and software implementation can create inconsistent results, so compare intermediate losses and thermal resistances, not only the final ampacity.

Frequently asked questions