IEC 60287 — Cable Current Rating from the Thermal Model
IEC 60287 calculates the continuous current rating of a cable from first principles: the losses that heat the conductor, and the thermal resistances between the conductor and the surrounding medium. Where IEC 60364-5-52 reads a rating out of a table for a reference installation, IEC 60287 computes it for the installation you actually have.
What is IEC 60287?
IEC 60287 is the IEC series for calculating the current rating of cables at 100 % load factor. Part 1-1 gives the rating equations and the losses; Part 2-1 gives the thermal resistances; Part 3-1 covers reference operating conditions and the selection of cable type.
The physics is a steady-state heat balance: the conductor sits at its permitted temperature, the losses generate heat, and the heat must escape through the insulation, the sheath, the serving and the medium around the cable. The rating is the current at which that balance holds exactly.
When is IEC 60287 used rather than a table?
- a trench as built — several circuits at real spacings and depths, not the two-cable reference arrangement of a table;
- soil thermal resistivity that differs from the 2,5 K·m/W the buried tables assume, or a route where the soil changes;
- duct banks and pipes, where the air gap and the duct wall add thermal resistance;
- medium-voltage cables where dielectric loss and sheath loss matter, which the low-voltage tables ignore because at 1 kV they are negligible;
- any case where an engineer has to justify a rating rather than cite a table row.
Clause 523.3 of IEC 60364-5-52 explicitly allows the current to be “determined by calculation” — and this is that calculation.
The rating equation (clause 1.4)
The permissible current follows from the temperature-rise expression of IEC 60287-1-1 clause 1.4.1.1, which balances the conductor temperature rise above ambient against the losses and the thermal resistances in their path.
Δθ conductor temperature rise above ambient (K) · I current in one conductor (A) · R a.c. resistance per unit length at maximum operating temperature · Wd dielectric loss per unit length · λ1, λ2 loss factors of sheath/screen and armour · n number of load-carrying conductors · T1…T4 thermal resistances of insulation, bedding, serving and the external medium.
Solving that for I gives the rating. Note where each loss enters: the dielectric loss appears with a factor of one half against T1, because it is generated throughout the insulation rather than at the conductor. Clause 1.4 also carries the instruction for partially dried-out soil: calculate both cases and use the lower rating.
The losses (clauses 2.1 to 2.4)
| Quantity | Clause | What it accounts for |
|---|---|---|
| A.C. resistance of the conductor R | 2.1 | The d.c. resistance at operating temperature, raised by the skin effect (ys) and the proximity effect (yp). Table 1 holds the resistivities and temperature coefficients of the metals; Table 2 holds the experimental coefficients ks and kp. |
| Dielectric loss Wd | 2.2 | Loss in the insulation, a.c. cables only, from the capacitance, the operating voltage and tan δ. Table 3 gives relative permittivity and loss factors for HV and MV insulations. |
| Sheath and screen loss factor λ1 | 2.3 | Circulating and eddy-current losses in the metallic sheath or screen — strongly dependent on whether single-core cables are bonded at both ends and on the formation. |
| Armour loss factor λ2 | 2.4 | Loss in armour, reinforcement and steel pipes, which is why a steel-wire-armoured single-core cable behaves quite differently from an unarmoured one. |
| Solar radiation | Table 4 | Absorption coefficient of the cable surface, for cables in direct sunlight. |
Thermal resistances T1 to T4 (Part 2-1)
T1, T2 and T3 belong to the cable; T4 belongs to the installation. T4 is where the trench, the spacing, the depth, the soil and the neighbouring circuits enter the calculation — and it is usually the dominant term for a buried cable.
| Resistance | Clause | Between |
|---|---|---|
| T1 | 4.1.2 | one conductor and the sheath |
| T2 | 4.1.3 | sheath and armour |
| T3 | 4.1.4 | the outer covering (serving) |
| T4 — free air | 4.2.1 | cable surface and ambient air |
| T4 — single buried cable | 4.2.2 | cable surface and the ground surface |
| T4 — groups of buried cables, not touching | 4.2.3 | with mutual heating between circuits |
| T4 — groups of buried cables, touching, equally loaded | 4.2.4 | — |
| T4 — buried pipes, troughs, ducts | 4.2.5 to 4.2.7 | including the air space inside a duct |
Part 2-1 clause 5 additionally gives the digital forms of quantities the older editions presented as graphs — the geometric factor G for belted cables among them — which is what makes the whole calculation programmable rather than a chart-reading exercise.
Input data the calculation needs
- Cable construction: conductor material and cross-section, insulation type and thickness, screen or sheath material and dimensions, armour, oversheath — from the cable datasheet;
- Permitted conductor temperature for the insulation, and the ambient temperature of the medium (air or ground) at the installation;
- Installation geometry: laying depth, spacing between circuits, formation of single-core cables, duct dimensions and material;
- Soil thermal resistivity, and whether partial drying-out has to be considered;
- System data: voltage and frequency for the dielectric and sheath losses, and the bonding arrangement of the screens.
Construction dimensions are manufacturer data. A rating calculated on assumed dimensions is an estimate, and a tool should say which numbers came from a datasheet and which from a default — the ampacity calculator here prints both.
Calculation workflow
- Fix the permitted conductor temperature and the ambient temperature; Δθ is their difference.
- Compute the a.c. resistance R at that temperature, with skin and proximity effects (2.1).
- Compute the dielectric loss Wd (2.2) — negligible at low voltage, not at MV and above.
- Compute the loss factors λ1 and λ2 for the sheath and armour (2.3, 2.4), respecting the bonding arrangement.
- Compute T1, T2, T3 from the construction and T4 from the installation (Part 2-1, clause 4), including mutual heating for a group.
- Solve the clause 1.4 expression for I.
- For a buried route with possible drying-out, repeat for the dry condition and take the lower rating (clause 1.4).
- Check the result against the fault duty separately — a continuous rating says nothing about short-circuit withstand, which is IEC 60949 and the currents from IEC 60909.
IECCalc calculators for IEC 60287
- #004 Cable ampacity — continuous current rating The clause 1.4 calculation with the losses of Part 1-1 and the thermal resistances of Part 2-1, on a trench drawn to scale with real spacings, depths and soil resistivity. Checked against the resistance tables of IEC 60228 and the property tables of IEC 60287-1-1.
- #008 LV cable sizing by the Annex B tables The faster route for a standard low-voltage installation, per IEC 60364-5-52.
- #001 Voltage drop The other constraint on the same cross-section.