Cable Current Rating Calculator (IEC 60287)
Cable rating tables only cover the installation conditions somebody chose to tabulate. IEC 60287 gives the method instead: balance conductor, dielectric, sheath and armour losses against the thermal resistances out to ambient, then solve for current. That lets you vary burial depth, soil resistivity and ambient continuously, which is what real projects need.
Choosing something to solve for makes Continuous current rating an editable target and computes the chosen input from it.
Formula
| dT | permitted rise, conductor limit minus ambient (K) |
| Wd | dielectric loss per unit length (W/m) |
| T1, T3, T4 | thermal resistances of insulation, serving and surroundings (K.m/W) |
| L1, L2 | sheath and armour loss factors |
| n | load-carrying conductors |
| u | twice the burial depth over the cable diameter |
What this model assumes, and where it stops
Assumptions
- Steady state at 100% load factor - the cable has been at this current long enough to settle.
- A single circuit with no other heat sources nearby.
- Uniform soil of the stated thermal resistivity, with no drying out around the cable.
- Balanced loading across the conductors.
- Sheath and armour loss factors are supplied rather than derived from the bonding arrangement.
Limitations
- Soil thermal resistivity is the dominant uncertainty and the hardest thing to know. It varies with moisture, and a cable that dries out the soil around itself can migrate from 0.9 to well over 2.5 K.m/W, which is a thermal runaway mechanism this steady calculation cannot show.
- Groups of circuits derate each other. This models one circuit; mutual heating between cables in a trench needs the group treatment in IEC 60287-2-1 or a field solution.
- Sheath and armour loss factors are inputs here. Deriving them properly depends on the bonding arrangement - solid bonded, cross bonded or single-point - and single-core armoured AC cable in particular can lose enormous amounts in the armour.
- The dimensions asked for are geometric idealisations. Real stranded conductors, screens and fillers differ, and manufacturer data will be more accurate than anything reconstructed from cross-section alone.
- Cyclic loading gives a higher permissible rating than this continuous figure; IEC 60853 covers that and this page does not.
- This is the calculation method, not a rating table and not a compliance check. Installation standards impose their own derating factors and rules that no formula reproduces.
When you need a 3D field solution instead
Closed-form models like the one above hold on idealised geometry. These are the cases where they stop being good enough and a full 3D electromagnetic and thermal solution is the only way to get a trustworthy answer:
- Groups of circuits in a trench or duct bank, where mutual heating decides the rating and the single-cable formula does not apply.
- Soil drying around a loaded cable, a coupled moisture and heat transport problem.
- Ducts, backfill and thermal stabilised sand, where the geometry is layered and irregular.
- Sheath and armour losses for a specific bonding arrangement, which are electromagnetic problems rather than thermal ones.
- Crossings, joints and terminations, which are local hot spots outside the uniform-section model.
- Cyclic and emergency ratings, where thermal mass rather than steady state sets the limit.
Common questions
Why is my calculated rating lower than the catalogue table?
Almost always the soil. Tables state a reference soil thermal resistivity - often 1.0 or 1.5 K.m/W, sometimes 2.5 - and your ground is drier. The default 95 mm2 case here rates 339 A at 1.0 K.m/W but only 227 A at 2.5, a one-third loss with no change to the cable. Check the table’s reference conditions before concluding either number is wrong.
How much does burial depth matter?
Less than people expect: going from 0.8 m to 1.5 m depth costs about 5% of the rating in the default case, because the external thermal resistance grows only logarithmically with depth. Soil resistivity and mutual heating from neighbouring circuits dominate; depth is mostly a mechanical-protection decision.
Will a bigger conductor fix my hot cable?
Probably not much. In the default buried case 82% of the total thermal resistance is the ground itself, so halving the conductor losses attacks only the remaining fraction of the temperature rise budget shared with it. Thermally stabilised backfill, more spacing from other circuits, or a shallower dry-out-resistant route usually buy more than the next conductor size.
References
- IEC 60287-1-1 - Electric cables - calculation of the current rating - current rating equations and calculation of losses
- IEC 60287-2-1 - Calculation of thermal resistance
- IEC 60853 - Calculation of the cyclic and emergency current rating of cables
- Neher & McGrath - The calculation of the temperature rise and load capability of cable systems, AIEE 1957