Anatomy of a Power Cable: Cable Design Layer by Layer, from Conductor to Oversheath

Every tender asks for "a cable", but no two tenders ask for the same one. Voltage, conductor, screen, armour, sheath and standard combine differently every time. To design quickly and correctly, it helps to see a power cable for what it is: a stack of concentric layers, each with a job, each dependent on the layers beneath it.

This article walks through a typical extruded power cable from the centre outwards — what each layer is for, what drives its design, and where things typically go wrong.

1. Conductor

The conductor carries the load current. The main choices are material (copper or aluminium), class (solid, stranded or flexible) and shape (round, compacted, sector-shaped or, for very large HV sizes, segmental). The size is driven by three different requirements — continuous current, short-circuit current and voltage drop — and knowing which one dominates tells you where to optimise. More in IEC 60228 explained.

2. Conductor screen (MV and above)

From medium voltage upwards, a semi-conductive layer over the conductor smooths the electric field around the individual strands. Without it, field peaks at the strand surfaces would trigger partial discharges and age the insulation from the inside.

3. Insulation

The insulation holds back the voltage. PVC dominates at low cost and low voltage; XLPE is the default for most power cables today because it tolerates higher operating and short-circuit temperatures; EPR adds flexibility; halogen-free compounds serve buildings, tunnels and transport. At low voltage, insulation thickness is largely a mechanical matter. At medium and high voltage it is set by electrical stress — which is why moving up one voltage class changes the whole cable.

4. Insulation screen (MV and above)

A second semi-conductive layer over the insulation means the insulation sits between two smooth, equipotential surfaces. Bonded or strippable? It sounds like a detail, but it affects jointing practice and accessories, so utilities often specify it.

5. Metallic screen or sheath

The metallic screen confines the field, carries charging current and — the key design driver — carries earth-fault current. Copper wires, copper tapes, lead sheaths, metal laminates and aluminium sheaths all exist for different reasons: fault capacity, radial water tightness, weight, cost. A bigger fault level means more metal in the screen, and on single-core circuits that can mean more losses and a lower current rating.

6. Fillers, binder and inner sheath (multicore cables)

Fillers and binders round off the laid-up cores so the next layer sits on a circular surface. An inner sheath or bedding protects the cores from the armour. These "invisible" layers still cost material and add diameter.

7. Armour

Armour gives mechanical protection and sometimes an earth path. Steel wire is the default for multicore buried cables; aluminium wire is used on single-core AC cables, where magnetic steel would cause heavy losses; steel tape offers crush resistance; braids suit flexible and shipboard cables. Armour sizes are selected by the diameter beneath them, so an upstream change can quietly push the design into a different armour size.

8. Oversheath

The outer sheath protects everything beneath from moisture, abrasion, chemicals, sunlight and, in some markets, termites and rodents. PVC, polyethylene and halogen-free compounds each have their place. Like armour, sheath thickness usually depends on the diameter underneath.

The key insight: a cable is a dependency chain

1. Conductor size sets the conductor diameter.
2. Voltage (and at LV, conductor size) sets the insulation.
3. The diameter over insulation sets the screen and laid-up diameter.
4. The diameter under armour selects the armour.
5. The diameter under the sheath sets the sheath.
6. Everything together gives outer diameter, weight, bending radius, drum length and bill of materials.

Change one input — a larger conductor, a higher voltage, one more core — and every layer above moves.

Where things go wrong in practice

• Copy-and-edit designs. A "similar" cable is copied in a spreadsheet and a few cells are changed. The armour or sheath that was right for the old diameter is now quietly wrong.
• Mixing single-core and multicore rules, such as steel armour on single-core AC circuits.
• Ignoring the screen in the rating. Fault-driven screen sizes change losses and current rating on single-core circuits.
• Datasheets drifting from designs. The datasheet is edited by hand and no longer matches what the plant will make.

Designing to the standard, every time

The standards define most of these rules. Encoding them once — and applying them to every new design — turns cable design from a manual, error-prone task into a repeatable one.

That is what CablePLM does. Each standards pack encodes the rules of its standard as a versioned calculation method. Enter voltage, conductor, screen, sheath and standard, and you get the complete layer-by-layer construction, the bill of materials, the datasheet and IEC 60287 current ratings — with every value marked as standard data or estimate.

Next: IEC 60502-1 for low-voltage power cables.

Want to see your own cable range built this way? Book a demo.